Geographic Information Geographic Information SystemsSystems

SGO 1910, SGO 4030SGO 1910, SGO 4030

October 18, 2005October 18, 2005

QuizzesQuizzes

Class average: 24,8Class average: 24,8 Two problem questions – I concede! Two problem questions – I concede!

19. Efforts to promote an international standard for 19. Efforts to promote an international standard for ellipsoids has led to the wide acceptance of the North ellipsoids has led to the wide acceptance of the North American Datum of 1927 (NAD27) (False, WGS84 American Datum of 1927 (NAD27) (False, WGS84 or NAD83) but… ”many older data still adhere to or NAD83) but… ”many older data still adhere to earlier standards, such as NAD 27” (p. 116)earlier standards, such as NAD 27” (p. 116)

24. The modern history of GIS dates from the early 24. The modern history of GIS dates from the early 1950s, when computers were developed. (False – ”the 1950s, when computers were developed. (False – ”the modern history of GIS dates from the early 1950s, modern history of GIS dates from the early 1950s, when the price of sufficiently powerful computers fell when the price of sufficiently powerful computers fell below a critical threshold” p. 18)below a critical threshold” p. 18)

20. Geographic techniques can be applied to 20. Geographic techniques can be applied to non-geographic spaces. Truenon-geographic spaces. True

””But many of the methods used in GIS are also But many of the methods used in GIS are also applicable to other non-geographic spaces, applicable to other non-geographic spaces, including the surfaces of other planets, the including the surfaces of other planets, the space of the cosmos, and the space of the space of the cosmos, and the space of the human body that is captured by medical human body that is captured by medical images” (p. 8)images” (p. 8)

27. If you were going hiking in the forest, it 27. If you were going hiking in the forest, it would be most useful to take along a map with would be most useful to take along a map with a small representative fraction. FALSEa small representative fraction. FALSE

1:50000 > 1:10000001:50000 > 1:1000000 30. Digital representations of geographic 30. Digital representations of geographic

phenomena are formalized through phenomena are formalized through photographic models. FALSEphotographic models. FALSE

Revised ScheduleRevised Schedule Week 42 (October 18)Week 42 (October 18) GIS Data CollectionGIS Data Collection Chapter 9Chapter 9 GPSGPS

Week 43 (October 25)Week 43 (October 25) Geographic DatabasesGeographic Databases Chapters 10Chapters 10

Week 44 (Nov. 1)Week 44 (Nov. 1) Geographic AnalysisGeographic Analysis Chapters 14, 15Chapters 14, 15

Week 45 (Nov. 8)Week 45 (Nov. 8) Mid-term Quiz IIMid-term Quiz II Map ProductionMap Production Chapter 12Chapter 12

Week 46 (Nov. 15)Week 46 (Nov. 15) GIS and SocietyGIS and Society Chapter 18Chapter 18

Week 47 (November 22)Week 47 (November 22) NO CLASSNO CLASS

Week 48 Week 48 Final Exam Dec 1Final Exam Dec 1

UncertaintyUncertainty

The more scientific knowledge we gain, the more uncertain we are likely to be: “Richness of representation and computational power only make us more aware of the range and variety of established uncertainties, and challenge us to integrate new ones” (Longley et al. 2005, p. 152).

Data Acquisition:Data Acquisition:Getting the Map Getting the Map

into the Computerinto the Computer

Data captureData capture

Primary (direct measurement, e.g. remote Primary (direct measurement, e.g. remote sensing and surveying)sensing and surveying)

Secondary (derivation from other sources; Secondary (derivation from other sources; digitizing, scanning, etc.)digitizing, scanning, etc.)

Data transferData transfer

Input of data from other systems (via Internet, Input of data from other systems (via Internet, CD ROMs, tapes, etc.)CD ROMs, tapes, etc.)

GIS maps are digitalGIS maps are digital

Real maps: traditional paper maps that can be Real maps: traditional paper maps that can be touchedtouched

Virtual maps: an arrangement of information Virtual maps: an arrangement of information inside the computer; the GIS can be used to inside the computer; the GIS can be used to generate the map however and whenever generate the map however and whenever necessary. necessary.

