Make several rasters more uniform

1. You could try merging the rasters into one:

   • From the toolbar:

     Raster > Miscellaneous> Merge

   • From the Processing Toolbox:

     GDAL/OGR > Miscellaneous > Merge

   • From the GDAL console:

     gdal_merge.py -o merged.tif input1.tif input2.tif
     

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2. Or build a virtual raster:

   • Raster > Miscellaneous> Build Virtual Raster

The reason for this mosaiked appearance is that each single image is drawn with gray scale stretched from image min to image max, and NOT across the global min/max. When you merge all images into one, well, there is only one min/max. Same is true for VRT, since VRT treats all images as one (when you have a look into the VRT file you will see the common statistics).

When both, merge and VRT, are not an option the following script might help.

In the first loop I iterate over all layers, picking the rasters and get an estimate of their min/max interval. This is the way like QGIS itself work. From these min/max values I calculate the global min/max

In the second loop the renderers for all raster layers are configured such that the gray scale is stretched across the interval global min/max.

gmin = 9999
gmax = -9999
layers = []
# loop over all layers, take rasters and estimate min, max values
for layer in iface.mapCanvas().layers():
    if isinstance(layer, QgsRasterLayer):
        # change percentages and sample size to increase or decrease accuracy
        min_max = layer.dataProvider().cumulativeCut(1, 0.02, 0.98, theSampleSize=250000)
        gmin = min(gmin, min_max[0])
        gmax = max(gmax, min_max[1])
        layers.append(layer)

# for all rasters create a single band gray scale renderer with 
# gray scale stretched across the interval [gmin..gmax]
for rasterlayer in layers:
    renderer = QgsSingleBandGrayRenderer(rasterlayer.dataProvider(), 1)
    # take the first band (0)
    ce = QgsContrastEnhancement(rasterlayer.dataProvider().dataType(0))
    ce.setContrastEnhancementAlgorithm(QgsContrastEnhancement.StretchToMinimumMaximum)
    ce.setMinimumValue(gmin)
    ce.setMaximumValue(gmax)
    renderer.setContrastEnhancement(ce)

    rasterlayer.setRenderer(renderer)

# refresh canvas to show changes
iface.mapCanvas().refresh()