Category Archives: QGIS

Sampling raster values at point locations in QGIS – an update

In a by now fairly old post I described how to sample raster values at point location in QGIS. The method I described used the ‘Point Sampling Tool’ addon. However, the function creates a new point layer, which only contains the values extracted from the raster layer. None of the fields in the original point layer is copied to the new one. It is possible to join the attribute table of the new vector point layer with the original attribute table afterwards using a spatial join as explained in that post. However, this will not work if your point data includes points with exactly the same coordinates.

Since I wrote that post, QGIS has come a long way. Continue reading

Using GDAL from within QGIS: trouble shooting

Some notes to self about steps I had to take to make GDAL work from within QGIS. Both where compiled from source and run without problems. However, there are still a few issues with running gdal from the QGIS processing toolbox. This may have to do with the fact that I installed both in a non-default location (in the /usr/local/ folder), but in any case, the steps below solved the problem for me. Continue reading

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

A new great version of QGIS

Are you one of those people eagerly waiting for all those new features in QGIS that have been mentioned in the last couple of months on various forums and blogs? Well, the waiting is (almost) over. QGIS 2.2 is already available in OSgeo4W, Fedora, and Debian, while binary packages are being updated and will be available any time (or might already be available by the time your read this) on qgis.org. Continue reading

Creating a QGIS color map from text file

I recently found the interesting USGS map of global ecosystems. The data can be viewed online but can also  be downloaded as geotif files, which is great as about every GIS program on the planet can read that format. Unfortunately the color legend is included as a *.lyr file, which can only be used in ArcGIS.

But the good people of USGS also provide a dbf file with, amongst others, the mapping unit, names of the ecosystem units and a color code. In ArcGIS this table can be linked to the raster layers, a features which is not (yet) implemented in QGIS. However, it is easy enough to create a color map from this file, which you can use in QGIS. It is just a text file. Continue reading

The all new shiny QGIS 2.0 has arrived

And it is looking great. I have been using the master (development) version for some time now, so I can tell you out of experience that this release rocks! Some of my favourites are the greatly improved map composer, great new styling features, integration of Sextante in QGIS core and much improved stability. For a list of major changes and new functions, go here. But, of course, don’t take my word for it, but check it out yourself and get a copy from the QGIS website. Continue reading

Sharing geotiff raster files

One of my favourite raster formats to share is the geotiff, mostly because I always thought it is one of the most portable formats. But yesterday I got an email from a colleague that the geotiffs I shared looked strange in ArcMap, with hardly any values to show and an apparent range of values of 1,79769e+308 to 2,22507e-308.
Continue reading