An Introduction to GPS

With support from:

NSF DUE-0903270

Prepared by:

in partnership with:

John McGeeJennifer McKee

Geospatial Technician Education Through Virginia’s Community Colleges (GTEVCC)

Supplemental Information

This presentation contains supplemental information that should be incorporated with the GPS presentation from the 2010 VCCS Geospatial Institute. The 2010 presentation is located here: http://gep.frec.vt.edu/VCCS/materials/PPTs/2.1-Intro_%20to_GPS.pptx

Outline

• Review: How does GPS Work?• Review: Differential processing

How GPS Works

One satellite…

How GPS WorksIf the GPS receiver only obtains signals from 1 Satellite, then it “knows” that it is located somewhere on this sphere…

How GPS Works

How GPS Works

If the GPS receiver only obtains signals from 2satellites, then it “knows” that it is locatedsomewhere where these 2 spheres intersect

How GPS Works

How GPS Works If the GPS receiver obtains signals from 3 satellites, then it “knows” that it is located somewhere where these 3 spheres intersect (2 points)

Using 3 satellites, the receiver assumes that you are at sea level…

How GPS Works

A fourth satellite is required to determine the exact location and elevation.

How GPS Works

What do you need to know about GPS?

This is only a review… reference the 2010 institute workshop

presentation for additional info.!

Different “Grades” of GPS receivers

• Recreational Grade GPS– Accurate to within 5 meters (could be better, but don’t rely

on it)– Suitable for hunting, recreational, and some business uses– Lowest cost (smallest, and easiest to use): ~$100-$800

• Mapping Grade GPS – Accurate to within 1 meter (3 feet)– Requires differential processing (from a base station)– Suitable for many natural resource applications, city

planning– $800-$7,000

• Survey Grade GPS– Accurate to within 1 cm – Suitable for building bridges…– $15,000 -$30,000

We did some of this last year!

We are going to ‘do this’ this year’!

We are going to ‘observe this’ this year!

Selective Availability

Signal Accuracy

There are 2 types of GPS Signals:P-code: (“Precise” code) This is only available to the military and some

selected public officials. Very precise, not degraded.C-code: (“Civilian” Code). Less precise Signal can be degraded (by scrambling the signal)

especially in times of conflict. This is what the GARMIN Legends (and all public GPS

receivers) work with…

Mapping and survey grade GPS receivers use this too!

Differential Correction

• There are already established base stations established around the U.S. (and the world…)

• Surveyors have determined the precise location of these base stations.

• Each base station has a GPS receiver, which collects incoming (scrambled) signals.

• The true (surveyed) location is then compared to the GPS coordinates.

• The correction values are then sent to other GPS receivers in the field.

Differential Correction – GPS Base stations

Base stations can be “large or small”, “urban or rural”, “permanent or temporary”!

Real Time Differential Correction

Exact known (surveyed) coordinates differ from GPS coordinates at this location = exact amount of error!

GPS receiver in the fieldcollecting points, routes, etc.

Differential Correction Signal

Base station w/ GPS receiver at known

location:

Post ProcessingDifferential Correction

Software communicates w/

base station…2. Field worker returns to office. GPS data is processed to correct for error

Internet

1. Field worker collects data w/ GPS receiver(differential correction enabled receiver)

WAAS

• The Wide Area Augmentation System (WAAS) is a differential GPS system that is being constructed to support GPS accuracy in aircraft.

• WAAS also provides additional accuracy “on the ground”• The GPS receivers that we are using are WAAS

compatible• Pro: Real time differential processing model• Con: ~3 meter accuracy, WAAS satellite not always

available (unreliable coverage)

WAAS

Most (but not all) GPS receivers are WAAS compatible. 95% of GPS receivers on the market today are WAAS compatibleThe GARMIN Venture HC is WAAS compatible

GPS planning software...

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

GPS Satellite Visibility: BlacksburgJuly 25, 2010

What can you do with a GPS?

• Collect and store points (positions) These are called WayPoints. Field corners, insect infestation areas, crop

damage, individual trees, trail heads, creek crossings, point source pollution, camping sites, and don’t forget “your car”!

• Download the points onto your computer and integrate them with other mapping programs

Waypoints

001

Corner2

Point3

Latitude: 37° 16’ 18”Longitude: W80° 28’ 45”

Elevation: 2108 feet

What can you do with a GPS?

• Collect and store the path that you have walked / driven

• These paths are called TRACKS.• Calculate the distance of a track (i.e.

perimeter around a field)• Calculate AREA measurements within a TRACK

(after walking around a field or parking lot...)• Save and Download TRACKS onto your

computer.

Tracks (just start walking…)

What

Tracks (just start walking…)

Latitude: 37° 16’ 18”Longitude: W80° 28’ 45”Elevation: 2108 feetTime: 13:22.15Date: 05/08/2009

Each track point has important information associated with it...“Virtual bread crumbs”

Track points can be collected:•Based on a time period (every 10 seconds)

•Based on distance (every 20 feet)•Or a combination of time and distance (every 10 secs. or 20 feet, whichever comes first).

Tracks

• You can “track your way back...”*• You can use the track data to estimate area /

perimeter*• You can use the time stamp in the trackfile to

“georeference (or geotag)” photographs!*

* We’ll do this later!

What can you do with a GPS?

• Collect and store ROUTES• Routes are similar to TRACKS, but are created

by associating a series of Waypoints• Tracks are straight lines... • Routes can be handy for measuring “square

fields” and “straight lines”• You can measure the length and area

(acreage) of a Route.

Routes

1. Establish Waypoints at strategic locations2. The GPS Receiver “Connects the dots” 3. Area and perimeter measurements are generated

#4

#2

#5

#3

#1

Routes vs. Tracks

Yellow lines = RouteRed lines = TrackRed dots = Track points

What can you do with a GPS?

• The GOTO (or “Find”) functionUsing the ‘GOTO’ function, the GPS will guide you to a predefined Waypoint (you choose which one…) using an electronic compass and “pointer”

• The GOTO/FIND function is like using “Autopilot”You can program the GPS to “beep” when you are within a certain distance of a selected Waypoint

• Tide Tables• Many of the marine GPS’s have built in tide

tables. They provide tidal information and ranges for any date and any place…

• The GARMIN Venture HC does not have tide table information…

• Extra bell & whistle = extra $!

What can you do with a GPS?

• Speed GPS’s calculate your ground speed as you walk,

run, drive, or fly

What can you do with a GPS?

What can you do with a GPS?

• Elevation In addition to providing you with your latitude and

longitude, GPS provides you with elevation information. Elevation is not as accurate as X,Y information.

Some GPS’s have built in barometric altimeters (to increase accuracy of z values). This option costs a bit extra!

• Measure Area / perimeter– Farmers can use a GPS to measure the area of a

pasture or a field of corn…– Natural Resource Agents can measure the area of

a proposed conservation easement…– Educators (and students!) can measure the area of

impervious surfaces (or green space) around their campus’s and communities...

What can you do with a GPS?

Any Questions?

John McGee Ph.D.Geospatial Extension Specialist

jmcg@vt.edu(540) 231-2428

Jennifer McKeeGeospatial Project Developer

jmckee@vt.edu(540) 231-9115

Virginia Geospatial Extension Program

http://www.gep.frec.vt.edu