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At what scales can the Web Mercator coordinate system be used to measure area?

October 05 2009 | 0 comments
Categories: Map Data

Hello cartographers!

I have a search and rescue application that requires me to have accurate area measurements for search area segmentation.

I want to use the ESRI World Topographic Map as a my basemap. To avoid projection on the fly, my data should be in the same coordinate system as the basemap, but I know Web Mercator doesn't maintain area.

I'm wondering if there are (large) scales at which the Web Mercator coordinate system can be used to accurately measure area? Is it possible to map these areas?

Any thoughts?


Mapping Center Answer:

A rule of thumb is 1:250,000 -- of course that depends on the projection and the location.  For example, aq Mercator projection near the high latitudes is grossly exaggerated, so even at 60 degrees north, I wouldn't use it for any measurements.  In the mid-latitudes, it would be better, and there woudl be no scale distortion along the Equator.  To be sae, you might set the cut off for mid-latituides a little lower, say 1:100,000.

In fact, this distinction between when a map can be accurately used for measurement and when it cannot is what we use as the dividing point between large and small (or medium) scale maps in Map Use, as dicussed in Chapter 2: Map Scale.

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