Basic Concepts (GPS and GIS) Part 4

Preparing to Correlate GPS Data with Map Data

{_} Take out the USGS topographic quadrangle (a topo map, usual scale 1:24,000) of the appropriate general area. The receiver can display a geographic position in several coordinate systems. Under "Configuration ~ Coordinates" you will find

• Degrees, Minutes, and decimal fractions of minutes (Deg &Min)

• Degrees, Minutes, Seconds, and decimal fractions of seconds (Deg, Min & Sec)

• Ordinance Survey of Great Britain (OSGB)

• Universal Transverse Mercator (UTM)

• Earth Centered, Earth Fixed (ECEF)

• Trimble Grid

{__ } Choose "Deg & Min."

{__ } Under "Configuration ~ Datum" you will find a list with a large number of choices, stored in alphabetical order. You can scroll through the list by holding down either the "up" key or the "down" key. Choose Bahamas18 (NAD-27) with the "CMD" key.

{__ } Then choose WGS84 (World), which is the fundamental GPS datum. WGS84 identifies the World Geodetic System developed in 1984.

{_} Finally, note the datum of the map you are using. The datum is usually found in the lower left-hand corner (e.g., NAD27, the North American Datum of 1927). Under "Configuration ~ Datum" set the GPS receiver to operate in this datum, shown as "N-Am. 1927 Conus" if this is the datum of your map—as it is with many USGS topographical maps.

To set these options with the Geo3 you use Fn & OPTION—Setup ~ Configurations ~ Coordinates. The Geo3 contains an annoying number of Datum choices. If you hold down an arrow key it scrolls through them— not quickly, but it beats wearing out your thumb and the switch. A shortcut: the datum choices are in alphabetical order; the list wraps around the end (i.e., "A" comes after "Z").

The other options you need to set in this section are under Fn & OPTION—Setup ~ Configurations ~ Units.

- Custom Setup -

1 .Distance Units

2.Angular Units

3.Velocity Units


5.Alt Reference

6. North Reference

{_} Under "Configuration ~ Units ~ Custom Setup" you will find this menu:

{_} Under "Distance Units" choose Kilometers. Distance units available on the GeoExplorer include Yards, Meters, Kilometers, NauticalMiles (6080 feet), Miles (statute, 5280 feet), "Internatl Feet" (international feet, where an inch is 0.0254 meters, exactly), and "U.S.Survey Feet" (where a meter is considered to be 39.37 inches, exactly).

{_} Examine your map to determine the appropriate distance units for the upcoming fieldwork. Feel free to change this value later if you should wish visual output in some other units.

{_} Under "Configuration ~ Units ~ Custom Setup ~ Altitude Units" choose Meters or Feet, depending on the map you will be using.

{_} Under "Configuration ~ Units ~ Custom Setup ~ Altitude

Reference" choose Geoid (MSL). The "altitude reference" may be set to either MSL (which is elevation above Mean Sea Level) or HAE (which is Height Above the reference Ellipsoid—the theoretical mathematical surface that approximates the surface of the Earth). Since at this time you probably do not know the relationship between the HAE and MSL at your location, you are selecting MSL.

{__ } Under "Configuration ~ Units ~ Custom Setup ~ North Reference" choose "North, True" or "South, True" depending on the map you will be using.

Double-Checking the Configuration

{__ } Now do a summary check of the configuration. Under the menu choice "Main ~ Configuration" check the following critical options:

• Rover Options ~ Dynamics: Land

• Rover Options ~ Pos Mode: 3D

• Rover Options ~ Elev Mask: 15

• Rover Options ~ SNR Mask: 4

• Rover Options ~ PDOP Mask: 6

• Rover Options ~ Antenna Ht: 1.00 (or your waist height in meters)

• Rover Options ~ Log DOPs: Off

• Rover Options ~ Velocity: Off

• Rover Options ~ File Prefix: (do not change)

• Rover Options ~ Not in Feature Rate: Off

• Coordinates ~ Deg & Minutes (and decimal fractional parts thereof)

• Datum: (set to your map)

• Units ~ Custom ~ Distance: (set to your map)

• Units ~ Custom ~ Angular: Degrees

• Units ~ Custom ~ Velocity: Kilometers per Hour

• Units ~ Custom ~ Altitude Units: (set to your map)

• Units ~ Custom ~ Altitude Reference: Geoid (MSL)

• Date & Time ~ Set Local Time: (adjust to local time)

• Date & Time ~ Time Disp: Local 12 Hours

• Battery Usage: (reset if battery freshly charged)

Use this checklist each time you take the GeoExplorer into the field — modifying it as necessary.

