I have to calculate area from a georeferenced tiff image. I want to find the whole area in the raster. The two methods I tried was : 1.Using the area measurement tool in QGIS toolbar. 2.Vectorized the raster . Extracted the number of pixels. Calculated pixel resolution in QGIS. Multiplied the number of pixels with area of one pixel.

The area measured in the 2 methods gave two different values. Could you tell me any another method other than Calculating area of rasters in QGIS?

  • 1
    Some more information would be helpful, e.g. what data do you have, what areas do you want to find, how large is your area of interest, how often do you have to repeat this procedure? – Erik Mar 26 '18 at 10:54

A different approach that you can explore is the command r.report from GRASS in the processing toolbox.

The GRASS man page is here: https://grass.osgeo.org/grass74/manuals/r.report.html

And these are the screenshots from QGIS 2.18.14. In the picture below the raster has only one category value (1) and the result is in km^2 (k).

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  • HI, This somehow gives me a different value from which I calculated in QGIS(using measurement tool and polygon digitization). Why could that be ? – Deepthi Apr 5 '18 at 12:23
  • How different? The raster math in GRASS depends of the resolution of the current geographic region, while a vectorization might be different. You do not say anything about the projection and resolution of your tiff file nor about the input to the r.report command. If the difference is signifiative, it might be worth a new question – Marco Apr 5 '18 at 19:28
  • The resolution of the tiff image[which is an orthophoto] that i used for calculation was 5cm. No input info of projection or resolution was given in the command. The area from polygon and r.report has significant variation. – Deepthi Apr 13 '18 at 6:25

Some more details, especially on your data and the goal of your research, would still be appreciated, in order to be more specific. Anyway, there are two ways to acquire the desired data:

  1. Load the geotiff into QGis, add a vector layer and manually digitize all the areas you need. Then make sure your vector layer is using the correct UTM-CRS for your area, then open the field calculator and add an integer-field with the expression $area. This gives you the area in square-meter. Works for both drawings and remote sensing data.
  2. Classify your image. Though more work-heavy, this should give you a more precise outcome and - if intended - allows you to build a timeline for an area, using the same training areas again and again. Works only with remote sensed data.

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