I have a satellite image from Landsat TM. I want to clip my area so i created a polygon and thru Qgis --> raster clipper I am trying to clip my area of interest. I noticed that the results have different values. e.g My first image has minimum 0, maximum 179 and mean 36.966. after clipping the result has values minimum 6, maximum 112 and mean 51.55. Is there something obvious that I can not see? Did the values change because the size of image is different after clipping?
When clipping the image it is likely that you removed the edge of the image. The edge of Landsat TM imagery is assigned 0 in all bands. This will result in 0 no longer being the minimum and a significant increase in the mean value across the raster. Furthermore, I would assume that you have also clipped the image to no longer include clouds, which would change the maximum value dramatically.
It would be worth checking if specific pixels have maintained their values by comparing the original with the clipped using the Identify tool, or a similar approach.
Small changes in pixel values is possible, due to potential resampling, but that depends on the exact method of clipping and resampling.