reclassification

  1. Using Raster Calculator statements to simplify data
    (previous page)
  2. Using a Reclassification tool
    This is referred to sometimes as a density slice or simply “slice,” and reduces data to usable information. In QGIS, the reclassification tools require you to build a table, which is not easy. Instead, we’ll use a plugin called Raster Reclassifier.

    1. First, let’s again visualize the results of the simplification in the Layer Styling Panel (without changing the data)
      1. open the layer styling panel for the demo_DEM layer
      2. you can see the min/max values at the top
      3. for “Interpolation” above the colors, choose “discrete”
      4. for “Mode” below the colors, choose “Equal Interval”
      5. switch the Mode to “Quantile”
        what’s happening?
    2. The Raster Reclassifier plugin can be used to make binary, multiple, and index layers.
      1. Choose the input layer and number of classification bins (the example below is 5 classes)
        1. choose the input layer and the output file (good names are good metadata)
          Actually….a better name might be “DEM_index_eqint5.tif” right? So you know on which layer the index is based…….
          raster reclassifier plugin box
          (note: we’re going to have some trouble because the tool rounds the minimum)
        2. Select the method and number of categories, and press “extract” to get the values added to the table.
        3. add the new reclassification values (here 1 thru 5).
      2. Save the table (and the plot, see below), so you can try it again, but also as good metadata!
      3. The methods are equal interval (each “bin” of elevation is the same, like our index of 59 m in the previous) and quantile (where the same area or number of cells will fall into each bin).reclassification of a dem by quantile and equal interval
         
    3. In ArcGIS Pro
      1. Choose how you want the histogram divided (here is some ESRI help on how these methods work) The two most common methods are equal interval and equal area (known in ESRI-speak as “quantile”). The illustration above uses “equal interval.” These choices are also available for symbology of any layer (without changing the data)

        These are the choices from the Reclassify tool, and similar choices are found in the Slice tool. The reclassification you choose has a huge impact on the nature of the output. Note the change in results and histograms from the data found in simplify project (to see the effect, I created histograms for the different reclass types illustrated below left in an older version of the software. I don’t think you can do this anymore).
    4. Reclassify is the only good way to deal with discrete, non-numeric layers (like land use, e.g., pasture, roads, forest, lakes, etc) that have to be combined by hand (“manual” reclassification”)
    5. Metadata are complicated. If you use a toolbox, it creates the geoprocessing history, but it is impossible to recover and difficult to read in that format (try it). Better to save the reclassification scheme directly from the tool, by clicking the “save” button. This operation stores an ArcMap table. You must highlight the classes you want to save before you save, or the table will be empty (way to go ESRI).