Averaging GPS Data in My World to Refine the Positions of Field Sites

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1 Steven Branting June 2010 Averaging GPS Data in My World to Refine the Positions of Field Sites The inherent inaccuracy of low-end GPS units (often limited to feet) can pose serious issues for field investigators who are attempting to retrace their steps to an exact site and do not have access to a sub-meter receiver. This lesson focuses on the use of My World buffers to create Venn diagrams that, in essence, average the lat/long data from a set of waypoints taken in sequence at a single position. The following process can be completed with a simple Garmin etrex and produce acceptable results for ground investigations. STEP ONE: At a selected single location and without moving the GPS receiver, mark at least three (3) separate waypoints. This data can be downloaded into My World via the GPS function found under the EDIT tab in the main window. More points can be marked. See STEP FOUR for a caveat. NOTE: Turning the GPS unit off and on between markings maximizes the margin of error for this project. STEP TWO: Disaggregate the data points by using the SELECT BY VALUE function, thus creating a separate layer for each data point. This example set has 6 points, with three being used. 1

2 STEP THREE: Create a buffer for each data point using the MAKE BUFFER AROUND function under the ANALYZE tab. The size of the buffer zone should be at least ½ of the navigation accuracy of the GPS at the time the points were taken. 20 feet All GPS units will alert the user to the accuracy of the point being calculated before marking occurs, as seen in the example on the right. Using the sample, the buffers would be set at 10 feet. In most instances, this will adequately care for a set of 3-6 points if need be when collecting data. The greater the number of points gathered to fix the relative position of an artifact, the greater the accuracy of the final product. Remember the caveat. 2

3 STEP FOUR: Use the INTERSECT function under the ANALYZE tab to create a subset of locations within a shared buffer zone. NOTE: My World cannot intersect more than two (2) layers at a time. After creating the first intersection, create the final product by intersecting with the third layer. If you use more than three (3) points, repeat the process until all of the points have been intersected in sequence. In the example above, buffers of the three the lat/long coordinates produced a search site that is a mere 3.7% of the size of any individual buffer area, which is the difference between searching 4,200 square feet and 154 square feet. The map can then be laid over a topographical map, aerial photo or digital elevation map of the site to further refine and define the location. 3

4 a sample application of the lesson: At selected site on a school campus, a canister with a removable top can be buried in such a way that it would not be readily noticed in a grassy area. In My World students can build a map of the three (3) points without a GPS unit. Students can be given the lat/long coordinates of three (3) waypoints (in decimal degrees). To give the data a chance to triangulate with graphic clarity, the teacher may wish to move the GPS unit 5-6 feet around the chosen site. Tell them the navigation accuracy of the points when you took them. Set up the data using STEP ONE above. While My World will portray the position to any number of numbers after the decimal point, your GPS units will most likely provide data such as , REMINDER: West longitude is entered as negative value, as it lies in quadrants II and III in the Cartesian grid used to map the planet, with the Prime Meridian with an X value of 0. Using the EDIT tab, the student will use the CREATE A NEW, EMPTY LAYER function and selected the dot option. Each set of coordinates can be set into the map by simply clicking inside the map window and using the lat/long windows at the bottom of the screen to refine the position to the desired point on the map to match the data you provided to the students. The students should return to the CONSTRUCT mode and create the buffer. They will repeat the process for each point and then go to STEP FOUR above. You should ensure that your school s My World data library includes city or county parcel and center line (street and road) data sets. Many utility companies have aerial photos available, often at 6 resolution. Students can then focus on the exact spot where 4

5 the canister is buried. Students can print out their maps and, going in pairs, try to find it, using the MEASUREMENT tool as an aide. Fill the canister with goodies. lesson corollary Prepare slips of paper on which just one (1) of the coordinates mentioned above is printed. Divide the number of students in your class by three (3) and hand out that many slips of the first coordinates, the same with the second and third. Adjust for classes not having an enrollment divisible by three. Say to the students: You each now have a slip of paper with a single set of latitude and longitude readings. To complete this lesson, you must first find two other people in this room to make up a team of three so that you each has a different set of data. You now have 1 minute to create your teams. Go to page 4 of this lesson to complete the tasks. demonstration: Given the points taken at a single archaeological site: The search site has been focused to a mere 34 square feet. 5