PROJECT 7F
This project will illustrate how taking data at the wrong time can result in loss of data. A route was driven at a time when only five satellites were visible. The same route was driven later when seven satellites could be tracked. The graph showing the PDOPs and number of satellites during the two-hour time frame that covered both data collection sessions is shown as Figure 7—11. You can note that a bit before 4:00 P.M. (1600 hours) the number of satellites is five and the PDOP is six. Just after that time the PDOP drops to approximately two and the number of satellites increases to seven.
Figure 7-10. Simulating obstructions blocking the southern sky.
You would expect, in general, that a higher number of satellites would result in a lower PDOP, since a greater number of satellites allows for more choices of four-satellite constellations. Figure 7—12 shows a Sky Plot of that same time period. In the following steps you will be able to see the effect of choosing a better time to take data.
{__ } In Pathfinder Office, navigate to __:\GPS2GIS\Planning and open the file From_UK_1. COR. Set up View ~ Layers ~ Features so that each Not In Feature fix is shown with a moderately thick red dot and that the dots are not joined. Bring up the background files one.tif through six.tif. From the Position Properties window locate the First fix. Note that it was taken at about 3:30. When was the last fix taken?_. Notice the sizable break in the GPS track near the southern end. More than half a mile seems to be missing just north of the highway interchange.
{_} Open From_UK_2 (only), which constituted the same drive taken starting about 40 minutes later.10 Now the trail of fixes is more complete. In fact, if you zoom up on the track where it appears to loop, you see that it shows an unexpected side trip that the car made: it turned into a bank driveway, circled the bank, turned right (heading north), then left on side roads to rejoin the main road going south. (The previous trip had done this as well but you couldn’t see it because of the data loss.)
Figure 7-11. PDOP & Nsats of when to take data and when not to.
{_} Look back at Figure 7—11. Starting the data collection session 40 minutes later made a real difference in the satellites available and the PDOP. And this made a significant difference in the data fixes that were collected!
As with the rest of Pathfinder Office, Quick Plan has features we have not experimented with. Check out the rest of the menu options if you wish. Consult the Help files. Consult the manual.
Figure 7-12. Sky Plot of when to take data and when not to.
OVERVIEW
Navigation with GPS Equipment
GPS was not developed primarily to allow users of GIS to obtain better data. Rather, the purpose, which we have virtually ignored in this text, was to provide better navigation and instantaneous position information.The software built into the GeoExplorer allows you to perform a number of other navigation functions.
As you may remember, a waypoint (WPT) is a single, 3-D position that can be stored in the memory of the receiver as a special point with a number and a name. A waypoint is a beginning point, a destination, or simply a "point on the way" to somewhere. The GeoExplorer receiver can store up to 99 way points.
A waypoint can be set up in the receiver by:
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- Navigation - |
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1 .Start Navigate |
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2.To Waypoint |
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3. From Waypoint |
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4.Waypoint Setup |
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5.Display Format |
• manually entering the coordinates of a point, or
• copying the current position of the receiver.
Obviously, before you enter any waypoints manually you must be certain that the datum, coordinate system, units, and altitude reference are set properly in the receiver.
Once a waypoint is stored, you can navigate to it by reading various screens on the receiver.
To look at how the GeoExplorers do navigation (while sitting in your easy chair), do PROJECT 7G. To practice (outdoors) using the navigation abilities of GPS, do PROJECT 7H.
STEP-BY-STEP
Navigation with GPS Equipment
PROJECT 7G
In this project we will look at the various options for navigation.
{__} Redo (or at least reread) the last few steps of PROJECT 1B that dealt with navigation using the GeoExplorer. The section starts with "Did the Earth Move?"
{__ } In the office or lab, set up your GeoExplorer: Under "Configuration ~ Units ~ Custom Setup" set the distance units to kilometers, the altitude reference to MSL, the north reference to true north, and the angular units to degrees.
{_} Under "Configuration ~ Coordinates" choose "Deg/Min/Sec."
{__ } From "Position" on the main menu, determine the longitude of the "Old Position." Write this value down.
{__ } Go to the "Navigation" screen. It looks like this: {__ } Select "Waypoint Setup." The resulting screen appears:
{_} Select "Edit WPT." The up and down arrow keys change the number of the waypoint to be modified. Press the up arrow key until you find an unused waypoint (one with zero longitude and zero latitude). Press "CMD." Note the number of the waypoint (Wnn) that you are working with.
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- Waypoint Setup - |
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1 .Add Here |
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2.Edit WPT |
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3. Delete WPT |
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4.Clear All |
{_} Give the waypoint the name "NP" for "north pole": Use the up-down arrow keys to highlight the top line. Press (or press and hold) the right arrow key to move the highlight to a single position to the right of the waypoint number. Press the up arrow until an "N" appears. Use the arrow keys to place a "P" next to it. (If you want to spell out "North Pole" and your thumb is up for it, go ahead. The lowercase letters are after the "Z.")
At this point you want to move on to the next field ("Lat"). If you press the down arrow key you merely get the next letter. If you press "CMD," you are asked if you want to save the waypoint. If you press "Esc" you are returned to the "Edit WPT" screen. So how do you move to the next line? This is where the previously unused "enter" key comes in. (It’s the key at the upper right with the bent arrow on it.) Press it.
{_} Edit the "Latitude" using the same technique. Make it 90° North.
{_} Edit the Longitude. Make it the same as your current longitude (as indicated by "Old Position," which you previously wrote down). Make sure you pick "East" or "West" correctly. (In virtually all the United States, pick "W.") (What state is northernmost? What state is westernmost? What state is easternmost?)11
{_} Edit the Altitude. Make it zero.
{__ } Press "CMD." Save the waypoint.
