Saturday, November 10, 2012

Fast tip: Changing the geotransform with GDAL python

Preparing the next post, I have found a file with a wrong geotransform, but not an easy tool to do it.
Coding it is as easy as opening the datasource with the update option and setting the new geotransform as follows:
ds = gdal.Open( fileIn, GA_Update )
ds.SetGeoTransform(geotransform)
ds = None
Where the geotransform must be a tuple like (-180.5, 1.0, 0.0, 90.5, 0.0, -1.0). Take a look to the documentation for more information.

I have created a small program to make it even easier, called changeGeotransform.py:
from osgeo import gdal
from osgeo.gdalconst import *
import sys

def changeGeotransform(fileIn, geotransform):
    """
    Takes a dataset, and changes its original geotransform to an arbitrary geotransform
    """
    ds = gdal.Open( fileIn, GA_Update )
    #Exit if there is an error opening the dataset. GDAL will output the error message
    if ds is None: 
        sys.exit()
    ds.SetGeoTransform(geotransform)
    ds = None

def helpText():
    print "To change the file Geotransform, the command is python changeGeotransform  , where the new geotransform must be like '(left_value,delta_x,rotation_x,top_value,rotation_y,delta_y)'"
    sys.exit()
def geotransformError(inputGeotransform):
    print "Error reading the geotransform: " + inputGeotransform + " it must be like (-180.5, 1.0, 0.0, 90.5, 0.0, -1.0)"
    sys.exit()


if __name__ == "__main__":
    """
    Program mainline
    """
    # Parse command line arguments.
    argv = gdal.GeneralCmdLineProcessor( sys.argv )
    if len(argv) != 3:
        helpText()
    fileIn = argv[1]
    try:
        geotransform = eval(argv[2])
    except Exception, ex:
        geotransformError(argv[2])
    if type(geotransform) != type(()):
        geotransformError(argv[2])
    changeGeotransform(fileIn, geotransform)
To execute, just type:
python changeGeotransform file_name new_geotransform
 where the new geotransform must be like:
(left_value,delta_x,rotation_x,top_value,rotation_y,delta_y)
The program basically tests if the inputs are correct and executes the code I have commented before.
Of course, only the files with drivers able to write the format can be changed. Type  gdalinfo --formats to know which drivers do you have installed.

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