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I have been working with some rasters and would like to know how to calculate the area occupied by one element of a raster in Qgis.

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What do you mean by "element"? A cell? An area of same value? –  underdark Feb 19 '13 at 20:31
    
area with the same value :) –  Nadia Feb 19 '13 at 20:39

2 Answers 2

up vote 2 down vote accepted

with 3 steps you can achieve what you want:

  1. Vectorize raster based on the values you want to calculate
  2. Open your new vector, and create a new column. Name it 'Area'.
  3. Use the field calculator to populate the column with the variable $area
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I'm assuming you just need to know the area occupied by each value. You can get this value by getting the statistics per value for the raster. Specifically, you want the total number of cells with a particular value. Once you have those totals, you can just multiply them with the cell size of your raster to get the area.

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Remembering that cell size is the square of the cell width...a common mistake since most GIS packages report cell width rather than cell area. –  Dan Patterson Apr 9 '13 at 13:49

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