gps trainingmanual

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GPS Training Manualfor NFA Field Supervisors

Prepared by TSD, NFA

January 2006


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Preface

The manual is in three main parts. The first part introduces the user to GPStechnology. This is intended to make the user grasp the basic principles ofGPS technology, its versatility but also be made aware the limitations.

The second deals with Datum (geoids & ellipsoids) and projections. Many GPSusers fail to relate to existing base maps just because they lack basicunderstanding of this concept. The intention is thus to make the trainees getfamiliar with these concepts.

In the third and most important part are a series of hands on exercises.More emphasis will be put using GPS in real field conditions and collectingdata that relates to supervisors day to day work.


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1. GPS Technology ...................................................................................................................................5

Who is the user? .................................................................................................................................5 What is GPS?........................................................................................................................................5

1.1. The three Segments of GPS...................................................................................................6

The Space segment: ............................................................................................................................6 The control segment ...........................................................................................................................8 The User segment .............................................................................................................................10

1.2. How GPS works?.......................................................................................................................10

Position of Satellites ........................................................................................................................10 Distance to Satellites.......................................................................................................................11 Two-dimensional Trilateration........................................................................................................12 Three -dimensional Trilateration...................................................................................................13

2. GPS Limitations .................................................................................................................................14

Sources of Errors .............................................................................................................................14 Dilution of Precision (DOP) and Visibility ....................................................................................16 Solutions ..............................................................................................................................................17 Differential Correction ....................................................................................................................18

3. Datum and Projections ....................................................................................................................20

Projection of a Sphere onto a Cylinder. .......................................................................................24

4. Exercises .............................................................................................................................................29

Collecting data with a GPS...............................................................................................................29

Main GPS features: ...........................................................................................................................29 Map reference....................................................................................................................................29 Data capture .......................................................................................................................................30 Exercise Ia, Familiarising With Garmin GPS functional keys..................................................31Switch on the GPS.............................................................................................................................33 Garmin V Display Screens ................................................................................................................34 Exercise II, Navigating With Garmin V .......................................................................................35 Navigation and data capture exercise ..........................................................................................35 Exercise III, Averaging and storing a point with GPS V .........................................................36Exercise IV, Identifying GPS Functional Limitations ...............................................................38Exercise V, Building a standard data coding System (Participatory approach) ..................39Exercise V, Data uploading & Downloading using 1)MN DNR and 2), Mapsource ...............40

5. Further Reading ................................................................................................................................41

GPS Grades and Costs ......................................................................................................................41 The Differential GPS (DGPS) .........................................................................................................42 Civil user (or Recreational) Versus Professional GPS................................................................42Accuracy ..............................................................................................................................................42 Other Considerations .......................................................................................................................43 Confidence: The Final Differentiator ...........................................................................................44


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Table of figures

FIGURE 1.AN EXAMPLE OF A NAVSTAR SATELLITE ....................................................................................5 FIGURE 2.SATELLITE CONSTELLATION.........................................................................................................7 FIGURE 3.GROUND CONTROL SEGMENT ...........................................................................................................8

FIGURE 6.AN EXAMPLE OF A (2D)TRILATERATION ...................................................................................14 FIGURE 8.POOR SATELLITE CONSTELLATION.....................................................................................................1


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PART I

1. GPS Technology

Who is the user?GPS technology is rapidly changing how people find their way around the

earth (on land, at sea and in the air) for all sorts of activities. GPS

applications range from specialised fields like military, resource managers,

surveyors to any one else who wants to know where they are where they have

been or where they are going e.g., the nearest airport, nearest hospital, or

locate points of interest for fun, for fishing etc.

What is GPS?GPS is an acronym for Global Positioning System. The system is comprised of

a constellation of satellites (figure 1) or space vehicles (SVs) that

continuously transmit coded radio information to GPS receiver units. The

receiver units utilise this information to calculate their1 locations on earth.

Figure 1. An example of a NAVSTAR satellite

1 Note that the calculated position is the position of the GPS receiver antennae


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GPS technology is owned by the U.S department of defence. The U.S has

invested billions of dollars in the launch of the GPS satellites and their

maintenance. Prior to the 1980s GPS use was a preserve of the U.S military

and its allies. Up to day the US Department of Defence reserves the right to

down grade GPS signals of those it thinks are a threat to the US security.

1.1.The three Segments of GPS

The GPS technology owned by U.S Department of Defence is known as

NAVSTAR2 (an acronym for Navigation Satellite Timing and Ranging) and is

composed of the space segment, a control segment (the ground stations), and

a user segment (GPS receiver and its user).

The Space segment:The space segment consists a constellation of 24 satellites. The satellites are

in what is known as a high orbit i.e., 120,000 miles (19,300 km) above the

earth surface. Operating at such high altitude allows the signals to cover a

greater area (figure 2). The satellites are in six orbital planes (4 satellites

each), equally spaced (60 degrees apart). This step-up provides 4-8 SVs

visible from any point on earth.

