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I am working on a quite simple spatial potential analysis using QGIS. I have some ArcGIS knowledge, but I am new to QGIS. My task is to identify the functional inner city of a region based on different criteria like density or distance to public transport. Therefore I created polygon layers representing different criteria. All the polygons have values between 0 and 1. The next step is to rasterize the vector layers and use the raster calculator to make an additions for each raster cell.

The problem I am facing is that the layer extents are different. Therefore all the raster outputs also differ in extent and cell size. As a result I cannot use the raster calculator to make the cell calculations.

So what I need to do, is to find a way to rasterize the polygone layers into raster sets with the same extend and the same cell size. ArcGIs offers the options to set a specific “Analysis extent” and a specific “Analysis cell size” for further raster calculations, which is basically what I am looking for in QGIS. I hope you guys can help me!

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Two methods come to mind: You can use the gdal utility, gdal_rasterize, with the -a option to specify the attributie column, and the -te and -tr options to set the output extents and resolution the same for each raster. See details on the GDAL website.

Alternatively, if you work in GRASS, then you first set the desired extents ("region settings" in GRASS terminology) with g.region. Then, when converting the vectors to rasters using, that region setting is honored for each new raster. Here's the manual page. Almost all GRASS raster modules work within a preset region.

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Thank you very very much for the quick answer. I needed some time to get familiar with qdal and working with a commando line but in the end it worked out. it still does not feel like an very intuitive way in comparison to arcgis' userinterface but a solution is better than no solution ;) – Luc May 31 '13 at 14:07
I like your "commando line" typo! As to what is intuitive, everyone has his own intuition... Now if you need to do a whole batch of vector to raster conversions, you could easily wrap your gdal command into a loop, and do the whole list of critera vectors in one go. I think that might be better than a friendly user interface. – Micha Jun 1 '13 at 19:09

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