GIS Data ConversionGIS Data Conversion

Traditionally the Traditionally the most time-consuming and most time-consuming and expensive part expensive part of a GIS projectof a GIS project

Involves a one-time Involves a one-time costcost Digital maps can be reused and shared.Digital maps can be reused and shared. Requires maintenanceRequires maintenance (eg. updating) (eg. updating)

GIS data can beGIS data can be

Purchased.Purchased. Found from existing sources in digital form.Found from existing sources in digital form. Captured from analog maps by Captured from analog maps by GGEOCODING.EOCODING.

Finding Existing Map DataFinding Existing Map Data

Map librariesMap libraries Reference booksReference books State and local agenciesState and local agencies Federal agenciesFederal agencies Commercial data suppliersCommercial data suppliers

Existing Map DataExisting Map Data

Existing map data can be found through a map Existing map data can be found through a map library, via network searches, or on media library, via network searches, or on media such as CD-ROM and disk.such as CD-ROM and disk.

Many major data providers make their data Many major data providers make their data available via the Internet.available via the Internet.

StatenskartverkStatenskartverkhttp://ngis.statkart.no/katalog/java/katalog.asphttp://ngis.statkart.no/katalog/java/katalog.asp

RasterdataRasterdata TemakartTemakart VektordataVektordata PrimærdataPrimærdata ProsjekterProsjekter

1. Accessing GIS Data1. Accessing GIS Data

Example: Costa RicaExample: Costa Rica

Data CollectionData Collection

One of most expensive GIS activitiesOne of most expensive GIS activities Many diverse sourcesMany diverse sources Two broad types of collectionTwo broad types of collection

Data capture (direct collection)Data capture (direct collection) Data transferData transfer

Two broad capture methodsTwo broad capture methods Primary (direct measurement)Primary (direct measurement) Secondary (indirect derivation)Secondary (indirect derivation)

Data Collection Techniques Data Collection Techniques

RasterRaster VectorVector

PrimaryPrimary Digital remote Digital remote sensing imagessensing images

GPS measurementsGPS measurements

Digital aerial Digital aerial photographsphotographs

Survey Survey measurementsmeasurements

SecondarySecondary Scanned mapsScanned maps Topographic Topographic surveyssurveys

DEMs from mapsDEMs from maps Toponymy data Toponymy data sets from atlasessets from atlases

GEOCODINGGEOCODING

Geocoding is the conversion of spatial Geocoding is the conversion of spatial information into digital form.information into digital form.

Geocoding involves capturing the map, and Geocoding involves capturing the map, and sometimes also capturing the attributes.sometimes also capturing the attributes.

Primary Data CapturePrimary Data Capture

Capture specifically for GIS useCapture specifically for GIS use Raster – remote sensingRaster – remote sensing

e.g. SPOT and IKONOS satellites and aerial photographye.g. SPOT and IKONOS satellites and aerial photography Passive and active sensorsPassive and active sensors

Resolution is key considerationResolution is key consideration SpatialSpatial SpectralSpectral TemporalTemporal

Secondary Geographic Data Secondary Geographic Data CaptureCapture

Data collected for other purposes can be Data collected for other purposes can be converted for use in GISconverted for use in GIS

Raster conversionRaster conversion Scanning of maps, aerial photographs, documents, Scanning of maps, aerial photographs, documents,

etcetc Important scanning parameters are spatial and Important scanning parameters are spatial and

spectral (bit depth) resolutionspectral (bit depth) resolution

Vector Primary Data CaptureVector Primary Data Capture

SurveyingSurveying Locations of objects determines by angle and distance Locations of objects determines by angle and distance

measurements from known locationsmeasurements from known locations Uses expensive field equipment and crewsUses expensive field equipment and crews Most accurate method for large scale, small areasMost accurate method for large scale, small areas

GPSGPS Collection of satellites used to fix locations on Earth’s Collection of satellites used to fix locations on Earth’s

surfacesurface Differential GPS used to improve accuracyDifferential GPS used to improve accuracy

Vector Secondary Data CaptureVector Secondary Data Capture Collection of vector objects from maps, photographs, Collection of vector objects from maps, photographs,

plans, etc.plans, etc. DigitizingDigitizing

Manual (table) Manual (table) Heads-up and vectorizationHeads-up and vectorization

Photogrammetry – the science and technology of Photogrammetry – the science and technology of making measurements from photographs, etc.making measurements from photographs, etc.