2Other Systems are 1), GLONASS (Global Navigation Satellite System) of Russia which operates on

similar parameters to the U.S. based GPS and 2), GALILEO the EU non-military system which is expected

to be fully operational in 2008.


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Figure 2. Satellite Constellation

The satellites are powered by solar energy and are equipped with backup

batteries in case of solar energy failure (solar eclipses etc). They travel at

speeds of 7,000 miles an hour which allows them circle the earth once every

12 hours (two complete rotations every day).They have small rocket boosters

to keep them in the correct path.


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The control segmentAny satellite can slightly travel out of orbit, so the ground control segment

keeps track of satellite orbits, altitude, location and speed. The groundcontrol segment consists of a Master Control Centre, and a number of widely

separated monitoring stations. The Master Control facility is located at

Schriever Air Force Base (formerly Falcon AFB) in Colorado (figure 3).

The monitor stations measure signals from the SVs, which are incorporated

into orbital models for each satellite. The models compute precise orbital

data (ephemeris) and SV clock corrections for each satellite. The Master

Control station uploads ephemeris and clock data to the SVs. The SVs then

send subsets of the orbital ephemeris data to GPS receivers over radio

signals (Figure 4)

Figure 3. Ground Control Segment


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Figure 4. SVs, Ground Control and GPS Receiver Data Follow


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The User segmentThe user segment consists of the GPS receiver and the user . Navigation in

three dimensions is the primary function of GPS. Navigation receivers aremade for aircraft, ships, ground vehicles (figure 5), and for hand carrying by

individuals. Apart from navigation, GPS receivers are used for positioning,

time dissemination, and other research.

1.2.How GPS works?

The GPS calculates its location based on two things; 1) the position of three

or more satellites and 2), the distance to those satellites. The GPS receiver

figures both of these things out by analyzing high-frequency, low-power radio

signals from the GPS satellites. Better units have multiple receivers, so theycan pick up signals from several satellites simultaneously.

Position of Satellites

Position of satellites isn't particularly difficult because the satellites travel

in very high and predictable orbits. The GPS receiver simply stores an

Figure 5. An example of a user Segment,

Navigating ones way using a back ground map


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almanac (a time table) that tells it where every satellite should be at any

given time. Things like the pull of the moon and the sun do change the

satellites' orbits very slightly, but the Department of Defence constantly

monitors their exact positions and transmits any adjustments to all GPSreceivers as part of the satellites' signals.

Distance to Satellites

Radio waves are electromagnetic energy, which means they travel at the

speed of light (about 186,000 miles per second, 300,000 km per second in a

vacuum). The receiver can figure out how far the signal has travelled by

timing how long it took the signal to arrive. Distance from a given satellite

object equals the velocity of the transmitted signal multiplied by the time it

takes the signal to reach the receiver (i.e., Velocity X Travel Time =

Distance). Note that the GPS signal is a radio wave (speed of light) and is

thus a constant (186,000 miles3).

In order to make this measurement, the receiver and satellite both need

clocks that can be synchronized down to the nanosecond. To make a satellite

positioning system using only synchronized clocks, you would need to have

atomic clocks not only on all the satellites, but also in the receiver itself. But

atomic clocks cost somewhere between $50,000 and $100,000, which makes

them too expensive for everyday consumer use. To overcome this problem,

the GPS clock (an ordinary quartz clock) is constantly reset to match the

highly price satellite clocks by utilizing information from incoming signals

from four or more satellites. In away, the GPS receiver gets atomic clock

accuracy "for free".

3Needs to be adjusted for atmospheric disturbances. The Earth's atmosphere slows the electromagnetic

particularly as it goes through the ionosphere and troposphere.


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Travel time measure

To measure signal travel time, the satellite begins transmitting a long, digital

pattern called a pseudo random code4 at a particular time (let's say midday).

The receiver begins running the same code also exactly at midday. When the

satellite's signal reaches the receiver, its transmission of the pattern (code)

will lag a bit behind the receiver's playing of the pattern. The receiver then

compares the two codes to determine how long it needs to delay (or how much

to shift) its code to match the satellite code. The length of the delay is

equal to the signal's travel time. The receiver multiplies this time by the

speed of light to determine how far the signal travelled.

A GPS receiver's job is to locate four or more of these satellites, figure out

the distance to each, and use this information to deduce its own location. This

operation is based on a simple mathematical principle called trilateration.

Trilateration in three-dimensional space can be a little tricky, so we'll start

with an explanation of simple two-dimensional trilateration.

Two-dimensional TrilaterationImagine you are somewhere in central Uganda and you are TOTALLY lost. For

whatever reason you have absolutely no clue where you are. You find a

friendly local and ask, "Where am I?" He says, "You are 87.5 km form

Packwach. This is important information but it does not tell you much of

where you are.

