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I have generated a raster as a *.tiff file in R, with four classes. I'd like to calculate the area occupied by each raster class. Each class has a value (-5, -4, -3, -2) - I expect that I could also count the number of pixels in each raster class and multiply it with the cell size (0.23, I believe).

How can I calculate the area per class?

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    Is your raster projected or lat-long? Because if lat-long then your cells will be different areas... – Spacedman Feb 23 '17 at 15:03
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Even if your raster is lat-long and therefore varying cell area, you can do this in R by adding the areas that correspond to cells with your values using the base tapply function:

First set up some dummy data:

> r = raster()
> r[] = sample(-5:-2, ncell(r), replace=TRUE)

Then do this:

> tapply(area(r), r[], sum)
       -5        -4        -3        -2 
126376977 126301228 125943687 129736879 

area(r) creates a raster where each cell has its area as its value. tapply then sums this over areas grouped by the value in r.

  • This is great, but in what units are the results expressed? square meters? – Jackk Mar 11 at 10:45
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    For projected coordinate systems its the square units of the projection - square metres or square feet or whatever. For unprojected coordinate systems (eg lat-long, where the cell surface area varies) its square metres as computed by the raster::area function. – Spacedman Mar 11 at 11:52
  • Great, so I guess the raster::area function yields an approximation of metric area based on lat-lon. Do you reccomend reprojecting the raster to a metric CRS for greater accuracy? – Jackk Mar 11 at 13:18
  • That's a potential new question - depends on a lot of things. – Spacedman Mar 11 at 14:31
  • There is an error message when I use it on a UTM raster: "This function is only useful for Raster* objects with a longitude/latitude coordinates". Values differ strongly from the answer @Christopher gave. – Jens Mar 13 at 16:12
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Here's another option that should work. Replace -5 with the value for each class. It determines the number of pixels in each class and then multiplies that by the area of each pixel (i.e., the resolution squared).

vals <- getValues(myraster) length(subset(vals, vals == -5)) * res(myraster)^2

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