Interpolation and contouring ============================= Module contributed by Paolo Cavallini - `Faunalia `_ .. note:: This chapter shows how to use different backends to calculate different interpolations. Interpolation --------------- The project shows a gradient in rainfall, from south to north. Let's use different methods for interpolation, all based on vector ``points.shp``, parameter ``RAIN``: .. warning:: Set cell size to :kbd:`500` for all analyses. - :menuselection:`GRASS --> v.surf.rst` - :menuselection:`SAGA --> Multilevel B-Spline Interpolation` - :menuselection:`SAGA --> Inverse Distance Weighted` [Inverse distance to a power; Power: 4; Search radius: Global; Search range: all points] - :menuselection:`GDAL --> Grid (Inverse Distance to a power)` [Power:4] - :menuselection:`GDAL --> Grid (Moving average)` [Radius1&2: 50000] Then measure variation among methods and correlate it with distance to points: - :menuselection:`GRASS --> r.series` [Unselect Propagate NULLs, Aggregate operation: stddev] - :menuselection:`GRASS --> v.to.rast.value` on ``points.shp`` - :menuselection:`GDAL --> Proximity` - :menuselection:`GRASS --> r.covar` to show the correlation matrix; check the significance of the correlation e.g. with http://vassarstats.net/rsig.html. Thus, areas far from points will have less accurate interpolation. Contour --------- Various methods to draw contour lines [always step= 10] on the *stddev* raster: - :menuselection:`GRASS --> r.contour.step` - :menuselection:`GDAL --> Contour` - :menuselection:`SAGA --> Contour lines from grid` [**NB:** in some older SAGA versions, output shp is not valid, known bug]