COGO – Coordinate GeometryCOGO – Coordinate Geometry

Managing Data Capture Managing Data Capture ProjectsProjects

Key principlesKey principles Clear plan, adequate resources, appropriate funding, and Clear plan, adequate resources, appropriate funding, and

sufficient timesufficient time Fundamental tradeoff between Fundamental tradeoff between

Quality, speed and priceQuality, speed and price Two strategiesTwo strategies

IncrementalIncremental ‘‘Blitzkrieg’ (all at once)Blitzkrieg’ (all at once)

Alternative resource optionsAlternative resource options In houseIn house Specialist external agencySpecialist external agency

SummarySummary

Data collection is very expensive, time-consuming, Data collection is very expensive, time-consuming, tedious and error pronetedious and error prone

Good procedures required for large scale collection Good procedures required for large scale collection projectsprojects

Main techniquesMain techniques PrimaryPrimary

Raster – e.g. remote sensingRaster – e.g. remote sensing Vector – e.g. field surveyVector – e.g. field survey

SecondarySecondary Raster – e.g. scanningRaster – e.g. scanning Vector – e.g. table digitizingVector – e.g. table digitizing

DigitizingDigitizing

Captures map data by tracing lines from a map Captures map data by tracing lines from a map by handby hand

Uses a cursor and an electronically-sensitive Uses a cursor and an electronically-sensitive tablettablet

Result is a string of points with (x, y) valuesResult is a string of points with (x, y) values

DigitizerDigitizer

The Digitizing TabletThe Digitizing Tablet

Digitizer cursor transmitsa pulse from an electomagneticcoil under the view lens.

Pulse is picked up bynearest grid wires undertablet surface.

Result is sent tocomputer afterconversion tox and y units.

map

DigitizingDigitizing

Stable base mapStable base map Fix to tabletFix to tablet Digitize controlDigitize control Determine coordinate transformationDetermine coordinate transformation Trace featuresTrace features Proof plotProof plot EditEdit Clean and buildClean and build

Selecting points to digitizeSelecting points to digitize

ScannerScanner

Scanning Scanning

Places a map on a glass plate, and passes a Places a map on a glass plate, and passes a light beam over itlight beam over it

Measures the reflected light intensityMeasures the reflected light intensity Result is a grid of pixelsResult is a grid of pixels Image size and resolution are importantImage size and resolution are important Features can “drop out”Features can “drop out”

Scanning exampleScanning example

This section of map was scanned, resulting in a file in TIF format that was bytes in size. This was a file of color intensities between 0 and 255 for red, green, and blue in each of three layers spaced on a grid 0.25 millimeter apart. How much data would be necessary to capture the features on your map as vectors? Would it be more or less than the grid (raster) file?

Field data collectionField data collection

Pen Portable PC and GPSPen Portable PC and GPS

Data TransferData Transfer

Buy vs build is an important questionBuy vs build is an important question Many widely distributed sources of GIMany widely distributed sources of GI Key catalogs includeKey catalogs include

US NSDI Clearinghouse networkUS NSDI Clearinghouse network Geography NetworkGeography Network

Access technologiesAccess technologies TranslationTranslation Direct readDirect read

Attribute data Attribute data

Logically can be thought of as in a flat fileLogically can be thought of as in a flat file Table with rows and columnsTable with rows and columns Attributes by recordsAttributes by records Entries called values.Entries called values.

