[gdal-dev] How to locate where the raster min and max values are?

[Even Rouault]

Slightly less tricky using gdal.ApplyGeoTransform() (note the explicit 
cast of line[0]/col[0]) as a int, to avoid a numpy type to be used, 
which confuses gdal.ApplyGeoTransform) :

import numpy as np
from osgeo import gdal
ds = gdal.Open('byte.tif', gdal.GA_ReadOnly)
rb = ds.GetRasterBand(1)
img_array = rb.ReadAsArray()
vmin = img_array.min()
(line, col) = np.where(img_array==vmin)
line = int(line[0])
col = int(col[0])
print(f"col={col}, line={line}")
gt = ds.GetGeoTransform()
X, Y = gdal.ApplyGeoTransform(gt, col, line)
print(f"X={X}, Y={Y}")

Le 30/10/2024 à 01:41, Even Rouault via gdal-dev a écrit :
>
> if you are just interested in any point where the minimum is reached:
>
> import numpy as np
> from osgeo import gdal
> ds = gdal.Open('P3412A.tif', gdal.GA_ReadOnly)
> rb = ds.GetRasterBand(1)
> img_array = rb.ReadAsArray()
> vmin = img_array.min()
> (line, col) = np.where(img_array==vmin)
> line = line[0]
> col = col[0]
> print(f"col={col}, line={line}")
> gt = ds.GetGeoTransform()
> X = gt[0] + gt[1] * col + gt[2] * line
> Y = gt[3] + gt[4] * col + gt[5] * line
> print(f"X={X}, Y={Y}")
>
> Le 30/10/2024 à 01:31, Rahkonen Jukka via gdal-dev a écrit :
>>
>> Hi,
>>
>> I would like to know the georeferenced coordinates of the min and max 
>> values of a DEM file. Even better if I could forward them into a 
>> vector file. If the minimum or maximum happens to be on a flat area 
>> like seabed I would be happy with the first pixel at the moment.
>>
>> By copy-pasting from How do I open geotiff images with GDAL in 
>> Python? - Stack Overflow 
>> <https://stackoverflow.com/questions/41996079/how-do-i-open-geotiff-images-with-gdal-in-python> 
>> and How to find the indexes of the minimum or maximum value(s) in a 
>> matrix using python ? 
>> <https://en.moonbooks.org/Articles/How-to-find-the-indexes-of-the-minimum-or-maximum-values-in-a-matrix-using-python-/> 
>> I think I managed to get the correct points as numpy indexes
>>
>> >>> import numpy as np
>>
>> >>> from osgeo import gdal
>>
>> >>> ds = gdal.Open('P3412A.tif', gdal.GA_ReadOnly)
>>
>> >>> rb = ds.GetRasterBand(1)
>>
>> >>> img_array = rb.ReadAsArray()
>>
>> >>> vmin = img_array.min()
>>
>> >>> vmax = img_array.max()
>>
>> >>> vmin
>>
>> -0.929
>>
>> >>> vmax
>>
>> 17.246
>>
>> >>>
>>
>> >>> np.where(img_array==vmin)
>>
>> (array([1504], dtype=intg64), array([1189], dtype=int64))
>>
>> >>> np.where(img_array==vmax)
>>
>> (array([1545], dtype=int64), array([2423], dtype=int64))
>>
>> >>>
>>
>> But now I have no idea about how to get the georeferenced coordinates.
>>
>> The task feels rather simple and I was sure that someone has already 
>> made an utility or a QGIS plugin, but all I have found yet is for R. 
>> I was thinking that perhaps some of the gdaldem modes could be 
>> misused for this purpose, but I believe they cannot. For QGIS I found 
>> advice to use an obvious but  clumsy method of polygonising the 
>> raster and finding the extremes from the vector data. And one 
>> OpenJUMP developer took the challenge and wrote a prototype with Java 
>> but it is not complete yet.
>>
>> -Jukka Rahkonen-
>>
>>
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/gdal-dev
> -- 
> http://www.spatialys.com
> My software is free, but my time generally not.
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>
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-- 
http://www.spatialys.com
My software is free, but my time generally not.
Butcher of all kinds of standards, open or closed formats. At the end, this is just about bytes.
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