Name: ___________________________

GPS Unit #: ______________________

Materials Needed: GPS unit, compass, pencil, Boulder topographic map

Your mission is to use satellite navigation to explore the University of Colorado campus. You will use the Garmin GPS (Global Positioning System) unit to find and identify a landmarks (statues, etc.) or building on campus, and then return here to the Benson. Earth Sciences Building.

The goal of this project is to gain familiarity with GPS navigation, review map and compass navigation, and compare the two.

1. Turn on the GPS units (press the red button). It will display the Satellite Status Page -- which shows the numbers of the satellites that are currently orbiting the earth within your line-of-sight. It is 'listening' for these satellites to determine at what Latitude, Longitude, and Altitude you are standing.
2. When it 'hears' a satellite's signal, it shows a little bar above the satellite number, and the satellite number within the circles on top is de-highlighted. The height of the bar indicates how strong the signal is. List below which satellite signals your GPS is receiving (the de-hilighted numbers), and sketch their positions at right:

Satellite Numbers: ______ ______ ______ ______ ______ ______ ______ ______

3. After it receives enough satellites' signals, it will go to the Position Page -- which shows your direction (a linear compass and your 'track' heading), speed, distance traveled, altitude, position in latitude (North/South) and longitude (East/West), and time. Notice how some of the numbers change fairly often. Which ones are changing, and can you guess why they might be changing even though you're standing still? ___________

_________________________________________________________________________________________

4. Now turn around in a circle. Why doesn't the compass needle change direction?? ____________________

After you walk a while, if figures out which direction you're heading, and shows that.

5. Go back to the Satellite Status Page (hit the PAGE button several times), and go stand right next to the Benson Earth Sciences building. Do you lose contact with any satellites? ________
6. Now we're going to practice navigating with the GPS unit. We want to travel on a SW (Southwest) course (compass bearing: 225^o) for 530 feet (about .1 mi.). What will happen to our Latitude (N/S) and Longitude (E/W) if we travel in this direction? _____________________________________________________
7. Use the magnetic compass to find your direction of travel (compass bearing of SW = 225^o). Turn the compass direction to 225 ^o, line up the red needle with North, and now you're facing SW. (225 ^o). Sight on an object in the distance at this bearing, and walk towards it, counting your steps. Check the compass bearing occasionally, and watch out for cars in the parking lot!

Watch the GPS unit as you walk. Does the compass on the GPS unit start moving towards SW, and the TRACK move towards 225^o. Is your latitude going down (less North), and your longitude going up (more West)??

8. Stand in the field to the south of the Benson Earth Sciences Building. Write the starting Latitude, Longitude, and elevation from the Position Page.

Start Latitude: ________________Start Longitude: _______________ Start Elevation: _____________

9. Reset the trip distance: on the Position Page, press the down arrow until the TRIP (distance) field is highlighted, then press ENTER, change it to 0, and press ENTER again.
10. Now it's time for the first leg of your scavenger hunt trip. We're going to ask the GPS unit to show us which direction to travel. We have recorded a "Waypoint" named "A#" in your GPS units; where # is the # of your GPS unit. This waypoint is just the location of a landmark on the CU campus. Press the PAGE button until you see the Main Menu Page. Then move down to the WAYPOINT LIST, and press ENTER. Be careful not to delete any of the existing waypoints. Highlight the waypoint A# and press ENTER. This is the Latitude/Longitude for the landmark that you will be walking to. BRG at the bottom tells the bearing (compass direction) you should travel to get to this location, and DST is how far away it is. How do you think it knows these things? ______________________________
11. Write the Latitude/Longitude of your destination Waypoint in the table below. Compare that Latitude/Longitude to your current location, and figure out approximately which direction you'll need to go.

Direction of Travel (e.g., NW) = ________

12. Now press the GOTO button, and then highlight your destination waypoint (e.g., 'A') and press ENTER. It will now display the Compass Page, which will help guide you to your destination (hit ENTER to get to Compass Page if you are on the Highway Page). At the top is the bearing you need to walk (BRG) and the distance left to go (DST). Write these 2 values in the table below. The compass pointer shows you the direction you need to travel (is it close to what you predicted in #11 above?). At the bottom, TRK shows you what direction you are actually travelling, and SPD is your current speed.

	Waypoint	Latitude	Longitude	BRG	DST to go	Trip Dist.
Landmark or Building Name, or description?   Estimate Latitude and Longitude of this feature from the topo map.