Database Management SystemsDatabase Management Systems

Data definition module sets constraints on the attribute valuesData definition module sets constraints on the attribute values Data entry module to enter and correct valuesData entry module to enter and correct values Data management system for storage and retrievalData management system for storage and retrieval DData definitions can be listed as a data dictionaryata definitions can be listed as a data dictionary Database manager checks values with this dictionary, Database manager checks values with this dictionary,

enforcing data validation.enforcing data validation.

The Role of ErrorThe Role of Error

Map and attribute data errors are the data Map and attribute data errors are the data producer's responsibility, but the GIS user producer's responsibility, but the GIS user must understand error.must understand error.

Accuracy and precision of map and attribute Accuracy and precision of map and attribute data in a GIS affect all other operations, data in a GIS affect all other operations, especially when maps are compared across especially when maps are compared across scales.scales.

Part II: Global Part II: Global Positioning Systems Positioning Systems

(GPS)(GPS)Sources of information:Sources of information:

http://www.trimble.com/gps/http://www.trimble.com/gps/

http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#DODSystem http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#DODSystem

GPS is a Satellite Navigation SystemGPS is a Satellite Navigation System GPS is funded by and controlled by the U. S. GPS is funded by and controlled by the U. S.

Department of Defense (DOD). While there are many Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. system was designed for and is operated by the U. S. military. military.

GPS provides specially coded satellite signals that GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. receiver to compute position, velocity and time.

Four GPS satellite signals are used to compute Four GPS satellite signals are used to compute positions in three dimensions and the time offset in positions in three dimensions and the time offset in the receiver clock. the receiver clock.

Space Segment Space Segment

The Space Segment of The Space Segment of the system consists of the system consists of the GPS satellites. the GPS satellites. These space vehicles These space vehicles (SVs) send radio signals (SVs) send radio signals from space. from space.

Space Segment (cont)Space Segment (cont) The nominal GPS Operational Constellation consists of 24 The nominal GPS Operational Constellation consists of 24

satellites that orbit the earth in 12 hours. satellites that orbit the earth in 12 hours. The satellite orbits repeat almost the same ground track (as the The satellite orbits repeat almost the same ground track (as the

earth turns beneath them) once each day. The orbit altitude is earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier over any point approximately each 24 hours (4 minutes earlier each day). each day).

There are six orbital planes (with nominally four SVs in each), There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. five degrees with respect to the equatorial plane.

This constellation provides the user with between five and This constellation provides the user with between five and eight SVs visible from any point on the earth. eight SVs visible from any point on the earth.

GPS Satellites

Name: NAVSTAR Manufacturer: Rockwell International

Altitude: 10,900 nautical miles Weight: 1900 lbs (in orbit)

Size:17 ft with solar panels extended Orbital Period: 12 hours

Orbital Plane: 55 degrees to equitorial plane Planned Lifespan: 7.5 years

Current constellation: 24 Block II production satellites Future satellites: 21 Block IIrs developed by Martin Marietta

Latest Development Latest Development

Galileo, Europe's contribution to the Global Galileo, Europe's contribution to the Global Navigation Satellite System (GNSS), is creating a Navigation Satellite System (GNSS), is creating a buzz in the Global Positioning Systems (GPS) buzz in the Global Positioning Systems (GPS) applications market. With its advantages of signal applications market. With its advantages of signal reliability and integrity, it is poised to drive European reliability and integrity, it is poised to drive European GPS applications markets. Unlike its US counterpart, GPS applications markets. Unlike its US counterpart, Galileo is envisioned as being independent of military Galileo is envisioned as being independent of military control and is expected to be harnessed for control and is expected to be harnessed for widespread commercial and civilian purposes. (Space widespread commercial and civilian purposes. (Space Daily, Dec. 18, 2003)Daily, Dec. 18, 2003)

Control SegmentControl Segment

The Control Segment consists of a system of The Control Segment consists of a system of tracking stations located around the world. tracking stations located around the world.

The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. 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.

User SegmentUser Segment The GPS User Segment consists of the GPS receivers The GPS User Segment consists of the GPS receivers

and the user community. GPS receivers convert SV and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. signals into position, velocity, and time estimates. Four satellites are required to compute the four Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. GPS dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time receivers are used for navigation, positioning, time dissemination, and other research. dissemination, and other research.

