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I have a Python script that I am using to load .asc raster files and then work with the contents therein. However, it isn't loading the correct information that is specified in the headers. The .asc file that I am using has the following for headers:

ncols        2131
nrows        2220
xllcorner    670813.505443912582
yllcorner    5480455.007973534055
cellsize     3.000109992417

But when I run the following simple script to obtain the values of xllcorner, yllcorner, and cellsize:

from osgeo import gdal

dempath = 'My_DEM.asc'
dem_file = gdal.Open(dempath)
dem_transform = dem_file.GetGeoTransform()
print(dem_transform)

I get the following as output:

(670813.5054439126, 3.000109992417, 0.0, 5487115.2521567, 0.0, -3.000109992417)

So as we can see, the yllcorner value that is loaded from the asc file isn't 5480455.007973534055, or any rounding off of it, that I would expect it to be. Instead I am getting 5487115.2521567, which is far off enough that it causes problems for me later on. I can import the correct header information with a variety of ways that are not using the GDAL Python wrappers, but ideally I'd like to keep using the GDAL Python wrappers. Is there something special I am forgetting to specify in my script like projection information? Is this a known bug in GDAL? How can I resolve this issue without having to write my own asc file parser?

3

No, it's not a bug. 5487115.2521567 is correct.

The GeoTransform doesn't give you the lower left, it gives you the origin. Which is the upper left. You can calculate the lower left from the origin + y pixel size * no. rows:

yllcorner = dem_transform [3] + dem_transform[5] * dem_file.RasterYSize
print(yllcorner)

5480455.0079735
| improve this answer | |
  • Today I learned something very new and useful about how the GDAL Python wrappers are actually loading these data in. Thanks so much for showing me what it's actually doing. – R. M. Spies Aug 14 '19 at 19:11

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