{__ } Find another blank waypoint. Edit it so that its name is "Eq" and that it represents a point on the equator (latitude zero degrees, longitude at the longitude of "Old Position," and altitude of zero.
In the next few steps you are going to explore the idea of a journey, first from your current position to the North Pole, then from your current position to the Equator. The sum of these two distances is interesting, in terms of the definition of the length of the meter.
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- Display Format - |
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I.Dist/Bearing |
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2.Track & XTE |
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3.East/North |
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4.Velocity/Time |
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- Old Navigation - |
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Dist: 5795.9Km |
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Turn: left 118 |
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Bearing: 0 Tn |
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Heading: 118 Tn |
{__ } Go to the "To Waypoint" screen. Press the up or down arrow keys until the "NP" waypoint appears. Press "CMD."
{__ } Go to the "From Waypoint" screen. Press the up or down arrow key until "00" appears. Note that it is named "LAST POS" and has the coordinates of the last position recorded by the receiver. Press "CMD."
{__ } Go to the "Display Format" screen which looks like this: Choose "Dist/Bearing." This will set the display so that distance and bearing are revealed during navigation.
{__ } Although you are planning to stay in one position for the moment, begin the navigation process with "Start Navigate." You will see something like:
Most of the items on the screen won’t make sense, because you are shielded from the satellites, and you are "proceeding" on the basis of "Old Navigation." But the distance should be the distance from "Old Pos" to the North Pole and the bearing should show zero degrees. The heading, based on "true north" (Tn) and the amount you are to turn to get on course, should be the same. Write down the distance measurement: (OldPos_to_NP=_km.)
{__ } Leave "Navigation" and configure the receiver to operate with a magnetic north reference. Then return and start the navigation again. The bearing now should reflect the magnetic variation (the difference between true north and magnetic north) in your part of the world. How much is it?
{__ } Make "Eq" the "TO" waypoint. "Start Navigation." What is the distance from your position to the equator? Add this distance to "OldPos_to_NP" which you wrote down earlier. The total should be about 10,000 kilometers. What is it?_12
{__ } Change the display format to "East/North" and restart navigation. Here you are told how far to proceed in each of three dimensions to reach the destination waypoint.
The designers of the Geo3 have gone to considerable lengths to make the unit "navigation friendly." One of the three main menu items (and hence the three white buttons) on the receiver is named "NAV." The unit even contains an internal compass—albeit a very imprecise one (it gives the direction the front of the unit is pointing to the nearest 45°). One thing the designers did not do is fix it so the last position is retained in the receiver after the power has been off. So you have to take it outside long enough to collect a position, to do the equivalent of the above. While you are outside waiting for the first fix to occur, use Fn-OPTION ~ Setup—Configurations (using CLOSE as necessary to get to the right place) to set Distance units to kilometers, North reference to True, and Degrees to DD*MM’SS.ss."
After you have done that, being sure that the unit calculated a position, come back inside and press the "NAV" button. You will get one of three file tabs that relate to the three navigation options the user has: Chart, Road, and Compass. Press NAV until you get "Road." Basically the Road option lets you visualize your present position (where you see a little person icon) with respect to a "target" symbolized as a pair of crossed flags. The emphasis is on providing information, both numerical and graphical, to aid the user in moving to the target.
To use this option you need to set up a target We will do that with a waypoint—the North Pole. To set it up, press the Option key, then highlight "New Waypoint" and press Enter. A screen with a waypoint name— something like OldGPS007—will appear. Our plan will be to establish the waypoint at the last GPS position and then edit it
Press Enter. Use the "keyboard" to enter NP, after backspacing over OldGPSetc. Close the keyboard Press OPTION, select "Here." This puts the old position into the Latitude, Longitude, and Altitude fields if they aren’t there already.
Press Enter. Make the Latitude 90°. Leave the Longitude alone. Make the Altitude zero. Press CLOSE until the Road screen reappears. Press OPTION, highlight "Select target" and press ENTER. Scroll down to "NP" and press ENTER. On the ROAD screen you will see the person icon in the center and the flags at one end of the road. If you point the unit in different directions you will notice the road rotates as well. Point the unit so that the flags are furthest from you, as though the person icon is walking forward toward the target. That direction should be approximately north.
Press OPTION and activate "Info windows," which lets you select what items of information you want displayed at the bottom of the Road screen. With the ENTER key put checks beside Distance, Bearing, and Heading, and return to the Road screen. Distance is the distance to the target Write it here:_. Heading is the direction the unit is pointed in. Since you aren’t moving and receiving GPS signals, the Heading is taken from the internal compass. Bearing is the direction you would have to go from your present position to get to the target It should say 360°, since you go directly north from anywhere to get to the north pole. Presstiie SYS button and changethe "Morth reference" to "Magnetic"
I have chosen these obvious (and impossible) targets to give you an intuitive idea of how the navigation process works. You may also navigate to features that you have collected. This is an important advantage of the Geo3, since databases need not only to be built, but to be updated. You can transfer a GPS file back into the Geo3 (even from a GPS) and then navigate to its features to make changes or corrections.
If you press the NAV button (or use Fn-OPTIONS) you can bring up the "Compass" screen. Besides the appearance, the primary difference between this and the "Road" choice is that there is no "Road Scale" and there is a Calibrate compass option. (Check the Trimble manual for how to do that if your compass seems confused.)
One might wonder why they bother with two such similar screens. One advantage is that you can put different "Info windows" on the two screens and flip back and forth between them. Since you can have only two or three windows out of more than a dozen possible, this provides a way to get more information while navigating.
The final navigation choice is "Chart" which is considerably different. Using Chart is left as an exercise in reading manuals and experimenting.
