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GPS/GIS Integration by Eva StrandGPS/GIS Integration by Eva Strand

• Global Positioning System (GPS) …..a global navigation system that everyone can use

What is GPS?

How does it work?

How accurate is GPS?

GPS/GIS integration?

Global Positioning System (GPS)Global Positioning System (GPS)

• Installed and maintained by U.S. Dept. of Defense

• Worldwide radio navigation system w/ constellation of GPS satellites and their ground stations

• NAVSTAR satellites (NAVigation Satellite Timing and Ranging)

• First launch – 1978

• Can be used for any application that requires location information.

Components of GPSComponents of GPS

NAVSTAR satellites

Earth’s surface

GPS receiver

GPS Satellite ConstellationGPS Satellite Constellation

GPS Satellites and OrbitsGPS Satellites and Orbits GPS ground control stationsGPS ground control stations

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How GPS works ?How GPS works ?

• GPS receivers calculate their• GPS receivers calculate their position by measuring the time it takes for the signal to travel from the satellites to the receiver• Satellite signals require a direct line to the GPS receiver• Signals can not go through water, soil, walls or others

How GPS works - 5 stepsHow GPS works - 5 steps

• Accurate measure of time for signal to travel from satellite to receiver

• Speed of light X travel time = distancell• Distance measurements to 4 satellites is

necessary to compute a 3-D position through triangulation

• Knowledge of satellite positioning• Corrections due to atmospheric influences and

other errors

TriangulationTriangulation

http://www.montana.edu/places/gps/

The second measurement….The second measurement….

http://www.montana.edu/places/gps/

A third measurement…A third measurement…

http://www.montana.edu/places/gps/

A Fourth measurement is needed for correction of timing offset.

Measuring travel time Measuring travel time

http://www.montana.edu/places/gps/

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Satellite signalsSatellite signals

• each satellite transmits two signals - L1 & L2 microwave frequencies/wavelengths (L1 = 1575.42 MHz and L2 = 1227.6 MHz)

• L1 - carries precision code (P-code) along with a coarse acquisition code (C/A-code)

• L2 - carries only the P-code

• P-code much higher frequency - generates a much more accurate position

• Atmospheric effects• Measurement noise• Ephemeris errors

GPS errorsGPS errors

• Satellite clock drift• Multi-path effects• Satellite geometry• Selective Availability (turned off May 2000)

Atmospheric refractionAtmospheric refraction MultipathMultipath

PDOP - Position Dilution of PrecisionSatellite geometrySatellite geometry

Good PDOP (< 6)Higher accuracy

Poor PDOPLower accuracy

Source Uncorrected Error LevelIonosphere 0-30 meters Troposphere 0-30 metersM N i 0 10

GPS error budgetGPS error budget

Measurement Noise 0-10 meters Ephemeris Data 1-5 meters Clock Drift 0-1.5 meters Multipath 0-1 meter

(Selective Availability 0-70 meters) removed in May 2000

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Differential GPSDifferential GPS

….is used to correct for errors

1) Real-time differential correction

2) Post-processing

Differential GPSDifferential GPS

• Involves use of two receivers - one that’s stationary (reference) and another rover

• errors are a compounding of factors, but receivers within 100km usually have same errors

• stationary receiver sits on a known location and back calculates what the timing should be, compares with what they are and develops “error correction” factor

• corrections are transmitted to rovers

• or differential corrections are made after field data collection - post-processing differential correction

Control for DGPS operationsControl for DGPS operations

• Base station networks - Forest Service, BLM, USGS, DOT, US Coast Guard

• WAAS – Wide Area Augmented System by FAA (Federal Aviation Administration )

• Omnistar and Landstar - commercial systems with base stations and satellite transmitters

• many new GPS receivers can accept corrections and many now with built-in radio receivers

• can be achieved by post-processing collected data

WAAS is based on a network of approximately 25 ground reference stations

Two geo-stationary satellites serving the WAAS area ( POR (Pacific Ocean Region) and AOR-W

How does WAAS work?How does WAAS work?

a ea ( O ( ac c Ocea eg o ) a d O(Atlantic Ocean Region-West)

The message is broadcast from the satellite on the same frequency as GPS (L1, 1575.42MHz)

The WAAS will improve basic GPS accuracy to approximately 7 meters vertically and horizontally

The WAAS systemThe WAAS system

Two geo-stationary satellites are presently

serving WAAS

WAAS ground station coverage is

approximately 200km around these stations

GPS Base Stations Web accessGPS Base Stations Web access

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GPS system Cost Accuracy Codes Data base

Sportsman unit(Garmin, Magellan..

