Spatial Error Related to Datums and Projections

Skill Drill: Creating Indicatrices Using the Buffer Tool

Table of Contents

  1. Spatial Error Related to Datums and Projections
  2. Skill Drill: Setting Up Your Workspace
  3. Skill Drill: Downloading Data from Natural Earth
  4. Skill Drill: Connect to Your Workspace Folder in ArcMap
  5. Creating a File Geodatabase
  6. Creating Feature Classes from Shapefiles
  7. Adding XY Data using the ArcCatalog Window
  8. Skill Drill: Creating Indicatrices Using the Buffer Tool
  9. Evaluate Distortion Patterns in Map Projections
  10. Measuring Scale Distortion
  11. Skill Drill: Evaluate and Measure Distortion
  12. Troubleshooting Datum Shift
  13. Repairing Corrupted Data Using the Define Projection Tool
  14. Skill Drill: Repairing Incorrect Coordinate System Definitions

In 1859, the French mathematician Nicolas Auguste Tissot introduced a method to visualize the distortions of map projections using regularly spaced circles. On the globe, each circle was precisely the same size. When one transforms the globe into a flat plane using mathematical equations, the size and shape of the circles reflect the distortion patterns of the map projection. In this instance, you use centroids feature class and the buffer tool to create a similar indicatrix.

ArcMap represents geographic data visually using the data frame, the center of the three primary windows of the ArcMap user interface (Figure 2.27). As you learned previously, the data frame uses a specific display projection, which is defined in the Data Frame Properties Coordinate System tab.

Figure 2.27: The pink rectangle delineates the data frame window in ArcMap. Double-click or tap twice to view the image in a larger size.

Begin by changing the data frame display projection so that Earth looks closer to a globe. On the data frame properties window, navigate to the Coordinate System tab. In the Projected Coordinate System folder, locate the World folder. Choose The World from Space. Double-click on it to open the Project Coordinate System Properties. Change the longitude of center to 180 (Figure 2.28). When ready, click OK, then click OK again. If a warning message appears, click Yes to close it. The map should appear closer in size and shape as the globe.

Figure 2.28: Setting the Longitude of Center to 180 centers the map on the pacific ocean. Double-click or tap twice to view the image in a larger size.

Earlier, you learned how to use the buffer tool. In ArcMap, create a buffer around the Tissot’s Indicatrices centroids. Set the buffer distance to five hundred kilometers. Save the output to your geodatabase and call the feature class indicatrix. Under Method, choose GEODESIC (Figure 2.29). When using the geodesic method, the ArcGIS software measures distance based on the elliptical model of Earth’s shape. As a result, the buffer distances remain accurate in all map projections.

Leave all other default settings and click OK.

Figure 2.29: Check to make sure that your settings match the image above. Double-click or tap twice to view the image in a larger size.

Your buffers should appear as regularly spaced circles around the globe. Take a moment to remove the centroid layer from the Table of Contents while leaving the indicatrix layer. Using the skills learned previously, rearrange the map layers and change the colors of the basemap so that the indicatrix layer is easily visible (Figure 2.30).

Figure 2.30: In this example, the graticule and countries layer were changed to a light grey. The cities were changed to grey stars. The graticule color is dark amethyst with a transparency of 50%. For clarity, the graticule layer was moved to the bottom of the drawing order. Double-click or tap twice to view the image in a larger size.