We had a pretty extraordinary rain storm in our region some weeks ago. Accumulated rainfall over a 24 hr period was between 20-100 mm in a region where the total annual precipitation is about 50 mm.! I got rain gauge data for the event and made isohyetal lines using only tools available in Quantum GIS.
First I imported the data table including XY locations and the precipitation with the Delimited Text plugin. Then, in order to limit the analysis more or less to the area covered by the rain gauges, I made a “convex hull”, the minimum polygon enclosing all points, and buffered that polygon by 10 km. (allowing that the interpolation algorithm will give approximate values outside the area covered by the gauges). Both of these operations are available in the Vector->Geoprocessing menu.
Now I fired up the GRASS plugin to do the interpolation. Using the v.in.ogr.qgis module, I loaded both the rain gauge point vector and the buffer polygon vector into a suitable GRASS Location/Mapset. First I converted the buffer polygon to a raster so that I could use it as a mask with v.to.rast. After adding the two GRASS layers to the map – the rain gauges, and the mask raster, I pulled up the r.mask module to force the next action to be limited to the buffer region. Then I ran v.surf.rst to produce an interpolated rainfall grid. I chose, of course, the precipitation column as the attribute field for doing the interpolation. The new precipitation grid was created in a few moments, and I closed the GRASS toolbox.
Now I activated the new GDALTools Raster plugin. Among the tools there is “Contours”. I ran this tool, choosing the GRASS precipitation raster as input. I left the default levels value at 10, and chose an output directory where the contours shapefile will be saved. I also checked “Attribute Name” and typed in “Precip”. The contours were created and here’s my resulting map: