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I have a question concerning area calculations in South-East Asia for West-East range that goes from India to Eastern Malaysia and a North-South range from Northern India to Indonesia (xMin,yMin 74.2203,-5.89636 : xMax,yMax 118.927,30.1183). I found two projections that might be suitable for this geographical region:

and

However, when I compare the results of both projections, I get large differences in total area (I used QGIS field calculator for area calculation):

  • 620,589.41 km^2 for Albers Equal

and

  • 689,941.23 km^2 for Pseudo Mercator

That is a difference of ard. 11%

Can anyone help me to understand the differences of these two projections and to know which one is more appropriate for my purpose?

Thanks so much in advance

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1 Answer 1

up vote 2 down vote accepted

Pseudo Mercator does not have real metres as units. At the aequator it fits to metres but the more to the poles you come, the greater the distorsions are. The measurement tool takes care of that, but the field calculator does not.

So you have to choose another CRS. Most UTM CRS are restricted to their defined area, and might produce wrong results outside.

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Ok, I just gave it another try with a Lambert Equal Area projection and it gives me almost the same results for total area as the Albers Equal Area (0.0005% difference). So I assume the differences for the Pseudo Mercator come from the distortions that you mentioned and it should not be used for area calculations (at least in my case) –  stefan Jan 28 '13 at 7:51
    
Or in any case, I'd suggest. EPSG:3857 is handy because it allows shapes to be viewed using the Open Layers plug-in, for example, but it's not a CRS that should be used for QGIS projects. In my view, of course. N. –  nhopton Jan 28 '13 at 8:45
    
I guess that "Equal Area" is a good choice in your case ;-) But I have not yet made any comparisons with other "real" projections. –  AndreJ Jan 28 '13 at 9:32

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