LAB 1

METHODS FOR LOCATING

YOUR FIELD DATA IN

GEOGRAPHIC SPACE

Geog 315 / ENSP 428

Lab 1 Schedule

Introduction to bio-physical field data collection (8:00-8:20am)

Locating your data on the earth: NAVSTAR Global Positioning System (8:20-9:20am)

-- 15-min Break --

Quiz (9:35-10:00am)

Measuring distance and azimuth(10:00-10:30am)

--15-min Break –

Planning the “field campaign” (10:45am-11:10am)

Introduction to Trimble Juno GPS units and Impulse laser rangefinder(11:10am-11:40pm)

Lab Objectives

Understand the spatial dimension of field data, and the costs and benefits of collecting it

Understand how the Global Positions System (GPS) works, its advantages, and its limitations

Understand how distance between points can be surveyed with laser rangefinders, and know when this technology is appropriate to use

Learn how to effectively plan for a field campaign to increase sampling efficiency and spatial and attribute data accuracy

Exposure to GPS units and laser rangefinders used in Lab 2

Collecting bio-physical field data

La Selva Biological Field Station

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1000 0 1000 Meters

Soils / SuelosOld alluvium / Aluvion viejoRecent alluvium / Aluvion recienteResidual

Stream-associated / Suelo de quebradasSwamp / Pantano

# Species present / Presencia del especies

N

Dipteryx panamensis

Elephant herd home range

http://www.save-the-elephants.org

Spatial information

• Most spatial information of interest is geographic –

can be placed on the earth

• Spatial information links a place with a property of that

place– The temperature is 40° C at

Latitude: 38° N, Longitude: 122° W

• Could also be at a specific time– The temperature is 40° C at Latitude: 38° N, Longitude: 122° W at

02/13/2009, 8 am

• Properties are variables that we measure – sensed

with our body or instruments

• Can be quantitative or qualitative

• The potential number of geographic places and their

properties is vast…

Some bio-physical variables of interest

Biological – plants, animals, fungi, etc.

Composition – presence/absence, abundance of species or groups, communities

Vegetation structure – biomass, height, diameter, percent cover, leaf area index

Physical environment variables

Moisture, temperature, light

Geological composition

Chemistry of soil, water or air (e.g., nutrients, pollution)

Geomorphology – shape of the land, including slope, aspect, elevation

Disturbance and threats

Natural: fire, wind-throw, pest attack, species invasion

Anthropogenic: deforestation, poaching, grazing

Sampling geographic information

Type

Single points

Transects along a line

Plots

Rapid assessment visit

Scale

Space – boundary area; geographic area or length of samples; distribution of samples

Time -- return interval. Once every year? Need to come back to location? Need to leave a monument?

Considerations

Preliminary assessment or long-term monitoring

Time

Money

Access

Global Navigation Satellite Systems

(GNSS)

Locating your data on the earth

Global Navigation Satellite Systems (GNSS)

► Most common approach to surveying locations is using Global Navigation Satellite Systems (GNSS)

► It uses range measurements based on radio signals from satellites

► Systems developed by USA, Russia, European community and China

► Developed, owned and maintained by the U.S. Department of Defense (US$400 million per year to maintain)

► Accurately determines horizontal location, elevation and speed

Almost anywhere on earth

day or night

any weather

► Free for public use!

NAVSTAR

Global Positioning System (“GPS”)

GPS segments

GPS satellite segment

32

http://science.nasa.gov/Realtime/jtrack/3d/JTrack3D.html

GPS user segment

GPS General Information

GPS satellites broadcast three different types of data using radio waves

1. Almanac data

- system health and rough orbits of all GPS satellites; tells receiver which satellites to “listen” for

2. Ephemeris data for the broadcasting satellite;

- allows a GPS receiver to accurately calculate the position of the broadcasting satellite

- satellite health, clock corrections, etc.

3. Coded signals

- Coarse Acquisition code, or C/A, and the Precise code, or P-code (C/A code used mainly in civilian applications)

Range = speed of light x travel time

Range = c(t1 – t0)

(c =299,792,458 meters per second)

t0

t1

A single satellite

range measurement

GPS code receiver

Assume that satellite and receiver are generating the same pseudo-random code at exactly the same time

Measurements to multiple satellites determines position

Sources of Error

Several factors can result in erroneous location

determination with GNSS (or GPS)

Source Range Error (m)

Satellite clock error 1

Satellite position error 1

Atmospheric & Ionospheric effects 4

Receiver error 1.5

Total ~7.5

Positional uncertainty (1)

Positional uncertainty (2)

Leads to positional

uncertainty…

Ionosphere

http://www.aiub.unibe.ch/ionospherehttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter1/ion.html

• Electrified region within the upper atmosphere• Can reflect, deflect and scatter radio waves – increase range

Source:http://www.garmin.com/aboutGPS/

Multipath signal error

Positional Dilution of Precision (PDOP)

Obstructions to satellite signals

Telescoping pole

Adjusting PDOP thresholds

Typically want PDOP < 6

Types of receivers (1)

Many types of receivers on the market – vary in price, features and performance

Most now are multi-channel (12) – can track up to 12 satellites

Ability to average points

Ability to change projection and datum

Display screen for features and maps

Memory to hold features and properties (attributes)

Ability to download data

Battery life

Differential corrections (more on this coming)

Antennas that reduce error

Types of receivers (2)

