I am trying to convert a polygon to raster and I noticed that the cell size of the resulting raster appears different for different coordinate systems, despite me setting the cell size the same. Both coordinate systems are in meters. Here's what I mean:

To begin, I set the polygon to raster cell size as 28 meters.

I run the tool with projected coordinate system of "WGS 1984 Web Mercator (auxiliary sphere)" and the resulting cell size is about 20.8 meters:


What's even weirder is that when I check the properties of that layer, it still says that the cell size is 28 and that the units are meters:


Now if I run the polygon to raster tool with the projected coordinate system of "NAD 1983 UTM Zone 16N", the cell size is as I expect, 28 meters.


Can anyone explain why the resulting cell sizes are different?


1 Answer 1


The Web Mercator "projection" (EPSG 3857 WGS 84 / Pseudo-Mercator) is not useful for measuring distance or area. From an Esri article about it:

Why is the Google/Bing/ArcGIS Online projection not acceptable for measurements?

Every map projection causes distortion of shapes, areas, directions, and/or distances. Some projections such as Robinson or Winkel Tripel attempt to minimize distortion across the world through some compromise of all those factors. Other projections (such as UTM and State Plane) are designed for focused areas of the globe in order to keep the distortion minimal.

The modified Mercator projection used by Google, Bing, and ArcGIS Online is not designed to minimize distortion at all. Instead, it was engineered for convenience in working with cached map tiles. This projection fit the entire globe (well, most of the latitudes anyway) into a square area that could be covered by 256 x 256 pixel tiles. The projection sacrifices some accuracy because it is based on a perfect sphere (the earth is better approximated by a spheroid), but the biggest problem is the heavy vertical and horizontal stretching at extreme latitudes.

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