How to create contour lines in open source GIS
I was given the task to create contour lines for three sites close to one another. The levels had been previously surveyed and existed as shapefiles. Here is what I did:
Programmes used (in this order):
GvSIG: Upload the level shapefiles. Then start the Sextante extension toolbox and choose: “Rasterization and Interpolation” - “Kriging”. Under “Parameters” choose the point input layer – the individual level shapefiles. The “Field” should be the one containing your height info. Select a filepath for the output “Result[raster]” and save it somewhere obvious – it will be used in the next step. In the “Raster output” tab choose “use extent from layer” and define your cell size (not too small). “Accept” - a raster image is added to your dataset which contains the height information Create as many raster images as required (in my case 3).
The next step takes place in MapWindow, because I discovered that although GvSIG produces them it has 2 major problems: a) The height of the individual contours does not seem to recognize them and rounds everything to the nearest meter – not very useful. b) GvSIG creates an outline around the extent of the contour shapefile, which has to be painfully cropped away.
Therefore we change to
MapWindow: Got to: “GIS tools” - “Raster” - “Generate a contour shapefile”. Choose: “use an external file...” and browse to wherever you saved the raster image produced by GvSIG, add it. You can choose an output shapefile or save the contours later. Then define your contour interval or “use fixed levels” - you will probably have to go through this process until you are happy with the result. “Generate” and the contour shapefile will be created and added. Create as many shapefiles as required (in my case 3).
Another problem I encountered was when I tried using MapWindow to crop the contour shapefiles to the boundaries of the three areas. One area consisted of several polygons and the “Clip shapefile with polygon” in “GIS tools” - “Vector” did not work for this one. I think it does work if your area consists of one polygon only, though.
Thus I put the boundaries and the contours into
GvSIG again and use the “View” - “Geoprocess toolbox” - “Geoprocesses” - “Analisis” - “Overlay” - “Clip” Select your Input- and Clip- covers and save the new shapefile – it will be cropped to match the boundary of the area.
OpenJump: Is handy to select individual polylines from your contours, copy these and export them to new shapes in order to be overlain to the rest and labelled. Right-click onto your layer - “Select current layer items”. Once that has been done, go “Edit” - “Replicate selected items” - they are then added to your dataset and can be saved as a new shapefile.