Problem: The Calculate Geometry tool produces incorrect results for basemaps
When using the Calculate Geometry tool to perform calculations on basemaps, inaccurate results are returned.
Online basemaps use the 'Web Mercator' projection, but the 'Mercator' projection is used for navigation. A straight line drawn on the map is the bearing to fly a plane or sail a boat from the starting point to the destination, however this severely distorts distance and area.
Solution or Workaround
The following are possible solutions to this issue.
- Assign a more suitable projection to the ArcMap data frame.
Please refer to the following Knowledge Base technical article, Select a suitable map projection or coordinate system. The article is linked to the Projections Table, which lists all supported map projections in ArcGIS for Desktop and their properties.
- Apply the selected projection to the ArcMap data frame.
1. Go to View > Data Frame Properties > Coordinate System Tab > Projected Coordinate Systems folder.
2. Locate the desired projection which produces the most accurate results.
3. Click Apply and OK.
- Get the accurate calculation output.
1. In the attribute table, go to Table Options, and select Add Field. Create as a Double field, and assign a name with no spaces and no more than 10 characters in length. For example, a name like Area_sqkm would be suitable to store areas in square kilometers.
2. Right-click the field header, and select Calculate Geometry.
3. In the Calculate Geometry dialog box, select the option ‘Use coordinate system of the data frame’, and select the desired units as the requirement.
- ArcGIS Help: Calculating area, length, and other geometric properties
- FAQ: Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator?
- HowTo: Select a suitable map projection or coordinate system