To follow up my last post on isohyetal lines, I want to check out the new Contours plugin authored by Lionel Roubeyrie and compare the results to contour lines created with GRASS. With no futhur ado here’s the result:
Two methods for creating contour lines
The green lines are my original isohyetal lines (made with GRASS). The blue lines are based on the same rainfall data with the Contour plugin, and choosing 12 levels of contours. The plugin doesn’t have an option to set interval, rather the number of levels. Thus the values will not be identical to the original run. In the center area the two sets of lines pretty much follow each other. What is obvious to me is the “confusion” around the edges. While the GRASS algorithm does fairly good extrapolation outside the area covered by data, the Contour plugin seems to just “cut off”. So the next test should be with a higher density of points and restricted to the region actually covered by data.
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#1 by marisn - March 5th, 2010 at 14:34
You are not comparing isoline creation but data interpolation/extrapolation tools, as isolines are derivatives of interpolated rainfall surface. Unless it’s known what kind of interpolation algorithm and it’s configuration parameters are used in QGIS plugin, such comparison is incomplete.
#2 by Micha Silver - March 6th, 2010 at 17:17
Quite true, of course.
The first interpolation I did, with GRASS, was based on the v.surf.rst module, which uses regularized spline with tension. I didn’t find any specific details on the python Contours plugin, but I think it just does linear interpolation between points to create contour lines.
#3 by Lionel Roubeyrie - March 15th, 2010 at 16:51
Hi Micha,
the contour plugin uses the contour algorithm of the matplotlib package, and at the last news just makes linear interpolations, without extrapolation. It need a regular grid due to a matplotlib limitation, but with the last Contour plugin version you can use a irregular grid and convert it with the help of the natgrid package. Additionally, you can now change each level manually to fit your need. Hope it help to understand these differences.
#4 by Micha Silver - March 15th, 2010 at 21:23
Thanks, Lionel for clearing that up.