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I'm working on a dataset of house searches - 1 polygon for each area in which people want to find a house from an online search tool.

This produces many overlapping polygons. It would be great to get a basic understanding of the places that are most frequently searched.

You can do this in QGIS and other programs using transparency:

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But ideally, you would create a new layer with the search density for each cell in a grid. This is exactly what the "feature density" algorithm does in ArcView 3.2 and I've seen it work to great effect. It creates a raster map from a mass of overlaid polygons.

So the question is: is there an implementation of this algorithm for my polygon dataset for open source programs. I'm an R and QGIS user so one of those would be preferable. Many thanks!

  • mhh, Can't you just specify an individual integer value for each feature type and then rasterize your vector layer using your newly field? This should return you a raster, where the cells value is the sum of your fields integers and therefore kinda represents density of sth. – Curlew Jun 15 '13 at 6:53
  • Hi Curlew, that sounds like a sensible solution to me: simple work around. Could you be more specific about which rasterize function (and in which program) you are referring to though. Would it create a new raster layer for each polygon and then add the cell values, or add them all up automatically?. – RobinLovelace Jun 15 '13 at 13:25
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In GRASS GIS you can achieve the functionality as follows:

All this can be easily put into a single script. Using the standard GRASS GIS Script parser lines as well, it would auto-generate the GUI, too.

  • Thanks for the suggestion - I've tried to test this but for some reason GRASS doesn't like my data (it loaded other shape files fine). I'll try it again once I've altered the attributes that I think are causing the problem... In the meantime: any ideas how you specify the cell size of the raster output of this approach? That's one of the big advantages of the ArcView 3.2 algorithm. – RobinLovelace Jun 15 '13 at 13:28
  • What error do you get with that file(s)? For the cell size: obviously you can specify it also in GRASS GIS, see here: grasswiki.osgeo.org/wiki/Computational_region – markusN Jun 16 '13 at 6:49

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