FAQ: Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator?


Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator?


WGS 1984 Web Mercator and WGS 1984 Web Mercator (Auxiliary Sphere) use a conformal projection that preserves direction and the shape of data but distorts distance and area. Published in 1569 by Gerardus Mercator, the Mercator projection was created for use in navigation. A straight line drawn on a map in this projection provides a bearing by which one could fly a plane or sail a ship between two points.

Examine the illustrations below to view various map projections and their different properties. The maps display the continents plus a 15x15 degree graticule that covers the entire surface of the earth.

1) WGS 1984: The 15x15 degree graticule cells appear as squares. The map uses a modified Plate Carree projection to display the latitude and longitude values. Note that distances are very distorted in the east-west direction, north or south of the Equator. In this coordinate system, the North and South Poles appear as lines as long as the Equator. One degree at the Equator measures about 69.2 miles, or 111 kilometers on the ground, but a degree has a length of 0 at the North Pole and South Pole because the poles are points.
[O-Image] GCS_WGS_1984
2) WGS 1984 Behrmann (World): This is an equal area projection. Note that the 15x15 degree graticule cells are now compressed to show the smaller areas north or south of the Equator. Distortion in the east-west direction is the same as that in GCS_WGS_1984 in #1 above.
[O-Image] WGS_1984_Behrmann_World
3) WGS 1984 Aitoff: This compromise projection does a fairly good job of preserving both shape and area for the world. Notice that the poles in this projection are displayed as points rather than lines.
[O-Image] WGS 1984 Aitoff
4) WGS 1984 Azimuthal Equidistant: In this projection, distances are correct when measured from the center of the projection. This is also a true direction projection.
[O-Image] WGS 1984 Azimuthal Equidistant
5) WGS 1984 Web Mercator: This projection distorts data in the east-west direction, as do WGS 1984 and WGS 1984 Behrmann, but the worst distortion is in the north-south direction. Examine this image carefully, and note that in this projection Antarctica appears larger than the land masses of the other six continents combined.
[O-Image] WGS 1984 Web Mercator
For further information about map projections supported in ArcGIS Desktop, the properties of each projection, and the geographic areas for which each projection should be used, refer to the Projections Table linked to Knowledge Base article 24646, "Select a suitable map projection or coordinate system" at the link in the Related Information section below.

Related Information