[GIS] Finding Median, Max and Min values from Raster Layers in QGIS

qgisrasterspatial statisticsstatistics

I currently have 10 years worth of January raster datasets (working on just one month at the moment) of cloud fraction cover from MODIS, and am trying to create new 3 separate rasters which will give me the Mean, Maximum and Minimum values for all 10 years.

I am currently processing this in QGIS and have so far only been able to obtain the Mean layer by using the Raster Calculator by first summing all 10 layers (years) for January and subsequently dividing it by 10, to obtain a Mean value layer for 10 years.

I can't seem to find similar solutions here of how to obtain obtain the Maximum and Minimum values?

Best Answer

You should probably use per-cell calculators like r.series in the QGIS processing toolbox (GRASS commands), which can give you mean, max, min, as well as other statistics on your datasets.

Related Solutions

[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As whuber mentioned, often statistics found in the raster properties are sometimes approximate or are out-of-date.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.

[GIS] Getting multiband raster’s min and max values with Python in QGIS

Next code works with multi band raster. It uses QgsRasterDataProvider objects to calculate statistic through QgsRasterBandStats objects.

layer = iface.activeLayer()

extent = layer.extent()

provider = layer.dataProvider()

for band in range(1, layer.bandCount() + 1):
    stats = provider.bandStatistics(band, QgsRasterBandStats.All, extent, 0)
    min = stats.minimumValue
    max = stats.maximumValue
    print 'min: {:.2f}, max: {:.2f}'.format(min, max)

After running the code with RGB raster of next image:

I got printed, at Python Console of QGIS, next values:

min: 62.00, max: 255.00
min: 89.00, max: 255.00
min: 90.00, max: 255.00

They are identical to reported in Metadata Layer Properties.

Best Answer

Related Solutions

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

[GIS] Getting multiband raster’s min and max values with Python in QGIS

Related Question