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Saturday, January 4, 2014

Unifying Extent and Resolution of Rasters Using Processing Framework in QGIS

My post about modification of extent and resolution of rasters drew quite a bit of attention and I decided to make a small New Year's present to the community and create a QGIS Processing script to automate the process.

The script was designed to be used within Processing module of QGIS. This script will make two or more rasters of your choice to have the same spatial extent and pixel resolution so you will be able to use them simultaneously in raster calculator. No interpolation will be made - new pixels will get predefined value. Here is a simple illustration of what it does:
Modifications to rasters A and B


To use my humble gift simply download this archive and unpack files from 'scripts' folder to your .../.qgis2/processing/scripts folder (or whatever folder you configured in Processing settings). At the next start of QGIS you will find a 'Unify extent and resolution' script in 'Processing Toolbox' in 'Scripts' under 'Raster processing' category:

If you launch it you will see this dialogue:
Main window


Note that 'Help' is available:
Help tab
Lets describe parameters. 'rastersare rasters that you want to unify. They must have the same CRS. Note that both output rasters will have the same pixel resolution as the first raster in the input.
Raster selection window
'replace No Data values with' will provide value to pixels that will be added to rasters and replace original No Data values with the value provided here. Note that in output rasters this value will not be saved as No Data value, but as a regular one. This is done to ease feature calculations that would include both of this rasters, but I'm open to suggestions and if you think that No Data attribute should be assigned I can add corresponding functionality.

Finally you mast provide a path to the folder where the output rasters will be stored in 'output directory' field. A '_unified' postfix will be added to the derived rasters file names: 'raster_1.tif' -> 'raster_1_unified.tif'

If CRSs of rasters won't match each other (and you will bypass built-in check) or  an invalid pass will be provided a warning message will pop up and the execution will be cancelled:
Example of the warning message
When the execution is finished you will be notified and asked if rasters should be added to TOC:


Happy New Year!

P.S. I would like to thank Australian government for making the code they create an open source. Their kindness saved me a couple of hours. 

4 comments:

  1. Hi I don't seem to be able to download your achieve/script. Each time I click the hyperlink it just takes me to a page with the mega logo on it. Any assistance would be great.

    Thanks,

    Pete

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    1. Make sure that you use the latest version of the modern browser (like Firefox) and that the javascript enabled and allowed for the mega.co.nz (or mega.nz if you choose to use the new site). In the middle of the screen you will be able to see the button to download archive.

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  2. Hey. thanks for the Script :) I ran it and I got Algorithm Unify extent and resolution starting...
    'NoneType' object has no attribute 'GetProjection' See log for more details error. maybe you know how can I get it work?

    ReplyDelete
    Replies
    1. I just encountered the same problem. Did you find a solution?

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