Supposing you ask somebody else of where you are, and she says, "You are

183 km from Fort Portal. Now you're getting somewhere. If you combine this

information with the Packwach information, you have two circles that

intersect (i.e., some where between eastern RDC and Hoima).

4 A predictable but different pattern of values (random numbers) that the GPS receiver can track.


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If a third person tells you that you are 123 km form Kakoge, you can

eliminate one of the possibilities, because the third circle will only intersect

with one of these points. You now know you are exactly at Nyabyeya Forestry

College, Hoima (figure 6).

Three -dimensional TrilaterationFundamentally, three-dimensional trilateration isn't much different from

two-dimensional trilateration, but it's a little trickier to visualize. Imagine

the radii from the examples in the last section going off in all directions. So

instead of a series of circles, you get a series of spheres.

If you know you are 10,000 km from satellite A in the sky, you could be

anywhere on the surface of a huge, imaginary sphere with a 10,000 km radius.

If you also know you are 15,000 km from satellite B, you can overlap the first

sphere with another, larger sphere. The spheres intersect in a perfect circle.

If you know the distance to a third satellite, you get a third sphere, which

intersects with this circle at two points.

The Earth itself can act as a fourth sphere -- only one of the two possible

points will actually be on the surface of the planet, so you can eliminate the

one in space. Receivers generally look to four or more satellites, however, to

improve accuracy and provide precise altitude information.


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#Kakog e

#

#

Kakoge

Packwach

Fort Portal

87.5 Km

123.3Km183.1Km

Figure 6. An example of a (2D) Trilateration

2. GPS Limitations

Sources of Errors

This system works pretty well, but inaccuracies do pop up. For one thing, thismethod assumes the radio signals will make their way through the atmosphere

at a consistent speed (the speed of light). In fact, the Earth's atmosphere

slows the electromagnetic energy down somewhat, particularly as it goes

through the ionosphere and troposphere. The delay varies depending on

Nyabyeya

Forestry College,

Hoima


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where you are on Earth, which means it's difficult to accurately factor this

into the distance calculations. The ionosphere is the layer of the atmosphere

from 50 to 500 km that consists of ionised air (figure 7). This can affect

accuracy up to 10 metres. The troposphere, the lower part of the atmosphere(especially the ground level of from 8 to 13 km) experiences changes in

temperature, pressure, and humidity associated with weather changes. This

can result in about 1 metre error.

Problems can also occur when radio signals bounce off large objects, such as

tall buildings (a factor known as Multipath), giving a receiver the impression

that a satellite is farther away than it actually is. Multipath is difficult to

detect and sometime hard to avoid. This can result in 0.5 metre accuracy.

Figure 7. Signal travel through the ionosphere and Troposphere


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Other minor errors are due to software or hardware failures (e.g., satellites

sending out bad almanac data, misreporting their own position) or human

errors e.g. incorrect geodetic datum selection.

The major source of error of GPS position data, is unfortunately intentional.This when the US DOD intentionally degrades satellite signals meant for the

civil users considered a threat to US security. This process is called

Selective Availability (SA). The original potential accuracy of 30 meters is

reduced to 100 meters (two standard deviations).

Dilution of Precision (DOP) and VisibilityDOP is a description of the purely geometrical contribution of the

uncertainty in a position of fix. Geographical Dilution of PrecisionGDOP iscomputed from the geometric relationships between the receiver position and

the positions of the satellites the receiver is using for navigation.

For planning purposes GDOP is often computed from Almanacs and an

estimated position. Estimated GDOP does not take into account obstacles

that block the line-of-sight from the position to the satellites. Estimated

GDOP may not be realisable in the field (figure 6).

Figure 8. Poor satellite constellation


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DOP factors depend on the parameters of the position-fix solution. Standardterms for the GPS applications are:

PDOP = Position Dilution of Precision (3-D), sometimes the Spherical DOP.

HDOP = Horizontal Dilution of Precision (Latitude, Longitude)

VDOP = Vertical Dilution of Precision (Height).

TDOP = Time Dilution of Precision (Time).

While each of these GDOP terms can be individually computed, they are

formed from covariances and so are not independent of each other. A high

TDOP (time dilution of precision), for example, will cause receiver clock

errors which will eventually result in increased position errors

SolutionsSolution to bias- errors:Since the GPS calculated positions (false positions) oscillates about the true

position (figure 7), averaging closes in to the true position. Although

Figure 9. Good satellite constellation, sharp

intersection


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averaging reduces bias, good results might require averaging for a long time-

maybe more than three hours.

Differential CorrectionErrors caused by atmospheric phenomenon and due to Selective Availability5

(SA) can be corrected by differential correction. By differential correction,

bias errors at one location are corrected with measured bias errors at a

known position. Data recording by the reference station and roving receiver

must be performed during the same time frame. Differential corrections canbe applied on the fly, i.e., real time differential, or later, using post-

processing techniques i.e., Post processing differential correction.

