[GIS] Min and max values not correctly calculated for DEM using QGIS

demqgisstatistics

I'm working with DEM in QGIS and need to know the min and max values of my data.
In the Properties window, under the Metadata tab, the statistics section shows such values. However, I usually find some pixels with values higher than the max and lower than the min.

Is there another way to obtain these values?

I've tried this with several DEMs.

Best Answer

By default, QGIS does not calculate the full range for a raster. The range it calculates by default is 2% - 98%, see the 'Cumulative count cut' option in the 'Load min/max values' section of the dialog box. This is just the first option, which is why (I suspect) it is the default.

To get the full range, choose the option below it marked 'Min/max', then select 'Full' for Extent (default) and 'Actual (slower)' for Accuracy (the default is 'Estimate (faster)'). With these options all selected, click the 'Load' button at the bottom of this section of the dialog box. This loads the statistics and you will see them appear in the min max boxes in the appropriate part of the dialog box (depending on whether you are generating a single-band grey or pseudocolor palette).

Related Solutions

[GIS] How to sum up pixel values in QGIS

Create a polygon using Vector > Research Tools >Polygon from layer extent. here layer is the raster in question.
Activate/ Install Zonal statistics Plugin.
Use Zonal statistics plugin where the raster is the one whose sum is required and the polygon is the one you created from layer extent. Enter output column prefix (say stats_). Run the plugin.
Open the attribute table of the polygon. Now you have stats_sum column along with other columns with stats_ prefix.
I checked the values in GRASS GIS and the match is perfect.

[GIS] Understanding min and max values with Mosaic to New Raster

As whuber mentioned, often statistics found in the raster properties are sometimes approximate or are out-of-date. They are predetermined properties that can be misleading to the actual raster values.

Calculated your own min / max values from 100% of the actual data using NumPy arrays. See Working with NumPy in ArcGIS, and RasterToNumPyArray (arcpy). E.g.:

import arcpy

inrast = r'C:\data\inRaster.tif'
my_array = arcpy.RasterToNumPyArray(inrast)
print((my_array.min(), my_array.max()))

If you have missing values (NODATA) then a masked array is needed to get the correct stats:

import numpy as np
my_array = arcpy.RasterToNumPyArray(inrast)
my_masked_array = np.ma.masked_equal(my_array, arcpy.Raster(inrast).noDataValue)
print((my_masked_array.min(), my_masked_array.max()))

Also, you don't need ArcGIS to read rasters as NumPy arrays; e.g. GDAL or rasterio can do similar.

Best Answer

Related Solutions

[GIS] How to sum up pixel values in QGIS

[GIS] Understanding min and max values with Mosaic to New Raster

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