An Introduction to GPS
Prepared by:
John McGee
Jennifer McKee
With support from:
NSF DUE-0903270
in partnership with:
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/2.1Intro_%20to_GPS.pptx
Outline
• Review: How does GPS Work?
• Review: Differential processing
How GPS Works
One satellite…
How GPS Works
If 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 2
satellites, then it “knows” that it is located
somewhere 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
How GPS Works
A fourth satellite
is required to
determine the
exact location and
elevation.
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
–
–
–
We did some of
this last year!
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
We are going to ‘do
this’ this year’!
–
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 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.
Mapping and survey
C-code: (“Civilian” Code).
grade GPS receivers
use this too!
 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…
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
Base station w/ GPS
receiver at known
location:
Differential Correction
Signal
GPS receiver in the field
collecting points, routes, etc.
Exact known (surveyed)
coordinates differ from GPS
coordinates at this location
= exact amount of error!
Post Processing
Differential Correction
2. Field worker returns to
office. GPS data is
processed to correct for
error
1. Field worker collects data w/ GPS receiver
(differential correction enabled receiver)
Software
communicates w/
base station…
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 compatible
The GARMIN Venture HC is WAAS compatible
GPS planning software...
http://www.trimble.com/planningsoftware.shtml
GPS Satellite Visibility: Blacksburg
July 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
Corner2
Point3
Latitude: 37° 16’ 18”
Longitude: W80° 28’ 45”
Elevation: 2108 feet
001
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…)
Each track point has important
information associated with it...
“Virtual bread crumbs”
Latitude: 37° 16’ 18”
Longitude: W80° 28’ 45”
Elevation: 2108 feet
Time: 13:22.15
Date: 05/08/2009
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
#2
#3
#4
#1
#5
1. Establish Waypoints at strategic locations
2. The GPS Receiver “Connects the dots”
3. Area and perimeter measurements are generated
Routes vs.
Tracks
Yellow lines = Route
Red lines = Track
Red dots = Track points
What can you do with a GPS?
• The GOTO (or “Find”) function
Using 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
What can you do with a GPS?
• 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?
• 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!
What can you do with a GPS?
• 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...
Any Questions?
Virginia Geospatial Extension Program
http://www.gep.frec.vt.edu
John McGee Ph.D.
Geospatial Extension Specialist
jmcg@vt.edu
(540) 231-2428
Jennifer McKee
Geospatial Project Developer
jmckee@vt.edu
(540) 231-9115