Raster Interpolation

NB: There is currently a bug to do with raster interpolations methods. The following will only apply after this bug has been fixed.

There are 4 resampling methods available to the user in the sandbar analysis:

  • linear: tesselate the input point set to n-dimensional simplices, and interpolate linearly on each simplex.
  • cubic: return the value determined from a piecewise cubic, continuously differentiable (C1), and approximately curvature-minimizing polynomial surface.
  • nearest: return the value at the data point closest to the point of interpolation.
  • bilinear: perform linear interpolation first in one direction, and then again in the other direction. Although each step is linear in the sampled values and in the position, the interpolation as a whole is not linear but rather quadratic in the sample location.

Different Methods

linear, cubic, and nearest use those same methods directly from the scipy.interpolate.griddata in the Scipy module. Bilinear is an interpretation of a well-known bilinear interpolation algorithm. The implications of this are that linear, cubic and nearest are optimized, fast and well-supported whereas bilinear uses a much less optimized approach and is slower.

Using different methods

Making use of interpolation is relatively easy and takes two parts:

  1. Change <ResampleMethod> to the method you choose.
  2. Change <RasterCellSize> to be different from <CSVCellSize>. This will indicate to the software that resampling needs to happen.

See Below:

<?xml version="1.0" encoding="UTF-8"?>
<SandbarAnalysis>
  <MetaData>
    <Meta Name="date">2016-11-22T11:22:55.5653730-08:00</Meta>
    <Meta Name="system">MATTYMATT</Meta>
    <Meta Name="user">matt</Meta>
    <Meta Name="version">1</Meta>
  </MetaData>
  <Outputs>
    <Log>log.xml</Log>
    <BinnedResults>results_binned.csv</BinnedResults>
    <IncrementalResults>results_incremental.csv</IncrementalResults>
  </Outputs>
  <Inputs>    
    <TopLevelFolder>c:\Sandbars\Topo_Data\corgrids</TopLevelFolder>
 	<CompExtentShpPath>c:\Topo_Data\corgrids\ComputationExtents.shp</CompExtentShpPath>
 	<srsEPSG>PROJCS["NAD_1983_2011_StatePlane_Arizona_Central_FIPS_0202",...</srsEPSG>
    <GDALWarp>/usr/local/bin/gdalwarp</GDALWarp>
    <CSVCellSize>1</CSVCellSize>
    <RasterCellSize>1</RasterCellSize>
    <ElevationIncrement>0.1</ElevationIncrement>
    <ElevationBenchmark>8000</ElevationBenchmark>
    <ResampleMethod>bilinear</ResampleMethod>
    <ReUseRasters>False</ReUseRasters>
    ...