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I have a large raster file of total mangrove cover for the whole globe (obtained from NASA: http://sedac.ciesin.columbia.edu/data/set/lulc-global-mangrove-forests-distribution-2000/metadata). I'm trying to calculate the area of mangrove forest that fall within protected areas (that are defined by polygon layer).

I can calculate the number of pixels that fall within the polygon, however, as the area of the pixels changes at differing latitudes, how can I reliably convert these pixel counts into area?

I'm aware that flattening a sphere causes some sort of distortion, so I tried Define Projection tool to change to World Cylindrical Equal Area (from WGS84), but that just shrinks the whole raster down to an unusable scale.

If I set the coordinate system of the layer to World Cylindrical Equal Area before importing the raster, it preserves the scale, but after measuring the pixels with a ruler, I can see they change at different latitudes.

I've included some screenshots for clarification.

The first image is the global mangrove forest cover (dark green), the second and third images are the >Layer Properties >Source window of the mangrove.tiff raster file (unaltered). And the last image is a measurement of 1 pixel close to the equator.

mangrove forest (dark green) layer properties 1 layer properties 2 pixel

  • Did you try reprojecting your Mangrove raster? You can't use "Define Projection", as the data are not in the projection you're trying to assign to them (this is why it shrinks). I don't use ArcGIS but I'm fairly certain there is a "Reproject" tool. – Jon Feb 9 '18 at 0:06
  • @Jon there is a Project Raster tool that I think does that. This is the arcGIS summary "Transforms a raster into a new projection. This creates a new raster. To apply the transformation without creating a new file, use the Warp tool.". But I've tried that, and it gives me an error unless I use the Project Raster tool first. But that shrinks the whole map to a smaller scale. :/ – Samual Chance Feb 9 '18 at 0:31
  • What projection did you set as the output for the Project Raster tool? – Dan Feb 9 '18 at 4:29
  • Are you sure when you use Project Raster it's creating a shrunk version? It could be that your display map is still in unprojected WGS84 and Arc isn't projecting "on the fly." After you create a raster with Project Raster, close ArcGIS, reopen it, and load your reprojected file--this sets the display map's coordinates to those of your projected raster. If it's still shrunk, there is another problem. It would be helpful if you could post screenshots of your projected raster properties. – Jon Feb 9 '18 at 17:24
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There is no single projected coordinate system that will do well for measuring areas in all parts of the globe.

Consequently, I think you should work out which parts of the globe have mangroves (which you have already done) and look for a projected coordinate system in each of those parts which will give area measurements that are accurate enough for your purposes in that part of the globe.

You would then clip and project separate parts of your raster using a suitable projected coordinate system for each.

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