Navigation in three dimensions is the primary Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying aircraft, ships, ground vehicles, and for hand carrying by individuals. by individuals.

Precise positioning Precise positioning is possibleis possible using GPS using GPS receivers at reference locations providing receivers at reference locations providing corrections and relative positioning data for corrections and relative positioning data for remote receivers. Surveying, geodetic control, remote receivers. Surveying, geodetic control, and plate tectonic studies are examples. and plate tectonic studies are examples.

Here's how GPS works in five logical steps:Here's how GPS works in five logical steps:

The basis of GPS is "triangulation" from satellites. The basis of GPS is "triangulation" from satellites. To "triangulate," a GPS receiver measures distance using the To "triangulate," a GPS receiver measures distance using the

travel time of radio signals. travel time of radio signals. To measure travel time, GPS needs very accurate timing To measure travel time, GPS needs very accurate timing

which it achieves with some tricks. which it achieves with some tricks. Along with distance, you need to know exactly where the Along with distance, you need to know exactly where the

satellites are in space. High orbits and careful monitoring are satellites are in space. High orbits and careful monitoring are the secret. the secret.

Finally you must correct for any delays the signal experiences Finally you must correct for any delays the signal experiences as it travels through the atmosphere. as it travels through the atmosphere.

TriangulatingTriangulating

Position is calculated from distance measurements Position is calculated from distance measurements (ranges) to satellites.(ranges) to satellites.   

Mathematically we need four satellite ranges to Mathematically we need four satellite ranges to determine exact position.determine exact position.   

Three ranges are enough if we reject ridiculous Three ranges are enough if we reject ridiculous answers or use other tricks.answers or use other tricks.   

Another range is required for technical reasons to be Another range is required for technical reasons to be discussed later. discussed later.

Measuring Distance Measuring Distance

Distance to a satellite is determined by measuring how long a Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.radio signal takes to reach us from that satellite.   

To make the measurement we assume that both the satellite To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes and our receiver are generating the same pseudo-random codes at exactly the same time.at exactly the same time.   

By comparing how late the satellite's pseudo-random code By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how appears compared to our receiver's code, we determine how long it took to reach us.long it took to reach us.   

Multiply that travel time by the speed of light and you've got Multiply that travel time by the speed of light and you've got distance. distance.

Getting Perfect TimingGetting Perfect Timing

Accurate timing is the key to measuring distance to Accurate timing is the key to measuring distance to satellites.satellites.   

Satellites are accurate because they have atomic Satellites are accurate because they have atomic clocks on board.clocks on board.   

Receiver clocks don't have to be too accurate because Receiver clocks don't have to be too accurate because an extra satellite range measurement can remove an extra satellite range measurement can remove errors. errors.

2005 Nobel Prize in Physics2005 Nobel Prize in Physics

Two physicists (Hall and Haensch) shared the Two physicists (Hall and Haensch) shared the Nobel Prize in Physics for advancing the Nobel Prize in Physics for advancing the developmetn of laser-based precision developmetn of laser-based precision spectroscopy, a field htat opens the way to the spectroscopy, a field htat opens the way to the next generation of global positioning system next generation of global positioning system (GPS) navigation and ultra-precise atomic (GPS) navigation and ultra-precise atomic clocks.clocks.

Satellite Positions Satellite Positions

To use the satellites as references for range measurements we To use the satellites as references for range measurements we need to know exactly where they are.need to know exactly where they are.   

GPS satellites are so high up their orbits are very predictable.GPS satellites are so high up their orbits are very predictable.   

Minor variations in their orbits are measured by the U.S. Minor variations in their orbits are measured by the U.S. Department of Defense.Department of Defense.   

The error information is sent to the satellites, to be transmitted The error information is sent to the satellites, to be transmitted along with the timing signals. along with the timing signals.