Final Inside Activity

You are almost ready to take the GeoExplorer into the field. One thing remains to be done. While you are still inside, read through the directions for PROJECT 1B below completely to prepare yourself for the fieldwork. Develop a feel for the sort of data you will be collecting. Practice changing from screen to screen. Outside, with the wind blowing and the traffic roaring, is no time to discover that you don’t have a solid surface to write on or that you don’t know just what it is you are supposed to be doing. A little preparation now will pay big dividends later.

If you are going to collect data with the Geo3 you also need a square about 2.5×2.5 inches made of a couple of layers of aluminum foil. And a few inches of masking tape.

{__ } Read over PROJECT 1B below.


Now Outside

This is an exercise best done with two people. You will take the map, your notebook, and the GPS receiver outside to make observations. You will not yet place the data you collect into a computer file but you will learn a lot about the factors affecting data collection. (If you are not sure that the set tings on the receiver are those you put in during Project 1A, verify them against those in the previous section: "Double-Checking the Configuration.")

{__ } As you leave the classroom or laboratory to travel to the site for data collection, turn the unit on. If you carry the receiver exposed to the sky, it will begin to "acquire" satellites. It isreof important whichmenu appears on the display; whenever the receiver is on, it "looks"for satellites and calculates positions if it can.

{__ } Move to a spot outdoors, well away from buildings and heavy tree canopy. If it is reasonably level and not shrouded by nearby hills or mountains, so much the better. And if you can locate the antenna over a geodetic monument, for which you can find the official latitude and longitude (perhaps from the NGS web site,, super.

{__ } Look at the map to locate your approximate position.

{__ } Hold the antenna over the spot for which the coordinates are to be determined. The antenna in the receiver is just below the image of the sextant embossed in the plastic of the top of the receiver. Hold the unit as far in front of you as is comfortable, with the top part close to horizontal, tilted only enough so you can read the screen.

- GPS Status -

I.Sat Tracking

2.Sat Posn & SNR

3.Sat Hlth & URA

4.Sat Chnl Data

Actually, no position will be comfortable after a few minutes; you will want to pass the unit to your partner so you can drop your arm and let the blood drain back into your fingers. An alternative is to put the unit on the ground and crouch or sit down so you can read the screen. This is less fun in winter, or when there is poison ivy about. I never claimed fieldwork was easy. You might bring a table or tripod with you, or find a fence post. Be careful: the power cord makes it easy to bounce the receiver off the ground. It’s a tough unit but it is also expensive; do you really want to test it?

{_} Keep your head and body out of the way, i.e., don’t block the signal from a satellite to the receiver. You are opaque, as far as the high-frequency, short-length GPS waves are concerned. Remember, the receiver is looking for satellites as low as 15 degrees above the horizon. It’s easy to forget this and obstruct the antenna, causing the receiver to lose its lock on a satellite.

Tracking Satellites

{__} Starting with the Main Menu, navigate to the "GPS Status" menu and press "CMD." The following will appear: Highlight "Satellite Tracking" and press "CMD." The "Sat Tracking" screen will appear, listing some two-digit numbers. These are the designations, called PRN numbers,19 that your GPS receiver uses to identify the satellites. The numbers lie between 1 and 32, inclusive.

The numbers that appear now are those of the satellites that the receiver might be able to pick up, based on your position and time. They are usually those which are above the horizon and the specified elevation mask angle. The receiver determines which satellites are available by formulas built into its computer and by an almanac transmitted by each satellite which describes the general location of all the satellites.

Since you are outside, presumably the receiver is locked onto some satellites. The number of little shaded boxes in the lower left-hand corner of the screen indicates how many. The receiver needs to be receiving at least four satellites before location fixes are computed.20 Because the geometry when the elevation angle is set at 15° allows the receiver to consider satellites in about one-third of the sky, it may track eight satellites, or even one or two more.

One more bit of information may be learned from this screen. Small arrows—up to four of them—may be seen pointing to satellite numbers. These are the satellites which the receiver is using (or attempting to use) to calculate its position. To summarize the screen:

• If a satellite’s number appears on the screen, then, according to the almanac the satellite should be physically in the space above the user, at an angle greater than the setting of the elevation mask. This would theoretically make the satellite available for position finding.

