calling gdal_merge.py into another python script running GDAL processes

Question

I want to use gdal_merge.py to merge a series of .tif files prior to clipping the merged file to a shapefile boundary, but want to do so as part of another larger script that will execute a number of other processes. I am a python/GDAL newbie and am not sure how to go about calling gdal_merge.py into another script.

I cannot merely run gdal_merge.py on its own as it is but one step in a script that will hopefully execute a number of processes. Any thoughts on the best way to do this?

• Aaron: Welcome to the site Mike. As this question stands, I see four questions: How to generate a list of tiff files?; How to reproject a list of .tiff files? How to clip a list of tiff files?; How to call a script from within a script?. GIS SE is a focused Q&A site that is best suited for single questions. Please consider focusing this post into a single question. For more information on GIS SE, you can take the tour (gis.stackexchange.com/tour) or access our help center (gis.stackexchange.com/help).

Answer 1

The easiest way to do this is by importing the path where gdal_merge.py is located, in my case, /usr/bin/ -- substitute with the path to gdal_merge on your system, which, obviously, could be a Windows path too.

import sys
sys.path.append('/usr/bin/')
import gdal_merge as gm

You will now have to build up an array for sys.argv, as if you were calling gdal_merge directly, e.g.,

sys.argv = ['-o','outputfile.tiff','inputfile1.png', 'inputfile2.png', ....'inputfile10.png']
gm.main()

There is more information on this Stack Overflow post

There is also the __init__.py mechanism, but this requires the file you are importing to be in a sub-directory of wherever you are running your python file from.

• Mike: Thanks a lot for the response John, I'll give it a try and let you know how it goes.
• John Powell: Are you in a position to use gdalwarp instead (depends what you are doing, I guess, but often recommended over gdal_merge). If so, you could use the suprocess module as it is C and not Python. Look particularly at Popen.
• Mike: So here's the situation. I have to write a script that will reproject a series of tifs, merge them, and then clip them to a boundary. I have already written a script using gdalwarp to reproject (using the subprocess module per another poster). That said, I now have to pull off the other two steps in the same script and am not sure how to use subprocess for multiple processes. Furthermore, I wasn't aware that gdalwarp could merge a series of files. I'm sure this is fairly simple, but this is all brand new to me.
• John Powell: gdalwarp input_tiles output_tile will definitely mosaic tiles and you could call it using subprocess also. Unfortunately, we just had a power cut at work and my laptop battery is a bit low, so can't do any testing :(
• Mike: Awesome John, thanks. Will gdalwarp need to be called in 3 different subprocesses given that it will be running three different processes (reprojecting 4 tiles, mosaicing the four, and then clipping them) or can all these features be contained in a single line?

Answer 2

Edit:

Just read that this option was suggested in the comments already. Anyway, for completeness sake. Maybe someone can merge the answers?

Of course it also possible to use python's subprocess, e.g.

import subprocess
def merge(first, second, out_file):
 """
 This utility will automatically mosaic a set of images.
 All the images must be in the same coordinate system and
 have a matching number of bands, but they may be overlapping,
 and at different resolutions. In areas of overlap,
 the last image will be copied over earlier ones.
 :param first:
 :param second:
 :param out_file:
 :return:
 """
 ps = subprocess.Popen(
 ['gdal_merge.py', '-o', out_file,
 '-of', unipath.Path(out_file).ext,
 '-n', NO_DATA_VALUE,
 first, second],
 stdout=subprocess.PIPE
 )
 output = ps.communicate()[0]
 for line in output.splitlines():
 logger.debug("[*] {0}".format(line))