Three satellites could be used determine three Three satellites could be used determine three position dimensions with a perfect receiver clock. In position dimensions with a perfect receiver clock. In practice this is rarely possible and three SVs are used practice this is rarely possible and three SVs are used to compute a two-dimensional, horizontal fix (in to compute a two-dimensional, horizontal fix (in latitude and longitude) given an assumed height. This latitude and longitude) given an assumed height. This is often possible at sea or in altimeter equipped is often possible at sea or in altimeter equipped aircraft. aircraft.

Five or more satellites can provide position, time and Five or more satellites can provide position, time and redundancy. More SVs can provide extra position fix redundancy. More SVs can provide extra position fix certainty and can allow detection of out-of-tolerance certainty and can allow detection of out-of-tolerance signals under certain circumstances. signals under certain circumstances.

Position in XYZ is converted within the receiver to Position in XYZ is converted within the receiver to geodetic latitude, longitude and height above the geodetic latitude, longitude and height above the ellipsoid. ellipsoid.

Latitude and longitude are usually provided in the Latitude and longitude are usually provided in the geodetic datum on which GPS is based (WGS-84). geodetic datum on which GPS is based (WGS-84). Receivers can often be set to convert to other user-Receivers can often be set to convert to other user-required datums. Position offsets of hundreds of required datums. Position offsets of hundreds of meters can result from using the wrong datum. meters can result from using the wrong datum.

GPS errors are a combination of GPS errors are a combination of noise, bias, blunders. noise, bias, blunders.

Selective Availability (SA) Selective Availability (SA)

SA is the intentional degradation of the SPS SA is the intentional degradation of the SPS signals by a time varying bias. SA is signals by a time varying bias. SA is controlled by the DOD to limit accuracy for controlled by the DOD to limit accuracy for non-U. S. military and government users. non-U. S. military and government users.

SA was turned off in May, 2000! SA was turned off in May, 2000!

Bias Error sources Bias Error sources

SV clock errors uncorrected by Control Segment : 1 meterSV clock errors uncorrected by Control Segment : 1 meter Ephemeris data errors: 1 meter Ephemeris data errors: 1 meter Tropospheric delays: 1 meter. The troposphere is the lower part (ground Tropospheric delays: 1 meter. The troposphere is the lower part (ground

level to from 8 to 13 km) of the atmosphere that experiences the changes in level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements Complex models of tropospheric delay require estimates or measurements of these parameters. of these parameters.

Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual. delay leaving a ten meter un-modeled residual.

Multipath: 0.5 meters. Multipath is caused by reflected signals from Multipath: 0.5 meters. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometime hard to avoid. difficult to detect and sometime hard to avoid.

Blunders can result in errors of Blunders can result in errors of hundred of kilometers.hundred of kilometers.

Control segment mistakes due to computer or Control segment mistakes due to computer or human error can cause errors from one meter human error can cause errors from one meter to hundreds of kilometers. to hundreds of kilometers.

User mistakes, including incorrect geodetic User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to datum selection, can cause errors from 1 to hundreds of meters. hundreds of meters.

Receiver errors from software or hardware Receiver errors from software or hardware failures can cause blunder errors of any size. failures can cause blunder errors of any size.

Correcting Errors Correcting Errors

The earth's ionosphere and atmosphere cause delays in the The earth's ionosphere and atmosphere cause delays in the GPS signal that translate into position errors. GPS signal that translate into position errors.

    Some errors can be factored out using mathematics and Some errors can be factored out using mathematics and

modeling.modeling.   

The configuration of the satellites in the sky can magnify other The configuration of the satellites in the sky can magnify other errors.errors.   

Differential GPS can eliminate almost all error. Differential GPS can eliminate almost all error.

GPS technology has matured into a resource GPS technology has matured into a resource that goes far beyond its original design goals. that goes far beyond its original design goals. These days scientists, sportsmen, farmers, These days scientists, sportsmen, farmers, soldiers, pilots, surveyors, hikers, delivery soldiers, pilots, surveyors, hikers, delivery drivers, sailors, dispatchers, lumberjacks, fire-drivers, sailors, dispatchers, lumberjacks, fire-fighters, and people from many other walks of fighters, and people from many other walks of life are using GPS in ways that make their life are using GPS in ways that make their work more productive, safer, and sometimes work more productive, safer, and sometimes even easier. even easier.