• If an arrow appears next to the number, the satellite is being considered for position finding. If a position is found, the satellite is being used for position finding.

• The number of little boxes indicates the number of satellites that the receiver is tracking, that is, electronically locked onto.

The Geo3 display is much different—actually much easier. Go Fn & OPTION ~ GPS. By using the OPTION button you can toggle between the Standard screen and the Advanced screen. The Standard screen shows a skyplot of the satellites that are being used for position finding, that are being tracked, and that could be tracked. It also, most importantly, shows the coordinates of the antenna of the receiver. The Advanced screen shows a table of satellite information. A check mark indicates the satellite could be or is being used to compute positions.

{_} A few minutes may elapse before the unit locks onto enough satellites to begin giving position fixes. If more than 10 minutes go by with no position fix, change the PDOP to eight (8) and make sure you aren’t obstructing the signal.

{_} Once the GeoExplorer is tracking four or more satellites, select "Position" from the Main Menu and write down the latitude, longitude, and altitude. When locked onto four or more satellites, the receiver computes the position of the antenna about three times every two seconds. (If the word "OLD" appears on the screen it indicates that the value presented is one that was collected in the past—perhaps the immediate past—and that the receiver is not calculating new positions. Make certain that there are no obstructions blocking the signals.)

{_} Note the time. Plan to write down a new position reading in your notebook every minute, approximately on the minute, for the next quarter of an hour.

{_} In between writing position fixes in your notebook you should record some other information. Move back to the "Sat Tracking" screen. Note down the numbers of the satellites which appear there. Circle the numbers of the satellites the receiver is using to compute positions. Also note how many satellites the unit is receiving signals from. Write down the value identified as "PDOP."

{_} Now it is probably about time to go back to the "Position" screen to write down the next set of position coordinates. They should be close to, but not exactly the same as, those you wrote down a minute ago. The screen should not say "Old Position." If it does, you probably got your head in the way of a satellite signal.

{__} Now go to the "Sat Posn & SNR" screen (it’s under "GPS Status"). You will see several horizontal lines of information—one for each satellite being tracked. One item of information displayed for each satellite is "Elv"—an abbreviation for Elevation. If you could stand and point a straight arm directly toward the satellite, the elevation would be the angle, in degrees, that your arm made with the Earth, assuming the surface is level where you are standing. Zero degrees would represent a satellite at the horizon; ninety degrees would represent a satellite directly overhead.

{_} Whoops. Time to write down another lat-lon-alt position.

{_} Return to the "Sat Posn & SNR" screen. The column after "Elv" is identified as "Az" which stands for Azimuth. The Azimuth specifies the angle between due north and the satellite: Point your arm toward the north, then rotate your body clockwise until your arm is pointed at the satellite. The number of degrees your body rotated is the azimuth.

{_} Write down another position fix.

The last column on the "Sat Posn & SNR" screen is the "signal to noise ratio" (recall that it is an indication of the strength of the signal from the satellite). Acceptable values are greater than or equal to four—which is the value you set as the SNR mask. Values may range up to 35 or so.

{_} For each satellite being tracked, record its elevation, azimuth, and signal strength.

{_} Put your hand over the antenna (it is directly under the little sextant embossed in the plastic above the screen) and watch the signal strength drop.

{_} Determine where one or two satellites are in the sky, relative to your position. Try to interpose your body between the unit and a satellite to see if you can make the signal strength drop for a single satellite. In the middle latitudes in the United States there will generally be more satellites to your south than north

{_} After recording another fix, move to the "Sat Hlth & URA" screen. This displays the "health" of the satellite, as determined by information broadcast by the satellite itself, and the "User Range Accuracy" (URA)—a numerical indication of the accuracy one might expect when using this satellite to compute a position fix. Satellite health will be indicated by "OK," by "U" for unhealthy, or by "n/a" for "not available," indicating that no signal is being received.

"URA" may have values ranging from one to 1024. Values greater than 16 indicate that the DoD is corrupting the signal for the particular satellite and that any single point calculated by using that satellite could be in error by approximately 100 meters. This is not supposed to happen after May Day 2000 so the URA values should be on the order of two to three. The units of the URA number are meters, but since a given position is found using a combination of several satellites, the URA value of any particular one is of limited usefulness in estimating error.

The Geo3 does not display User Range Accuracy.

{_} Finish recording the 15 position fixes. Is the unit still tracking the same satellites? Is it using the same constellation of satellites to compute fixes? If not, write down the new information.

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