5Since the end of the cold war (May 200), the US is no longer applying SA but reserves the right to do so

when deemed necessary

X

100m

Figure 10:GPS false positions about its true position (i.e., due to atmospheric phenomenon & SA )

Z

Y

True position


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For Real-time differential, the master reference station, at a known

position, decimetre accuracy, transmits differential corrections to rover

units over an internal radio or an external one. Timing of the correction is so

critical, so the differential correction is time stamped before transmission.

The rate of change of the differential correction values is calculated and

transmitted to roving units. It is possible to get signals form private services

e.g., FM sub-carrier broadcasts, satellite links6, or private radio beacons for

real-time applications.

In Post processing procedure, a reference station records information to be

used to generate a correction file at same time as rover units collect data.Data is then processed on your personal computer by post processing

software (such as Grafnav, Postpoint, or Centipoint) to remove position

error7.

Correction files for a particular time period may also be available from public

and private agencies (e.g. bulletin board service) that record dGPS

corrections for distribution by electronic means.

With differential correction, position error is reduced to sub-metre.

6Currently, real time differential signal supplier for Sub-Saharan Africa is only Oministar of South Africa

7This a cheaper solution but needs dedicating personnel and equipment to ensured that whenever data is

being collected, base stations are up and running


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PART II

3. Datum and Projections

GPSs are increasingly becoming surveying, navigation and mapping tools.

Planning of these operations and mapping is easily done on flat surfaces. The

earth, on the other hand, has a highly irregular and constantly changing

surface (both land and sea) Thus irregularities (hills and valleys) along the

measuring surface can cause ambiguities in distance (and thereby,

location).To alleviate this situation, it has long been a common practice to

reduce all measurements to a more regular measuring surfacea reference

surface.

Geoid

The oldest reference surface used for mapping is known as the geoid. The

geoid can be thought of as mean sea level, or where mean sea level would be if

the oceans could flow under the continents. More technically, the geoid is an

equipotential surface of gravity defining all points in which the force of

gravity is equivalent to that experienced at the ocean's surface.

Since the earth spins on its axis and causes gravity to be counteracted by

centrifugal force progressively towards the equator, one would expect the

shape of the geoid to be an oblate spheroida sphere-like object with a

slightly fatter middle and flattened poles. In other words, the geoid would

have the nature of an ellipse of revolutionan ellipsoid.

As a reference surface, the geoid has several advantagesit has a simple

physical interpretation (and an observable position along the coast), and it

defines the horizontal for most traditional measuring instruments. Thus, for


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example, leveling a theodolite or sextant is, by definition, a process of

referring the instrument to the geoid.

Reference EllipsoidsUnfortunately, as it turns out, the geoid

is itself somewhat irregular. Because of

broad differences in earth materials

(such as heavier ocean basin materials

and lighter continental materials,

irregular distributions such as mountains,

and isostatic imbalances), the geoid contains undulations that also introduce

ambiguities of distance and location. As a result, it has become the practice

of modern geodetic surveys to use abstract reference surfaces that are

close approximations to the shape of the geoid, but which provide perfectly

smooth reference ellipsoids (Figure 11). By choosing one that is as close an

approximation as possible, the difference between the level of a surveying

instrument (defined by the irregular geoid) and the horizontal of the

reference ellipsoid is minimized. Moreover, by reducing all measurements to

this idealized shape,

ambiguities of distance

(and position) are removed.

There are many different

ellipsoids in geodetic use.

They can be defined either

by the length of the major

(a) and minor (b) semi-axes

(Figure 12), or by the

Figure 11: Smoothening out earthsurfaces

Figure 11.Ellipsoidal Parameters


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length of the semi-major axis along with the degree of flattening [f = (a-b) /

a]. The reason for having so many different ellipsoids is that different ones

give better fits to the shape of the geoid at different locations.

Geodetic DatumsSelecting a specific reference ellipsoid to use for a specific area and

orienting it to the landscape, defines what is known in Geodesy as a datum

(note that the plural of datum in geodesy is datums, not data!). A datum thus

defines an ellipsoid (itself defined by the major and minor semi-axes), an

initial location, an initial azimuth (a reference direction to define the

direction of north), and the distance between the geoid and the ellipsoid at

the initial location. Establishing a datum is the task of geodetic surveyors,

and is done in the context of the establishment of national or international

geodetic control survey networks.

By contrast to local datums, we are now seeing the emergence of World

Geodetic Systems (such as WGS84) that do try to provide a single smooth

reference surface for the entire globe. Such systems are particularly

appropriate for measuring systems that do not use gravity as a reference

frame, such as Global Positioning Systems (GPS). However, presently they are

not very commonly found as a base for mapping. More typically one

encounters local datums, of which several hundred are currently in use.