Location: Where am I?Location: Where am I?

The first and most obvious application of GPS The first and most obvious application of GPS is the simple determination of a "position" or is the simple determination of a "position" or location. GPS is the first positioning system to location. GPS is the first positioning system to offer highly precise location data for any point offer highly precise location data for any point on the planet, in any weather. That alone on the planet, in any weather. That alone would be enough to qualify it as a major would be enough to qualify it as a major utility, but the accuracy of GPS and the utility, but the accuracy of GPS and the creativity of its users is pushing it into some creativity of its users is pushing it into some surprising realms. surprising realms.

Navigation: Where am I going?Navigation: Where am I going?

GPS helps you determine exactly where you are, but GPS helps you determine exactly where you are, but sometimes important to know how to get somewhere sometimes important to know how to get somewhere else. GPS was originally designed to provide else. GPS was originally designed to provide navigation information for ships and planes. So it's no navigation information for ships and planes. So it's no surprise that while this technology is appropriate for surprise that while this technology is appropriate for navigating on water, it's also very useful in the air and navigating on water, it's also very useful in the air and on the land.on the land.

The sea, one of our oldest channels of transportation, The sea, one of our oldest channels of transportation, has been revolutionized by GPS, the newest has been revolutionized by GPS, the newest navigation technology.navigation technology.

By providing more precise navigation tools and accurate By providing more precise navigation tools and accurate landing systems, GPS not only makes flying safer, but also landing systems, GPS not only makes flying safer, but also more efficient. With precise point-to-point navigation, GPS more efficient. With precise point-to-point navigation, GPS saves fuel and extends an aircraft's range by ensuring pilots saves fuel and extends an aircraft's range by ensuring pilots don't stray from the most direct routes to their destinations. don't stray from the most direct routes to their destinations.

GPS accuracy will also allow closer aircraft separations on GPS accuracy will also allow closer aircraft separations on more direct routes, which in turn means more planes can more direct routes, which in turn means more planes can occupy our limited airspace. This is especially helpful when occupy our limited airspace. This is especially helpful when you're landing a plane in the middle of mountains. And small you're landing a plane in the middle of mountains. And small medical evac helicopters benefit from the extra minutes saved medical evac helicopters benefit from the extra minutes saved by the accuracy of GPS navigation. by the accuracy of GPS navigation.

Finding your way across the land is an ancient art and science. Finding your way across the land is an ancient art and science. The stars, the compass, and good memory for landmarks The stars, the compass, and good memory for landmarks helped you get from here to there. Even advice from someone helped you get from here to there. Even advice from someone along the way came into play. But, landmarks change, stars along the way came into play. But, landmarks change, stars shift position, and compasses are affected by magnets and shift position, and compasses are affected by magnets and weather. And if you've ever sought directions from a local, weather. And if you've ever sought directions from a local, you know it can just add to the confusion. The situation has you know it can just add to the confusion. The situation has never been perfect. never been perfect.

Today hikers, bikers, skiers, and drivers apply GPS to the age-Today hikers, bikers, skiers, and drivers apply GPS to the age-old challenge of finding their way. old challenge of finding their way.

““In 1994 Norwegian Borge Ousland reached In 1994 Norwegian Borge Ousland reached the North Pole after skiing 1000 kilometers the North Pole after skiing 1000 kilometers from Siberia alone and unsupported. For this from Siberia alone and unsupported. For this incredible challenge Børge carried a bible to incredible challenge Børge carried a bible to read, some Jimi Hendrix to listen to, and a read, some Jimi Hendrix to listen to, and a Trimble Scout GPS receiver to help find his Trimble Scout GPS receiver to help find his way.” way.”

