5

since I am abandoning windows to stay only with Linux, I started learning how to use QGIS. So far so good. However, trying to reproduce a python script from Arcpy to PyQGIS I noticed something. When I import the exactly same image in both software applications (ArcMap and QGIS), the min and max are totally different (see below). And that happens to every single image.

see image attached.

Did that happen to anyone already?

This may be a dumb question, but I have no clue of what is going on.

|improve this question
14

If your image comes without pre-computed statistics QGIS and ArcGIS will produce a quick estimate of what the ideal min/max for displaying the image is. This does not change the values you are seeing, just the color range. You can easily test this by comparing the pixel values.

If you want to see the computed band statistics you can right click on the layer in QGIS and select Properties --> Style --> Metadata. You will see that ArcGIS and QGIS have access to the same values, they just chose to display them differently.

As standard QGIS uses 2% to 98% of the image values instead of the absolute min/max. You can change this by right clicking on the raster layer and selecting Properties --> Load min/max values to change the cutoff or select the real min/max.

enter image description here

If you want to change the behaviour permanently you need to go to Settings --> Options --> Rendering --> Limits (minimum/maximum)

enter image description here

|improve this answer
  • 1
    I must add this works in most cases but not if QGIS hits the annoying bug - precision issue with no data value which is used in some raster formats like -1e+32 (as -1,000000003318..+32). In that case it is necessary to set additional (same but precise) no data manually to proper value and refresh/reload the values. – Miro Aug 17 '15 at 11:29
  • 1
    Another problem is, that the default "Accuracy - Estimate (faster)" option in the "Layer Properties" cannot be set to "Actual (slower)" per default in the Settings. Only this would then enable the display of the true min/max values of the raster per default. – openwater Nov 9 '17 at 17:54
  • 1
    @openwater you are right, and there's also a bug report for this issue. – Kersten Nov 10 '17 at 8:17
  • Thanks Kersten, yeah...I know...that's my report... This issue bugs me for a long time already. – openwater Nov 11 '17 at 9:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.