Datums and Geodetic Coordinates

It is common to assume that latitude and longitude are fixed geographic

concepts, but they are not. There are several hundred different concepts of

latitude and longitude currently in use (one for each datum).

It is important to bear in mind about local datums is that each defines a

different concept of geodetic coordinates latitude and longitude. Thus, in

cases where more than one datum exists for a single location, more than one


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concept of latitude and longitude exists. It can almost be thought of as a

philosophical difference.

For example if one were to measure latitude and longitude according to

WGS84 and compare it to the ground position of the same coordinates in the

ARC1960 system, the difference is about 300 metres in the northing and

about 30 metres in the easting. Note that over 90% of data mismatch by

GPS users is due to specifying a datum that does not correspond to that

of a base map.

Projection

A globe is the only representation of the earth that does not distort its

geometry-except, of course, its size. Unfortunately, for many purposes a

globe is an inconvenient way to display geographical relationships: e.g.,

shortest routes and equal distances are difficult to measure on curving

surface.

The process of transforming spheroidal geodetic coordinates to plane

coordinate positions is known as projection, and falls traditionally within the

realm of cartography. Originally, the concern was only with a one-way

projection of geodetic coordinates to the plane coordinates of a map sheet.

With the advent of GIS, however, this concern has now broadened to include

the need to undertake transformations in both directions in order to develop

a unified database incorporating maps that are all brought to a common

projection.


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Projection of a Sphere onto a Cylinder.The mercator cylindrical Projection projects the earth surface into a

cylinder that shares the same axis as the earth. The meridians are equally

spaced but the parallels are not (figure).

In Secant Cylindrical Projection In the secant case, the cylinder touches the

sphere along two lines, both small circles (a circle formed on the surface of

the Earth by a plane not passing through the centre of the Earth).

When the cylinder upon which the sphere is projected is at right angles to

the poles, the cylinder and resulting projection are transverse. Transverse

Mercator maps are often used to portray areas with larger north-south than

east-west extent.


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The Universal Transverse Mercator projection was developed to set a universal

world-wide system for mapping (and is probably the most commonly used map

projection). The cylinder of projection was rotated such that its axis passes

through the equator and was turned in sixty positions to create sixty zones

around the world, each six degrees in width.

Eastings begin at 500,000 on the centre line (central meridian) of each zone. In

the Northern hemisphere, Northings begin at the equator (0) and increase as

they move towards the pole. In the southern hemisphere, Northings begin at

10,000,000 at the equator (to eliminate negative numbers) and they decrease

as they move towards the pole. To determine where your location on the globe,

you must know which hemisphere and zone you are in, as co-ordinates will be

identical from each zone to zone without the zone number and zone grid latter

(e.g., 36 N 0493,245E / 40,085 N)

Fi ure 12


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UTM system divides the Earth into 60 zones each of 6 degree of longitudewide. The zones define the reference point for UTM grid coordinates withthe the zone. UTM zones extend from a latitude of 80o S to 84 o N. In thepolar regions the Universal Polar Stereographic (UPS) gridsystem is used. UTM zones are numbered from 1 to 60 startingat the International Date Line, longitude 180o, and proceedeast.

Each zone is divided into bands of 8o latitude lettered south tonorth beginning with C (omitting I and O) and ending with X.Latititudal band X, the only exception spans 12 degree. Whenusing UTM coordinates, these zone letters are included in thedescription as well as the band number e.g.

36 N '0493245

UTM '0043245

or

36 M '0493245

UTM 9940845


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Zone 36

CentralMeridian

N

S

Zone 35 Zone 37

Ce

ntralMeridian

CentralMeridian


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References and Further reading

Eastman J. Ronald, 1995, Idrisi for Window, Users Guide Version 1.0 Clark University,

USA

GPS Joint Program Office. 1997. ICD-GPS-200: GPS Interface Control Document.

ARINC Research.Available on line from United States Coast Guard Navigation Center.

Global Positioning System Standard Positioning Service Specification, 2nd

Edition, June2,

1995. Available on line from United States Coast Guard Navigation Center.

Hoffmann-Wellenhof, B. H. Lichtenegger, and J. Collins. 1994. GPS: Theory and

Practice. 3rd ed.New York: Springer-Verlag.

Institute of Navigation. 1980, 1884, 1986, 1993. Global Positioning System monographs.

Washington, DC: The Institute of Navigation.

Kaplan, Elliott D. ed. 1996. Understanding GPS: Principles and Applications. Boston:

Artech House Publishers.

Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New York: John Wiley & Sons.

National Imagery and Mapping Agency. 1997. Department of Defense World Geodetic

System 1984: Its Definition and Relationship with Local Geodetic Systems. NIMA

Navtech Seminars and GPS Supply 6121 Lincolnia Rd. Suite 400, Arlington, VA 22312-

2707 USA - (800) 628-0885 or (703) 256-8900). Fax: (703) 256-8988

TR8350.2 Third Edition. 4 July 1997. Bethesda, MD: National Imagery and Mapping

Agency. Available on line from National Imagery and apping Agency.

