G338/G538: Global Positioning Systems Lab

The purpose of this lab is to get hands-on experience using consumer grade Global Position System (GPS) units and to explore the errors typically encountered with these (consumer grade) units. In this lab you will do three things:

1) Collect a position for the National Geodetic Survey (NGS) marker on the North Side of Dunn Woods. You will compare your GPS location for this marker to the NGS recorded location for this point to assess the accuracy of the Garmin GPS unit.

2) Collect points for a polygon feature (Dunn Woods) - collect locations that bound Dunn Woods, specific locations are up to you. This polygon you create will be used to consider the issues associated with defining the boundaries of features, including the uncertainty in identifying boundaries/edges, and the spatial accuracy of recording these in the field.

3) Collect points for trees along the North side of 7th street (between Indiana and Woodlawn). You will use these points to determine prospective locations for new tree plantings.

Datasets for this lab can be downloaded from Resources (GPS_Lab.7z)

You will need to create three datasets (or feature classes), one dataset for each of the three objectives above (you can create either shapefiles or geodatabases, the format is up to you and either format will work fine for the lab).

NOTE: The ancillary datasets for the lab are in UTM Zone 16 North, WGS 1984 datum so make sure your GPS unit is set to this spatial reference before you start recording locations.

There are several methods of getting GPS data into a GIS dataset. The most common method is to download data from the GPS unit via a USB cable and convert to a vector data format (e.g. shapefile). Garmin GPS units come with software that allow you to download data from GPS units, but not to create shapefiles directly. A good (and free) tool that allows data in Garmin GPS units to be downloaded and exported to Shapefiles is "DNRGarmin" developed by the Minnesota Department of Natural Resources. However, we will use text file methods because the DNR Garmin utility is not installed in STC labs.

There are a number of methods to import XY data in a text file into a shapefile. One method is to enter data into an Excel spreadsheet, add the spreadsheet to your ArcMap session (using the Add Data utility) and then use the Add XY Data (under Tools menu) to convert the spreadsheet to a spatial dataset. The format of the Excel spreadsheet is one row of header information with an ID column (i.e. "ID") and one column each with "X" and "Y" labels (no quotes). Then one row of data for each of your GPS points. Then you will need to save this dataset as a Shapefile (right click on the dataset in Table of Contents and select Export) and set the Spatial Reference (Define tool under Projections and Transformations under Data Management).

Alternatively, here are documents that describe several other methods that you can use. They are included here to emphasize that with GIS software there are usually multiple methods of accomplishing a particular objective.

In ArcGIS, how do I create a shapefile from GPS coordinates in a text file?

In ArcGIS, how do I create a shapefile from GPS coordinates in a table?

In ArcGIS, how do I add individual GPS points to an existing shapefile?

So for this lab, write down the XY coordinate of each GPS point you collect and use one of the methods above to import into a shapefile or geodatabase. If you wish to try to DNR Garmin utility you are welcome to download it and try it out, but you are not required to do so (make arrangements with us to borrow the GPS unit). Record all GPS coordinates using point averaging, ~60 seconds/points for each position. Use UTM, Zone 16N, WGS 84 for all of the below.

1. (5 points) Collect three coordinates for the NGS survey marker. List them.

2. (5 points) a) What is the x and y range among the three points you captured? The range is simply the difference from the maximum and minimum values for both the X and Y coordinates.

3. (10 points) a) List the NGS stated location of the NGS survey marker at the north end of Dunn woods. b) What is the distance from each of your three points to the NGS recorded location? The distance can simply be calculated as the hypotenuse of a right triangle.

4) (20 points) Describe the sources of error that explain why your GPS coordinates for the NGS survey marker differ from the NGS recorded survey location.

5) (10 points) Why do you not acquire the exact same GPS coordinate each time you recorded the three positions you acquire for the survey marker? You can refer to content you provided in #4 if you wish.

6) (15 points) From the aerial photography, create a feature class composing the area of Dunn Woods. a) Calculate the area of this polygon. Create a second feature class from the GPS positions you recorded that bound Dunn Woods. b) List the area of this polygon. c) List reasons explaining why the value for A differs from the value of B.

7) (15 points) Make a map with a) Your three GPS generated locations for the NGS survey marker, b) the NGS stated position of the survey marker (in a different symbol), c) the sidewalk locations, and d) the air photo in background. Make sure all elements a, b and c are clearly visible.

8) (20 points) Make a new ArcMap session with just the TreePlot dataset and your GPS points for the tree locations on the North side of 7th street. You will use these points to find the locations within the tree plot area that are farthest from existing trees. To do this you will create a raster distance surface that calculates the distance from each cell to the closest three location. The spatial extent of this distance surface will be contained to the area bounded by the tree plot. The general steps are as follows:

1) Create point dataset from the GPS points you collect for the individual trees. Adjust locations of points if they fall outside the TreePlot area when you overlay with the TreePlot dataset.

2) Open Spatial Analyst Toolbar, set Analysis Mask to the TreePlot dataset, set the analysis extent to that of Tree Plot, set cell size to .5 meters

3) Create a Straight Line distance dataset to find the location that is farthest from existing trees

For question 8, make a map of the tree plot area with the distance surface and tree locations noted. In your lab document record what the maximum distance is within the tree plot area, which identifies the location farthest from existing trees.