Category Archives: GIS software

Rescale your raster data layer

Sometimes you want to rescale a raster layer, e.g., to reduce the number of categories, or to create a common scale for different raster layers. Very basic of course, so you can expect to find an appropriate function in any self-respecting GIS software. Just be aware that different terms are being used for the same thing, e.g., scale in gdal, rescale in GRASS and normalize in SAGA GIS. Below a few ways to do this using my favourite GIS programs: GRASS GIS, QGIS, SAGA GIS or gdal. Continue reading

Calculating the raster cell area of an unprojected raster layer

What if you get a raster layer with number of people per raster cell, like for example the population layer from Afripop,  and you want to convert it to a population density layer?

Well, obviously, you need to divide the number of people by the surface area of the raster cells. However, the surface area of the raster cells of an unprojected (lat/lon) are not constant; they decrease with increasing latitude. So what you need is a raster layer with the surface areas of the cells.

I thought I had seen a function in GRASS GIS to do this, but that might have been a typical case of the wish being the father to the thought. But anyway, it isn’t terribly difficult to calculate it yourself using the map calculator. Continue reading

Integrating Maxent, R and GRASS GIS

The maximum-entropy (Maxent) methods is one of the most widely used approaches for species habitat modelling. It has its own dedicated software, the Maxent software (written in java and therefore cross-platform). The software is easy to use and includes fairly a complete help file and tutorial. But things get better… Continue reading

New “Control Rendering Order” Option in QGIS

I just installed the latest QGIS development version (build 1.9.90-Alpha), and one of the changes I noticed was the option “Control rendering order” in the layers pane. I had to Google to check out what this new option was about. As Darren Cope explains in his blog, it allows you to decouple the way layers are organized in the legend (“Layers” pane) from the order they are rendered (drawn) on the canvas (controlled via the “Layer order” pane.). Continue reading

Create wld file for map image

It is easy to export a raster file as a georeferenced images with e.g., GRASS, QGIS or gdal. It becomes more complicated if you want to export a map, composed of different layers. In QGIS, you can create very nice maps with the ‘print composer’, but these maps are not geo-referenced. A solution by Underdark is to use the QGIS command line to create a snapshot of a map, which will give you an image of any size and with corresponding world file. It works like a charm, except for the “any size” part. The function, on my system, does not handle very large image sizes very well.

But why not creating the World file myself? Continue reading

Linking a lookup table to your vector layer

I was looking for a way to link an attribute table of a vector layer to a look up table, with a one to many relation. As discussed here and here, this does not seem to be possible in QGIS. One of the suggestions was to use the “eVis Event Browser”. This plugin allows you to link to external documents. Although a neat plugin, it is not the same as creating an one-to-many link to a lookup table. Continue reading