NAVSTAR GPS User Equipment Introduction. 1996. Available on line from United

States Coast Guard Navigation Center.

Parkinson, Bradford W. and James J. Spilker. eds. 1996. Global Positioning System:Theory and Practice. Volumes I and II. Washington, DC: American Institute of

Aeronautics and Astronautics, Inc.

Wells, David, ed. 1989. Guide to GPS positioning. Fredericton, NB, Canada:Canadian

GPS Associates.


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PART III

4. Exercises

Collecting data with a GPS

Main GPS features:

The alpha-numeric keys used to initiate modes and functions, to respondto software generated in put prompts, and to enter names, numbers,attribute, features, reference points and anything needed for data entry.

Display screen (may be crystal display) displays prompts from the system.

The antenna may be internal or external. Signals from SVs are via theantenna and the position displayed on the GPS screen is that of theantenna.

Power supply (battery or AC adaptor) for powering the unit.

Memory (RAM and for data storage). In some systems the main memoryand storage media are separate. In some systems data has to betransferred by hooking on to a PC while others systems can read and storedata on PCMCIA cards.

Getting started (Switch on the GPS unit):1. The unit might need to be initializedthe first time it is switched on. This

is to speed up the process of acquiring satellite signals. If you have notused your unit for long (e.g., 6 months) or have changed location (off thecontinent for example), satellite acquisition may take several minuteslonger.

2. The menu selectiongives you a number of options.

i) Settingsa) You may need to select a datumthat best suits your location

Map reference

b) The map referenceor map coordinate type mainly UTM (metres north andEast form reference point) or Geodetic (Degrees, Minutes and Seconds inrelation to Centre of the earth)

i) Location or Position option (This options displays coordinates of yourlocation on the screen)

ii) File manager / Library. Depending on the system, it is where you designthe format in which you want capture data and store it. For example,


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Point, Arc or Polygon attributes. In some systems you can even definethe data base fields.

ii) Most GPS have the Navigation and Route options. You need to knownthe grid reference of the starting point and end point. You may alsoput in a number of waypoints.

iii) Averaging: In some systems, averaging of data being recorded isindependent of averaging on the display screen.

Data capture

Before capturing data, be sure of the way data is going to be processed. Barein mind that, even with post processing facilities, common GPS receiversneed averaging for a bout half an hour. Thus, depending on the level accuracyneeded, mapping line features might require averaging a set of points alongthe line.

If data is going to be post processed, the Base stationhas to be running atsame time as data is being collected. Depending on the software, data forpost processing has to be of the right format (e.g., Centipoint needs PostData Raw format). In some cases, an accuracy of within 100 meters radius isacceptable. In instances when post processing is not needed, the unit is saidto be in stand- alone mode. Note that some expensive GPS have real timedifferentially corrected positions and averaging of very few minutes (notmore than five) is good enough.

For whatever receiver used, obstacles reduce the quality of GPS positioning.Make sure that the unit is clear of obstacles like tall buildings and big trees.

A clear view of the horizon gives the best satellite constellation for goodtriangulation.

Figure 12. Avoid obstruction


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Exercise Ia, Familiarising With Garmin GPS

functionalkeys

Find

Power

Quit

Enter Mark

Zoom in outPage

Menu

Rocker keypad


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Switch on the GPS


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Garmin V Display Screens

Garmin V has three main display screen (pages) vizthe GPS Satellite statusscreen, Map display Screens and the Trip Information Screen (co-ordinates

page). The Satellite Status Screen gives information on Satellite location and

satellite number (or space vehicle number), signal strength, receiver status.


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RReecceeiivveerr SSttaattuuss;;

Before use ensure that the GPS has acquired at least three satellites

(preferably four or more). A GPS that has not been in use for over six

months requires a bout 15 minutes to download new information about SVs.

Exercise II, Navigating With Garmin V

Navigation and data capture exercise1. Explore the available GPS settings especially datum, projection,

simulation mode and GPS mode

2. Find and display waypoints loaded in the GPS (by name & by nearest)

3. Navigate to Waypoints already loaded in the GPS (loaded using data

transfer cable fixed to a PC)


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4. Navigate to area of interest picked from a paper map

5. Navigate to area of interest from uploaded background maps and

shapefiles

6. Load and navigate to x,y co-ordinates using rocker keypad and virtual

keyboard

7. Navigate to object Ficus natalensis

8. Navigate to object Directional Trench (DT_002)

9. Navigate to Mound (MD) 05

10.Identify special point features in the field and save them as waypoints

11.Identify special line features in the field and save them as tracks

12.Identify special line features in the field and save them as a series of

points

Exercise III, Averaging and storing a point with GPS

V

1)

When the GPS is ready as described above, move to with in 3-5 metersfrom the building with the solar instillations.

