Mapping with GPS

Here are a couple of results added to the CAD map from a previous example to show some of the limitations.
The file was collected by walking south down the path outside ES, stopping at a light fixture--crossed hammers, with 5 sec interval collection points. I turned right at the "stadium" sculpture, stopping at a WWU directory.  I continued up the path east of the road, crossing the road to the athletic field, where I established an area feature and walked along the sidelines.
The track perimeter and the yellow path were run earlier the same day. Both files were differentially corrected from the Whidbey Island base station.
 

This file was collected while driving from the parking lot opposite Fairhaven College, past Sehome High, over I5, then out to Padden. At Padden I stopped to survey a Benchmark (blue diamond) then continued to I5 at the South Lake Samish Exit. I joined I5 to return to Western, but obviously lost the signal for part of the way. If I had been walking it would probably have been OK.

• The Global Positioning System is a system of navigation based on 24 satellites orbiting 11000 miles above the earth that send continuous signals to earth.
•  GPS receivers intercept the signals and “triangulate” the position and elevation of the receiver to varying degrees of accuracy.  Hand held receivers have a precision of about 5 - 100m, before correcting from base stations.
•  The GPS system works by measuring the travel time of the signal from each satellite visible to your receiver (<1/10 sec. travel time.) The system has a sophisticated method of measuring the time--see details in the exercise in the book.
• Travel times give us distances from the satellites, but how do we know where the satellites are in space? The satellites are put into precise orbits and their positions can be predicted as precisely as any other orbiting body, such as the moon. However, ground stations monitor slight variations. The ground stations then relay corrections back to the satellites that in turn transmit that information for the GPS receivers to make the necessary corrections. Then, because we know the position of each satellite precisely, the problem of triangulation is simply a matter of spherical geometry. The units we have need signals from at least 4 satellites to give an accurate position.
• There are a number of factors that can degrade the signal.
• The signal slows as it passes through the ionosphere. This problem is overcome in the expensive receivers by comparing the arrival of different frequencies and calculating the slowing of the signal. Cheaper units simply assume a certain slowing.
• The signal also slows as it passes through the atmosphere. Because water vapor has a large effect on slowing, we cannot predict an average effect so that remains a problem.
• A third type of error arises from time precision and by rounding in calculations. Timing is checked constantly and adjusted, but rounding errors are a function of the precision of calculation in your receiver.
• A final type of error is called a “multipath error”, for example, you may get reflections off buildings. More sophisticated receiver software will try to eliminate such signals by checking for unexpected positions.
• The military introduced errors into the clocks to give a rather random 100m swing in positions. This is called the "Selective Availability" or S/A error so that missiles cannot be targeted accurately--they turned it off during the Gulf War and Kosovo engagements so that our forces could use accurate positioning. They permanently disabled the system on May 1, 2000, so now this error should not be a problem unless you stray into a sensitive area.
• (This by e-mail 5.3.00 from NOAA.

DATA TEST PERFORMED BY NGS, NOAA
On May 2, 2000, SA was no longer present. The plots show that SA causes 95% of the points to fall within a radius of 48.3 yards. Without SA, 95% of the points fall within a radius of 4.5 yards.)

•  Some errors are eliminated using “Differential GPS.” A receiver at a known location calculates the difference between its “real” position and that determined by the satellite signals. It then broadcasts that information, which is picked up by field receivers that apply the correction. The closest ground station to Bellingham is on Whidbey Island, operated by the coast guard. This sends signals continuously, and the corrections are made automatically by your receiver. This capability, called “Real Time Differential GPS”, is built into to the Trimble backpack GPS unit and increases the precision to about a meter. (Higher precision is available in “survey grade” receivers.) This method is not included in the GeoExplorer 3, though an add-on receiver is available to provide this feature.
• “Post Processing” is the method we use to reduce errors in the handheld units. This requires you download the differential corrections from a ground station—Whidbey Island or Sedro Wooley—and perform the corrections on your PC using software developed for the purpose. You can also use another receiver as the base station, locating it at a known position, such as a benchmark.

Collecting data

• Creating a data dictionary that allows different objects to be recorded. This dictionary is then transferred to the GPS unit.
• The dictionary contains a list of features, e.g. “Soil Type” with Attributes that can have values. e.g. one attribute could be "Name" that you can type in using a clumsy screen on the unit. Or you could choose from a list if you had included one in the file. You could enter numerical data, such as thickness, porosity, or permeability, again if those were in your list of attributes.
• Points, lines, and areas can be plotted, along with the attributes listed in the dictionary.
• The data is then downloaded to the computer and corrected from the nearest base station.

The corrected data is then "exported" in ArcView format to be added to an existing project.

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