 

13. Use your magnetic compass to get you pointed in the right direction (use BRG), and start walking. As you walk, the GPS compass will show you which direction you need to turn (for example, if the pointer is pointing to the left, you should turn a little bit to the left). Don't make direction changes too quickly or often! When you are getting close to your waypoint (landmark), your GPS unit will start beeping. Press the PAGE button to see the message, and then PAGE again to go back. Remember -- make sure the DST gets fairly small (.01 - .03 mi.).
14. When you think you've found the right landmark or building, record the Trip Distance (press PAGE to get to the Position Page). Record the Landmark or building Name.
15. Take a look at the Moving Map Page on the GPS unit to see the path you traveled to this Waypoint. Press the PAGE button until you see a screen with ‘ZOOM’, a scale factor number, and ‘PAN’ at the top. Try different scale factors (try 0.5 mi. or 1 mi.) by highlighting the number at the top, pressing ENTER, and pressing the up and down arrows, until you can see your whole trip path. You can move around with the PAN function. There should be a squiggly line between your starting point and the letter for this landmark or building.
16. Now it’s time to return to the Benson Earth Sciences building. What direction do you need to walk? ___. When back at your starting point at Benson, write the Latitude, Longitude, and elevation from the Position Page.

How does this compare to the values you obtained in (8)? Calculate the change in meters in latitude, longitude, and elevation and write them down here. Which quantity changed the most? (Instructions on how to convert latitude and longitude differences into distances in meters and feet given at the end of this handout).

Compare your GPS locations for the landmarks with those obtained from the topo map. Discuss. (you will have to convert the latitudes and longitudes to common units (deg-min-sec, decimal degrees, whatever) before comparing)

Once you have a position from GPS in latitude, longitude, and altitude, you may want to convert this to other units of measurement. This is useful if you want to know the relative distance between two points in feet or meters. You can use the following conversion to find the distance between two GPS located points. This conversion assumes that the Earth is a sphere but that the distances between points are small enough that the ground can be considered flat.

latitude and longitude [in decimal degrees]

= degrees + (minutes/60) + (seconds/3600)

latitude difference in meters

= latitude difference in decimal degrees x 111,300 m/deg

longitude difference in meters

= longitude difference in decimal degrees x 85,300 m/deg (near 40° N)

total difference between two points

= square root of : (longitude difference2 + latitude difference2 + altitude difference2)

Some GPS software will make this conversion for you. However, it is useful to know how this conversion is made and to have an intuitive feel for what your GPS positions mean.

Frequently it is desirable to use coordinates that are nearly Cartesian in the field. The UTM (Universal Transverse Mercator) projection is frequently used in this case. The projection is a sideways Mercator projection 6° wide along a meridian; positions are expressed as northing and easting and are meters (or kilometers) north of the equator or east of the central meridian (plus 500 km). The declination of northing axis relative to true north is shown on USGS topographic maps, as are the UTM coordinates. Many newer USGS maps show the UTM grid across the map. Nearly all commercial GPS receivers will display UTM coordinates in addition to or instead of latitude and longitude.

It is tempting to read the numbers off a GPS unit and ignore just what in detail they mean. For many applications, this works fine. But this can produce serious errors in some applications. Although things like latitude, longitude, and elevation might seem well defined, in fact they have some slop. The Earth is more or less a flattened ellipsoid. You might think there would be a single best estimate of this shape, but if you try to estimate the dimensions of the ellipsoid from measurements over only part of the Earth, you find that you get slightly different ellipsoids for different areas. These yield different datums that define latitude and longitude. Nearly all USGS topographic maps are mapped to the CONUS 1927 datum for North America (also called the North American Datum of 1927). This was fine until satellite-based descriptions of the shape of the Earth were made and needed; the need for a single global datum led to the WGS 84 datum. GPS units usually use the 1984 World Geodetic System of 1984 (WGS 1984) as their default; this is virtually identical to the North American Datum of 1983 (GRS80 or NAD83). More recent USGS maps show the offset between the two datums; in Colorado this amounts to about 40 m. On most GPS receivers you can specify in which datum you wish your position reported.

A second error can result from misinterpreting the elevations that are being reported. GPS tends to work in height above a reference ellipsoid; this ellipsoid is a simple geometric figure that only approximates the Earth's surface. The geoid represents where sea level would be in any given spot; it is usually 10s of meters from the ellipsoid. For many applications it can be important to know whether your height is above the reference ellipsoid or above sea level.