2) On the Press the Menu buttononce.

3) A pop up menu will appear showing various commands. Use the down

arrow key on the rocker button to scroll down and select the Average

location option then press the enter button.

4) Let the GPS calculate you current potion by making 10 to 15 samples.

These appear in the filed labeled measurement count.

5) Select save option (if not already) and press the enter button

6) Now record the coordinates (Easting and Northing) on the filed form

provided.


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7) Scroll from the OK upwards 3 times and once to the right to select the

waypoint name and press the enter button. (The way point name is

usually a 3-digit number). A virtual keyboard appears.

8) Use the appropriate arrows up, down, left or right to select thecharacters making up the clients name from the virtual keyboard.

After highlighting the desired character, press the enter button and

the cursor will automatically jump to the next space. Continue till the

name is fully entered.

9) When you have finished, scroll to the OK option on the virtual

keyboard and the press the enter button.

10)The display goes one step backwards to a dialogue box. where you

recorded the coordinates from. Scroll down to select the OK option

and press the enter button.

11)Your information is now stored on both paper and in the GPS memory.

You are now ready to visit the next client!

Note

In all cases, please remember to write on the form the file name for

the point you save in the GPS. This will help us relate the GPS data to

what is recorded on the on the form during the cross checking

exercise.

You do not have to type in the full client name into the GPS. Use the

shorter name or abbreviation, which must also appear on the form or

note book.


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Exercise IV, Identifying GPS Functional Limitations1. How many points were you able to access or navigate to

2. How many points were you unable to reach and why were you unable to

navigate to them

3. How many satellites were available under object Fcus natalensis

4. How many satellites were available under object (DT_002)

5. What do you think is the cause of difference between 4 and 3?

6. What do you think are the solutions to the above limitations

7. Practice using offsets (using other navigational tools e.g. compass &

measuring)

8. Compare results of averaged points with un averaged points

Dos and Donts

Dos

Do follow all the instructions you were told/given If you find yourself on a page you dont understand, press the Quit or Page

button till you get to the coordinates page.

Donts

Do not experiment or tamper in any way with the GPS settings. You mayrender the GPS unusable or even damage the unit.

Do not drop, hit or shock in any way the GPS unit.

Do not leave the GPS unit on (running) unattended. You risk damaging theunit and running down the batteries in the field.

Though the unit is water proof and ruggedly built do not expose the GPSunit to rain

Do not hold the GPS unitby the strap. This may cause breakage or powerdisruption of the unit.


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Exercise V, Building a standard data coding System

(Participatory approach)Boundaries:

Boundary line description and status;i. opened

ii. changed bushyiii. date openediv. opened byv. disputed segment

Boundary types: River, road, paths, cut line, wetland, shoreline, fire line.

Planted areas:Forest Reserve description and Area allocation

Forest reserve area allocation and General area description;

i. Soils typeii. Terrain

Flat Undulating Hilly Mountainous

iii. Drainage Good Seasonally wet

Water loggedTotal reserve in hectares

1. Area plantablei. Roads

ii. Harvested

iii. Burntiv. Never planted beforev. Damaged by

a) Pest attackb) Firec) Windd) Land slidee) Animalsf) Drought

Planted area categories

i. by speciesii. by age

iii. by operationa) weeding

b) pruningc) thinning

d) fire line maintenance, harvesting

Area not plantable:Area not plantable


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i. Conservation areaa) Natural forest

b) Wetlandii. Settlements

a) Scattered homesteadsb) Villages

iii. Impedimentsa) Rocks

b) Sandc) Shallow lime bed rock layer

iv. Steep slopesv. Poor soils

Exercise V, Data uploading & Downloading using

1)MN DNR and 2), Mapsource


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5. Further Reading

GPS Grades and Costs

Receiver costs vary depending on capabilities. Small civil user receivers canbe purchased for under $200, some can accept differential corrections.Receivers that can store files for post-processing with base station files costmore ($2000-5000).

Receivers that can act as dGPS reference receivers (computing andproviding correction data) and carrier phase tracking receivers (two areoften required) can cost many thousands of dollars ($5,000 to $40,000).Military standards receivers may cost more or be difficult to obtain.

Other costs include the cost of multiple receivers when needed, post-processing software, and the cost of specially trained personnel.

Project tasks can often be categorised by required accuracy which willdetermine equipment cost.

Low-cost, single-receiver for civil users (10 metres. Goes up to 100+ meterwhen SA is on)

Medium-cost, can support differential processing (1-10 meter accuracy)

High-cost, single-receiver authorised users (20 meter and below evenwith SA on)

High-cost, differential carrier phase surveys (1 mm to 1 cm accuracy)


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The Differential GPS (DGPS)

In this section you will see how a simple concept can increase the accuracy of

GPS to almost unbelievable limits.

