Creating a Digital Elevation Model from GPS Data Using Interpolation Methods

Table of Contents

  1. Creating a Digital Elevation Model from GPS Data Using Interpolation Methods
  2. Setting up Your Workspace
  3. Preparing the Data
  4. Skill Drill: Downloading data using the Minnesota Department of Natural Resources GPS Application (DNRGPS)
  5. Creating a Surface Model Using the IDW Interpolation Method and the Geostatistical Wizard
  6. Creating a Surface Model Using the Spline Interpolation Method
  7. Creating a Surface Model Using the Kriging Interpolation Method
  8. Creating a Digital Elevation Model from a Geostatistical Layer
  9. Skill Drill: Creating a Map of the Results

Creating a Surface Model Using the IDW Interpolation Method and the Geostatistical Wizard

You will start this interpolation exercise with the Inverse Distance Weighted (IDW) method. Inverse Distance Weighting (IDW) is a deterministic interpolation method that uses known values within a user-defined search neighborhood to predict the value at a specific location. The known values are weighted by distance, so the local influence diminishes as it reaches the edges of the search neighborhood. One of the assumptions of IDW interpolation is that local variation is one of the characteristics that define the dataset. In the best case scenario for this method, sample points are evenly distributed throughout the area and are not clustered.   The essential parameters for IDW are the power and the search neighborhood settings. IDW is available as a stand-alone tool. However, deterministic methods like IDW and Spline do not provide a standardized measure of uncertainty. This lack can pose a problem when you need to defend the methods used in your model. The Geostatistical Analyst extension within the ArcGIS software can provide some measure of uncertainty, even among deterministic methods through the use of cross-validation. From the Geostatistical Analyst toolbar, select Geostatistical Wizard from the drop-down menu.  Under the Methods section, choose Inverse Distance Weighting, which is located under Deterministic Methods.

An image of the Geostatistical Wizard Option on the Geostatistical Toolbar

For the Source Dataset option, select the shapefile you have created.  In this example, I have called it elevation_data.shp. For the Data Field option, select altitude. Leave the Weight Field blank and click Next.

By default, the Garmin GPSMap64st receiver uses this name altitude for the elevation field. if you are using a different GPS receiver the elevation field name might be different.

An image of the Geostatistical Wizard IDW step 1

Maximize the Geostatistical Wizard window to view the results. You should now see a colored preview of your surface model along with a few model settings on the right. Take a moment to explore the data. Expand the list of weights to the right near the bottom of the General Properties pane. Then, click around on different places on the map. Notice how the values change and how the points nearby are symbolized to map. The colored points indicate which known data points are used to calculate the value of the pixel under the crosshair. There are many settings available on this window. At this time, you will use the default IDW settings. Once you are done exploring the data, click Next.

An image of the Geostatistical Wizard IDW step 2
This image shows the method properties window. Your results will appear different than this image. Click to view in a larger size.

On step 3 of the Geostatistical Wizard for IDW, a cross-validation is performed. A list of error values appears on the left and a scatter plot on the right. Below the scatter plot, you will find statistics for prediction errors.

An image of the Geostatistical Wizard IDW step 3
This image shows the results of the cross-validation using the Geostatistical Wizard. Your results will appear different than those found here. Click to view a larger size.

Open a blank Excel workbook and create a table recording the results from the default IDW surface model. Take note of the Root Mean Square Error and add it to the excel table. An example table is shown below.

 Default IDW
Power 2
Neighborhood Type Standard
Maximum Neighbors 15
Minimum Neighbors 10
Sector Type 1 Sector
Angle 0
Major semiaxis 76.54706
Minor Semi Axis 76.54706
Root Mean Square Error (RMSE) 2.624321

Your values may be different than the example above.

Click the Back button to return to the previous step in the Geostatistical Wizard. Record the remaining default IDW settings on your table including neighborhood type, maximum neighbors, minimum neighbors, sector type, angle, major semiaxis, and minor semiaxis. Now you will attempt to optimize the IDW interpolation. On step 2 in the Geostatistical Wizard, spend about five minutes experimenting with the IDW settings mentioned above to try to improve the RMSE value. A good place to start is to click the Optimize Power Value button to the right of the Power setting. While you are experimenting, occasionally return to step 3 in the Geostatistical Wizard to check on the RMSE value.

An image of the Geostatistical Wizard IDW step 2 optimized
In this example, I have optimized the settings by changing the power, the maximum neighbors, the minimum neighbors, the sector type, the major semiaxis, and the minor semiaxis. I kept the angle at 0 because I knew the values in this dataset tended to be similar in a north-south direction. You may need to use different settings to optimize your dataset. Click to view a larger image.

Once you are satisfied, record the results in your Excel table under a new column with the header Optimized IDW. See the example below.

 Default IDWOptimized IDW
Power 21.239543
Neighborhood Type StandardStandard
Maximum Neighbors 1510
Minimum Neighbors 105
Sector Type 1 Sector4 Sectors with 45 offset
Angle 00
Major semiaxis 76.54706100
Minor Semi Axis 76.5470650
Root Mean Square Error (RMSE) 2.624321?

Your values may be different than the example above.

Save your excel file in your final folder for later use. Once you have recorded the optimized RMSE value, return to ArcMap. In the Geostatistical Wizard, click Finish and then click OK.

An image of the IDW Prediction Map Results in ArcMap
The results of the Geostatistical Wizard are only temporary and will not exist outside of the map document.

A temporary geostatistical layer should now be added to your Table of Contents. This layer is a temporary representation of your surface model.