Spatial Error Related to Datums and Projections

Skill Drill: Repairing Incorrect Coordinate System Definitions

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

Though it may have a similar name, the Define Projection tool and the Project tool work very differently. The Project tool does not alter the original input data. Instead, it makes a copy of the data. Most of the geoprocessing affects the geometry stored in the new .shp file. The Define Projection tool does not create an output dataset. It also does not alter the geometry stored in the .shp file. The Define Projection tool modifies the original input data by overwriting the .prj file. If the .prj file is missing, it creates a new one. You should only use the Define Projection tool when you have a dataset that has an unknown spatial reference or an incorrect coordinate system defined.

Confusing these two tools is a common mistake that many readers often make. Mixing them up will corrupt your data. For example, suppose you wanted your populated places shapefile to use the same spatial reference as the streets and fire hydrant layers. Your intentions are good because you know that having layers with different spatial reference properties can introduce spatial errors, such as a datum shift. In this instance, you want all of your layers to be in the State Plane Coordinate (SPC) system used by the streets and fire hydrants. The correct procedure would be to create a new shapefile with the desired spatial reference properties using the Project tool. However, like many others before, you choose to use the Define Projection tool instead.

To demonstrate this problem, run the Define Projection tool on the populated places shapefile that you corrected earlier. This time, use the other spatial reference listed in the Layers folder that starts with NAD 1927 State Plane. You may notice a warning icon in the upper right (Figure 2.69). Click the yellow exclamation to read the warning. Then, close the warning and click OK to run the tool.

Figure 2.69: ArcMap tries to warn you when a dataset already has a spatial reference defined.

In your Microsoft Word document, write down the answer to the following questions:

  • What happened to the location of the ne_110_populated_places data layer on the map?
  • Why does the data not align with the layer from the geodatabase feature class?
  • What kind of spatial error is this?

Open the .prj file with Notepad and view the contents (Figure 2.70). As you can see, the textual information stored within has changed. The information now references a different datum, NAD 1927. It also includes much more information because the State Plane Coordinate (SPC) system contains more complexity than a geographic coordinate system.

Figure 2.70: PRJ files contain only text describing the spatial reference information. Double-click or tap twice to view the image in a larger size.

ArcMap reads the text in this file and assumes that the geographic coordinates stored in the .shp file come from the North American Datum of 1927 (NAD 1927). Recall that the Define Projection tool does not alter the .shp file. The geographic coordinates come from a different datum, WGS 1984. The result is a shapefile with an incorrectly defined spatial reference. To correct this error, rerun the Define Projection tool. Overwrite the incorrect .prj file with one that uses GCS WGS 1984 as the coordinate system (Figure 2.67).

When done, you can save your map document and close ArcMap. Take a moment to save your Microsoft Word Document and your Excel workbook. Be prepared to discuss your results at a later time. Back up your project folder to a safe location, such as Google Drive.