I would like to get some advice on the most efficient way to return a list of unique values of a discrete-valued raster using Python and GDAL.
I had thought that the most obvious way would be to examine the raster's attribute table, but if I do band.GetDefaultRAT()
on the band of a raster dataset that contains an attribute table (the table is visible in ArcCatalog, anyway), the result is always None
:
>>> rat = band.GetDefaultRAT()
>>> rat == None
True
In that case, I end up having to scan through each cell of the raster and build a list of unique values manually. Is this the only way to do it?
Or is there a way to build an attribute table with Python and GDAL, then query it for a list of unique values?
Answer
If I understood correctly, you can use np.unique function from numpy lib:
from osgeo import gdal
import numpy as np
ds = gdal.Open("myimg.ext")
band = ds.GetRasterBand(1)
array = np.array(band.ReadAsArray())
values = np.unique(array)
or you can one-shot it:
values = np.unique(np.array(ds.GetRasterBand(1).ReadAsArray()))
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