$100 -$300

5 – 25 m C/A

Sportsman unit DGPS

$600 2 – 5 m C/A

Pocket PC w. GPS card

$700 -$1000

5 – 25 m C/A x

Sportsman DGPS $600 2 – 5 m C/A Sportsman DGPS $600 2 5 m C/A

Geoexplorer III $4000 5 - 15 C/A X

Geoexplorer III DGPS

$4000 2 – 5 C/A X

Geo XM $4000 5 - 15 C/A X

Geo XT DGPS $6000 < 1 m L1 +C/A X

Trimble Pro-XRSDGPS

$15,000 0.1 – 1 m L1 c/a+p

X

Survey grade GPS $50,000 0.01–0.1m L1, L2 X

Trimble GeoXT

New GPS field mapping technologyNew GPS field mapping technology

GPS Flash Card that works with a

IPAQ Pocket PC

that works with a Pocket PC

GPS in Natural Resources…GPS in Natural Resources…

• Forest inventory• Weed mapping• Wildlife habitat • Vegetation monitoring• Lake and stream boundaries• Property corners

Geographic coordinatesGeographic coordinates

Coordinates are expressed as Latitude and Longitude

in Degrees, Minutes, Seconds.

Map coordinates on USGS mapMap coordinates on USGS map

UTM

Degrees, minutes

UTM zones of the worldUTM zones of the world

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UTM Coord. Moscow: Easting – 500,000 Northing: 5,175,000UTM zone 11 N, units meters

What do the UTM coordinates mean?

Remember:The northing describes the distance in meters from the equator

….. it has various bumps and indentations

Beyond a map projection we also need to specify a datum

The earth is not perfectly round…The earth is not perfectly round…

The surface generated

SpheroidSpheroid

by rotation of an ellipseabout one of its axes.

P

b

minor axis

Elements of an EllipseElements of an Ellipse

a = 1/2 of the major axis = semi-major axisb = 1/2 of the minor axis = semi-minor axisf = Flattening =

PP' = Axis of revolution of the Earth's ellipsoida - b

a

P'

amajoraxis

NNorth America

N Europe

The Geoid and two EllipsoidsThe Geoid and two Ellipsoids

O1

GEOID

O2

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…..an ellipsoid of specified dimensions which is associated with

Reference ellipsoid…..Reference ellipsoid…..

a geodetic reference system (mathematically defined model of the earth’s surface).

REFERENCE ELLIPSOID A (meters) f

Clark 1866 (NAD1927) 1/294.9786926378206.4

Reference Ellipsoid ConstantsReference Ellipsoid Constants

Geodetic Reference System 1980(NAD1983)

WGS84

1/298.257222101

1/298.257223563

6378137

6378137

Difference between NAD27 and NAD83Difference between NAD27 and NAD83

• GPS actively works in WGS84 datum• Resource GPS WGS 84 = NAD83• What datum should you use?• What projection should you use?

Id h S Id h T M NAD27

What projection and datum to use?What projection and datum to use?

Idaho State - Idaho Transverse Mercator NAD27Montana State Library – State Plane, NAD83, metersOregon Spatial Data Library – Lambert, NAD83, feetWashington GIC - Geographic, NAD83, ddNRCS - UTM, NAD83BLM - UTM, NAD27US Forest Service - UTM, NAD27 (mostly)Latah County - State Plane, Idaho westAlaska - Albers (Alaska Albers)

GPS references on the InternetGPS references on the Internet

• http://www.trimble.com • http://www.fs.fed.us/database/gps/welcome.htm• http://www.cla.sc.edu/GEOG/rslab/gps.html• http://www.montana.edu/places/gps• http://vancouver-webpages.com/peter/• http://www.cmtinc.com/gpsbook/• http://www.esri.com/news/arcuser/0103/different

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