Typical Error inMeters(per satellite) Standard GPS Differential GPS

Satellite Clocks 1.5 0

Orbit Errors 2.5 0

Ionosphere 5.0 0.4

Troposphere 0.5 0.2

Receiver Noise 0.3 0.3

Multipath 0.6 0.6

Civil user (or Recreational) Versus Professional GPSGood decisions require good information. For GIS users, the quality of information coming out of

their systems depends on the accuracy of the data going in. Confident decision making leaves no

room for doubt over data accuracy. The last thing GIS users need to worry about is whether the

data collected by GPS meets the accuracy requirements of the intended application.

Given the large number of products on the market, selecting the right GPS receiver can be

difficult. With budgets tightened everywhere, some GIS users have begun looking at and buying

less expensive recreational GPS products that are popular with outdoor enthusiasts. New

features, such as ruggedized cases and differential correction, and a price tag often below $500make these units attractive compared with higher-priced professional-grade GPS receivers.

But beware, as is true with most products, you get what you pay for. There is a significant

difference in the accuracy of location data acquired by recreational GPS receivers versus the

professional units. The 10-meter error typical of a recreational model won't cause a major

problem for a hiker in the woods, but such inaccuracy may not be acceptable for GIS applications.

Accuracy

Recreational and professional GPS units are designed and built for different purposes. A

recreational GPS unit is designed to acquire a location fix quickly without the need for pinpoint

accuracy because hikers can find their campsite once they get within 10 meters of it. GIS users,

on the other hand, typically require extremely accurate placement of features often to within a

meter or less so that data layers can be overlaid and intricate spatial relationships can be

determined.


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Although recreational products are not specifically designed for GIS mapping, they can be used

successfully in some applications. And for some GIS users, the recreational products may be the

most cost-effective choice. In choosing between a recreational and a professional GPS receiver,

GIS users should answer the following questions to be certain the selected unit will meet their

application needs.

* Do you need to integrate data seamlessly with a GIS?

If you will be converting GPS points to a specific GIS format, such as shapefile format, you

should purchase a professional-grade GPS receiver. Some newer units can even convert points to

popular GIS formats on the fly during downloading. Most recreational receivers cannot convert

data to other formats.

* Will you be collecting attributes along with location points?

Many GIS users have found that accurate attribute collection is just as crucial as location

acquisition. Only the professional GPS products offer customizable interfaces and routines for

detailed attribute collection.

* Is five-meter accuracy sufficient for your application?

A recreational GPS is typically able to achieve 10-meter accuracy in autonomous mode, but some

now can handle real-time differential correction capable of sharpening accuracy to five meters or

better. In this situation, the most cost-effective purchase may be the recreational unit.

* Is submeter accuracy required for your application?

For many GIS users, accuracy is measured in centimetres. In these cases, professional GPS units

are the only ones capable of performing the differential postprocessing required to achieve this

level of accuracy.

Other Considerations

After price, data quality and accuracy are the main differentiators between recreational and

professional units that influence the buying decision of a GIS user. Engineering, design, and

construction characteristics account for the variation in capabilities among GPS receivers.

Professional units have been engineered and built to acquire more accurate location coordinates.

Although many design features contribute to this higher level of performance, three factors

quality control, electromagnetic shielding, and antenna technologyset GIS-grade products apart

from recreational receivers.


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Quality ControlProfessional GPS units give users control over the quality of the position points

that are collected. Through a simple interface, the user can establish specific thresholds for

acceptable data quality. For instance, the user chooses the number of satellites and position

above the horizon needed to achieve suitable accuracy. The user can also program the receiver to

disregard any satellite signals that suffer from too much noise interference. These qualitycontrol settings essentially allow the user to filter out any potentially poor data that may degrade

the overall quality of the location coordinates, resulting in greater accuracy in the final dataset.

Electromagnetic ShieldingSignals from GPS satellites are very weak and can easily be degraded

by interference from nearby electronic devices such as laptop computers or personal digital

assistants (PDAs). Given the fact that many GPS receivers and GPS cards are linked to computers

and PDAs, this can pose a serious problem. High-end GPS products have built-in shielding

technology that minimizes the effects of stray electromagnetic signals from other equipment.

Antenna TechnologyWeak GPS signals requires a sensitive antenna, especially when receiving

transmissions in urban canyons and under tree canopies. The antennas provided with professional

grade GPS units are designed to pick up signals in almost any environment. More important, high-

end antennas protect against interference from multipath signals. These signals from GPS

satellites have been degraded by bouncing off buildings and other overhead features on their way

to the receiver on the ground. Multipath signals can significantly reduce the accuracy of location

calculations. However, antennas on professional receivers recognize and filter out multipath

signals.

Confidence: The Final Differentiator

For GIS users, settling for a receiver that collects data less accurate than is required by the GIS

application will cast doubts over management decisions based on the information coming out of the

system. While shopping for a GPS receiver, GIS users should honestly compare the needs of their

GIS application with the GPS receivers in their price range.

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