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I need to get an area of different polygons at continental scale (like third of Africa etc.). Usual functions like rgeos::gArea work only in the projection units.

  1. Is there any function which works correctly in real square meters on the geoid surface? I.e. real, geodetic area?
  2. If not, what projection should I use to get the least distortion?

I need to measure 6 really huge areas (like third of Africa), so UTM or national projections are out of question.

Can I use a single one projection that has minimal distortion area-wise across the whole Africa?

Right now I am thinking of Mercator projection used by google maps (EPSG:3857), but I am not really sure.

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Geodetic area calculations are not part of GDAL. You would probably need to write your own function to do it and exact solutions are computationally intensive.

You're better off using an Equal-Area projection, such as Albers, Azimuthal, or Lambert azimuthal (https://en.wikipedia.org/wiki/Category:Equal-area_projections). For largish regions in Africa, you're best bet is probably the lambert azimuthal equal area (Check with table partway down page: http://www.geo.hunter.cuny.edu/~jochen/gtech201/lectures/lec6concepts/map%20coordinate%20systems/how%20to%20choose%20a%20projection.htm)

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  • Thanks Jezibelle! Does the "Lambert azimuthal equal area" projection have an EPSG you can post here? I found this: nsidc.org/data/atlas/epsg_3408.html but Africa here seems distorted a lot so that's probably not what you meant..
    – Tomas
    Aug 11, 2015 at 16:53
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    I don't think that's it - should be 9820. Also, equal area projections preserve area, not shape, some distortion from a Mercator is expected.
    – Jezibelle
    Aug 11, 2015 at 16:57
  • Thanks! Unfortunately EPSG:9820 is not working in rgdal nor do I find any normal formula of it that could be used. So I got to search for "equal area" in qgis dialog and found EPSG:102022 (Africa Albers Equal Area Conic) which seems to do the job.
    – Tomas
    Aug 12, 2015 at 8:25

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