Recreational receivers - $200 to $1000

Mapping grade - $1000 - $10,000

Set PDOP, satellite elevation and signal-to-noise filters

Target satellites

Point averaging

Differential corrections

Data dictionary and data download

High-end survey grade

Better antennas

Can achieve centimeter accuracy

Differential correction

For each satellite, the roving (receiver) range is corrected by the observed range error at base station

Differential correction

Two common

types of

differential

GPS

Best performance

if base station

within 180 miles,

300 km

CORS - Continuously Operating Reference Stations

National Geodetic Survey (NGS), an office of NOAA's National Ocean Service, coordinates a network of Continuously Operating Reference Stations (CORS) –base stations

Each CORS site provides carrier phase and code range measurements for differential correction

GNSS - GPS and GLONASS supported

CORS data are available at their original sampling rate for 30 days, after that at reduced sampling rate

http://www.ngs.noaa.gov/CORS/

CORS

http://www.ngs.noaa.gov/CORS/GoogleMap/

Measuring distance and azimuth angles

Distance and azimuth measurements

Offset points: We can locate a fixed point with a GPS (GNSS) receiver and then calculate the horizontal position of other points relative to the GPS point with distance and azimuth (bearing) angle measurements

The GPS point in this context is called a control point

Azimuth: the clockwise angle from north, e.g. 45o

(northeast), 180o (south) – i.e., bearing

The accuracy of this technique depends on accuracy of instruments used to measure

Geographic position (e.g., GPS)

Angle (e.g., analog or digital compass)

Distance (e.g., tape measure, laser rangefinder)

Compass

Measures azimuth angles

0º to 360º

Magnetic declination

Earth has a geographic north and south pole – axis

upon which the planet spins

Earth is like a big magnet; liquid iron-nickel core

creates magnetic field

Compass needles point in the direction of the

magnetic field lines – “North” on a compass, or 0º is

magnetic north, not geographic north

Magnetic declination - angle between the compass

pointing direction and geographic north, or true

north

http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination

Distance

Tape measures

Laser rangefinder

Maximum

Range575 m

Accuracy 3 - 5 cm typical

Inclinometer

Range± 90 degrees

Inclinometer

Accuracy± 0.1 deg. typical

Horizontal distance

Want horizontal distance, or planimetric distance --

may need to slope correct

A

B

hd = horizontal distance

Θ = inclination angle

Cos (θ) = hd/sd (adjacent/ hypotenuse)

hd = sd * Cos (θ)

Θ can be measured with a inclinometer

…or laser rangefinder inclinometer

Vertical height

Some applications require measurement of height

Same concepts apply…

hd = horizontal distanceA

B

θ1

Tan(θ) = vh/hd (opposite/adjacent)

vh1 = hd * Tan(θ1)

vh1 = vertical height

vh2= vertical heightθ2

C

vhtotal = vh1 + vh2

Measuring tree height, La Selva, Costa Rica

Using the Laser Tech Impulse

Mapping a point

Forest

GPS control point &laser rangefinder

θ

x

y

Sin (θ) = x/hd (opposite/ hypotenuse)

x = Sin (θ) * hd

Cos (θ) = y/hd (adjacent/ hypotenuse)

y= Cos (θ) * hd

Trunkx = GPSx + x

Trunky = GPSy + y

0˚ N

Field Plot

Mapping a polygon (1)

Collecting corner x,y positions with a GPS receiver

GPS positional error

Field Plot

Mapping a polygon (2)

Collecting corner x,y positions with a GPS receiver

and differential corrections (DGPS)

DGPS positional error

Field Plot

Mapping a polygon (3)

Collecting corner x,y positions with DGPS and laser

rangefinder

Control pointDGPS positional error

Laser rangefinderpositional error

Closure?

Planning the “field campaign”

Project fundamentals

Define research question and goals

Consider a spatial perspective in questions

Ask how spatial data and sampling scheme help

answer this

Familiarize yourself with study area

Logistics of getting there

What type of obstacles – canopy cover, mountains

Permissions for access, other cultural issues

Project fundamentals (cont)

Resolution and accuracy needs

Spatial and temporal scale

Type of equipment: recreational or mapping-grade

GPS?

Tape measure or laser rangefinder?

Data collection methodology

Points, lines or areas

Coordinate system and projection

How data collected? Who?

How data stored – data dictionary, on a paper form

Long-term field plots

Field data

form

Data dictionary

A data dictionary is a "shopping list" of the features and their attributes that you want to map in the field

You create the data dictionary with the GPS vendor’s software (e.g., Pathfinder Office) prior to going into the field

You then upload the data dictionary to your GPS receiver

Once in the field, the data dictionary prompts you for information for each spatial feature (e.g., point, polygon) measured

Provides a standard format for data entry

Saves time!

Helps prevent input errors!

Mission planning software

GPS (GNSS) vendors generally provide mission planning software with receiver

Free software is from Trimble

Almanac information on satellites is available from Trimble, http://www.trimble.com/gpsdataresources.shtml

http://www.trimble.com/planningsoftware.shtml

Data dictionary

GPS satellites - 02/26/2010

Sky plot

Rohnert Park, CA 02/26/2010

Number of satellites – Santa Rosa, CA

Rohnert Park, CA 02/26/2010

PDOP – Santa Rosa, CA

Rohnert Park, CA 02/26/2010

Sonoma State Campus – June 2007