How to sum up pixel values in QGIS?

Question

I need to know the value of the sum of all pixels in a raster. However, in the Statistics section of the Metadata tab in Properties, under Sum of All Cells it always indicates 0.00. I have tried with different raster formats to no avail. What am I doing wrong? How can I get this value?

Answer 1

I hope I got you right; I tried to reconstruct your problem with an Raster of mine. The sum of all pixels in a raster is the x-pixels*y-pixels, right? This value you can get in 2 ways: 1)Right click on the actual File(a tiff in my case), the detail window should provide this information 2)Inside the Gis click Raster->Miscellaneous->Information. Select your file->edit->ok. Then you should the the two values, that have to be multiplied. The result is the sum of all pixels in a raster.

Answer 2

I understood the question as, How do I get sum of all pixel values of a raster? Something like running Statistics on the attributes of a vector. One can do this easily via programming. I can think of another way to do this via QGIS. It would be inefficient and slightly inaccurate, but it will work.

• Create a point layer representing each pixel of the raster. You can do this via Vector -> Research Tools -> Regular Points. Use number of points as x_size * y_size. So for a raster of 100 px width x 100px height, use 10000. Specify an offset of (pixel resolution)/2 , so you are sampling a the center of the pixel.
• Next, install the Point Sampling plugin. Using this plugin, you can sample the raster at all of the 10000 points and get an attribute value which is the pixel value.
• Run Vector -> Analysis -> Basic Statistics on the resulting vector to get the sum.

Answer 3

You could use gdalinfo to get the raster statistics, for example:

gdalinfo -stats filename.tif

This will give you the mean pixel value and the raster size in pixels. You could also get this data from the Properties -> Metadata tab.

Then it's a question of arithmetic: mean value x (height in pixels x width in pixels).

Nick.