TrackingTracking Commerce relies on fleets of vehicles to deliver goods and services either Commerce relies on fleets of vehicles to deliver goods and services either

across a crowded city or through nationwide corridors. So, effective fleet across a crowded city or through nationwide corridors. So, effective fleet management has direct bottom-line implications, such as telling a customer management has direct bottom-line implications, such as telling a customer when a package will arrive, spacing buses for the best scheduled service, when a package will arrive, spacing buses for the best scheduled service, directing the nearest ambulance to an accident, or helping tankers avoid directing the nearest ambulance to an accident, or helping tankers avoid hazards. hazards.

GPS used in conjunction with communication links and computers can GPS used in conjunction with communication links and computers can benefit applications in agriculture, mass transit, urban delivery, public benefit applications in agriculture, mass transit, urban delivery, public safety, and vessel and vehicle tracking. So it's no surprise that police, safety, and vessel and vehicle tracking. So it's no surprise that police, ambulance, and fire departments are adopting GPS-based AVL (Automatic ambulance, and fire departments are adopting GPS-based AVL (Automatic Vehicle Location) Manager to pinpoint both the location of the emergency Vehicle Location) Manager to pinpoint both the location of the emergency and the location of the nearest response vehicle on a computer map. With and the location of the nearest response vehicle on a computer map. With this kind of clear visual picture of the situation, dispatchers can react this kind of clear visual picture of the situation, dispatchers can react immediately and confidently. immediately and confidently.

TimingTiming Although GPS is well-known for locating, navigation, and tracking, it's Although GPS is well-known for locating, navigation, and tracking, it's

also used to disseminate precise time, time intervals, and frequency. Time also used to disseminate precise time, time intervals, and frequency. Time is a powerful commodity, and exact time is more powerful still. Knowing is a powerful commodity, and exact time is more powerful still. Knowing that a group of timed events is perfectly synchronized is often very that a group of timed events is perfectly synchronized is often very important. GPS makes the job of "synchronizing our watches" easy and important. GPS makes the job of "synchronizing our watches" easy and reliable. reliable.

There are three fundamental ways we use time. As a universal marker, time There are three fundamental ways we use time. As a universal marker, time tells us when things happened or when they will. As a way to synchronize tells us when things happened or when they will. As a way to synchronize people, events, even other types of signals, time helps keep the world on people, events, even other types of signals, time helps keep the world on schedule. And as a way to tell how long things last, time provides and schedule. And as a way to tell how long things last, time provides and accurate, unambiguous sense of duration. accurate, unambiguous sense of duration.

GPS satellites carry highly accurate atomic clocks. And in order for the GPS satellites carry highly accurate atomic clocks. And in order for the system to work, our GPS receivers here on the ground synchronize system to work, our GPS receivers here on the ground synchronize themselves to these clocks. That means that every GPS receiver is, in themselves to these clocks. That means that every GPS receiver is, in essence, an atomic accuracy clock. essence, an atomic accuracy clock.

MappingMapping Using GPS to survey and map it precisely saves time and money in this Using GPS to survey and map it precisely saves time and money in this

most stringent of all applications. Today, Trimble GPS makes it possible most stringent of all applications. Today, Trimble GPS makes it possible for a single surveyor to accomplish in a day what used to take weeks with for a single surveyor to accomplish in a day what used to take weeks with an entire team. And they can do their work with a higher level of accuracy an entire team. And they can do their work with a higher level of accuracy than ever before. than ever before.

GPS technology is now the method of choice for performing control GPS technology is now the method of choice for performing control surveys, and the effect on surveying in general has been considerable. GPS surveys, and the effect on surveying in general has been considerable. GPS pinpoints a position, a route, and a fleet of vehicles. Mapping is the art and pinpoints a position, a route, and a fleet of vehicles. Mapping is the art and science of using GPS to locate items, then create maps and models of science of using GPS to locate items, then create maps and models of everything in the world. Mountains, rivers, forests and other landforms. everything in the world. Mountains, rivers, forests and other landforms. Roads, routes, and city streets. Endangered animals, precious minerals and Roads, routes, and city streets. Endangered animals, precious minerals and all sorts of resources. Damage and disasters, trash and archeological all sorts of resources. Damage and disasters, trash and archeological treasures. treasures. GPS is mapping theGPS is mapping the world.world.