Forestry and Natural Resources
Unit23: Geographic Information System
Unit 23: Geographical Information Systems and GPS
Lesson 2 Using a Geographical Positioning System
Duration 2 Hours
Students will be able to:
1. Explain the purpose of a geographical information system and give examples of how they are
used.
2. Describe the major components of a GIS and their functions.
3. Explain how GIS can be used to test hypotheses about the spatial distribution of features and
their attributes.
Suggested Learning Activities:
23.2A Using a GPS: Use a hand held GPS unit to locate features on the maps you used for
lesson 1 of this unit.
23.2B Locate a USGS Benchmark: On your map locate a USGS benchmark and obtain its
position using a GPS unit. How close is the location given by the GPS to the location of
the benchmark on your map.
23.2C Create Your Own Destination: Locate a place on the map that you want to go to. Read
the coordinates off of the map using a ruler or straight edge. Enter the position of your
"destination" into the GPS unit. Go to your destination. How close to the position on
the map was your position when you arrived? List possible sources of error in this
process.
23.2D Treasure Hunt: Set up a treasure hunt using the GPS unit. Start by giving participants a
way point to navigate to. A piece of paper (clue) with the next way point is placed at the
location of the first waypoint. Have students record the position of the waypoint and
compare it to those on the clues. Students can also record the time it took to complete
the course, their average speed, and the compass bearing from one point to another.
Combine use of the GPS, map and compass in the hunt to make it more challenging.
Teaching Outline:
A. GPS (Global Positioning System) is a system of satellites developed by the U.S. Department
of Defense to provide consistent and accurate navigation information. GPS is useful for
navigation and for locating man-made and natural features on maps. GPS is also used
extensively to locate features in a geographical information system.
1. The system was originally designed for military applications.
2. Recreational users with 24-hour worldwide navigation coverage with accuracy to 15
meters (49 ft.)
3. A procedure known as selective availability is used to degrade the quality of GPS
information available to the public to about +/- 100 meters. This is to prevent the use of
GPS information for targeting purposes. Differential GPS uses ground based radio
signals to correct for selective availability.
B. How does GPS work?
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Unit23: Geographic Information System
1. A system of 24 satellites continuously send their precise position to earth via radio
signals. Using a technique known as ranging the GPS unit determines its distance from
one or more satellites.
2. By knowing the position of three or more satellites and correcting for the time differences
between signals, the GPS receiver can determine its present position anywhere on earth.
3. Three receivers are required to locate the receiver in 2 dimensions such as latitude and
longitude. A fourth will allow the unit to determine elevation.
4. By making various calculation the GPS unit can also determine your heading, and speed
when moving.
5. In combination with a map and compass GPS is a navigational aid providing navigational
accuracy far superior to celestial navigation using a sextant, or LORAN radio ranging
equipment.
C. The basic information given by the GPS are position and altitude.
1. Position is generally given in terms of latitude and longitude. For example, Davis
California is located at latitude and longitude. Latitude and longitude are given in
degrees, minutes and seconds. There are 360 degrees of longitude (180W, and 180E) and
180 degrees of latitude. (90 N, 90 S).
2. There are several other systems for determining location. The Universal Transverse
Mercator (UTM) system is the most commonly used alternative to latitude and longitude.
This system uses a grid system onto which global sections are projected to measure
position in specific zones.
3. Altitude may be in either feet or meters
D. GPS glossary
1. Almanac Data - information on the constellation of satellites transmitting data. The
almanac data must be acquired before GPS navigation can begin
2. Bearing - The compass direction from your position to a destination
3. Course Made Good (CMG) - The bearing from the your starting position to your present
position.
4. Crosstrack Error (XTE)- the distance you are off a desired course.
5. Desired Track (DTK)- the compass course between the from and to waypoints
6. Differential GPS (DGPS) - an extension of the GPS system that uses land-based radio
beacons to calculate position corrections.
7. Estimated Time of Arrival (ETA) - Estimated time at which you will arrive at your
destination.
8. Estimated Time Enroute (ETE) - The time it will take to reach your destination at present
speed.
9. Fix - an established position at a given point in time.
10. Grid - A coordinate system that projects the earth on a flat surface using square zones for
position measurements. For example Universal Transverse Mercator or UTM coordinate
system.
11. Ground Speed - The velocity of travel relative to a ground position.
12. Latitude - A north/south measurement of position perpendicular to the earth's polar axis.
0 degrees = equator, 90 degrees south is the south pole, 90 degrees north is the north pole.
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Unit23: Geographic Information System
13. Longitude - An east/west measurement of position in relation to the Prime Meridian, an
imaginary circle that passes through the north and south poles. Greenwich England.
14. Plot - to record ones position on a map. Some GPS systems include maps onto which
your position is plotted.
15. Position - an exact, unique location based on a geographic coordinate system.
16. Track (TRK) - the direction of movement relative to a ground position. Can be based on
magnetic or true north.
17. Velocity Made Good (VMG) the speed you are closing in on a destination along a
desired course.
18. Waypoint - marks an exact position fix so it can be recalled for future use. The GPS
stores waypoints electronically.
E. Basic Processes for Navigation
1. Acquisition - When the unit is started it must establish contact with the satellites and
update almanac information.
2. Enter a waypoint - You enter a position into the system's memory. It may be your
current position or a location you wish to navigate to.
3. Record a route - Enter or save a set of waypoints to establish a route to and from some
established starting and ending points.
4. Determine bearing - Determine the compass heading to steer to reach a desired waypoint.
5. Determine reverse route - the necessary bearing to reverse travel between a series of
waypoints making up an established route.
6. Determine Speed - Speed (S) is determined by taking the distance (D) covered and
dividing it by the time (T) taken to cover that distance. S = D/T. Most GPS units will
make this calculation for you.
7. Obtain graphic steering guidance. - The GPS unit will generally have a grahic display
designed to keep you on track. A typical display is a graphic of a highway. A line down
the middle of the highway represents the desired track. To stay on the desired course
you steer toward the center of the highway.
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Unit23: Geographic Information System
Sources of GIS Data
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Forestry and Natural Resources
Unit23: Geographic Information System
Components of a GIS
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