Python/GDAL get unique values in discrete-valued raster

Question

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?

• R.K.: Which version of GDAL are you using btw?
• nickves: stupid question but what is RAT?
• Markus M.: RAT stands for Raster Attribute Table.

Answer 1

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()))

• jpmc26: Why wrap band.ReadAsArray() in a np.array call? Doesn't it already return a numpy array?
• nickves: Yes it does. Wrapping it in a np.array has no performance drawbacks, as it is not a copy but it uses the same memory address and helps your IDE identify the object so you'll have auto-completion enabled.