Calculating the topographical index

The topographic index

Some GIS programs can calculate the topographic index, but the Topmodel-package provides a function too. If you use the R function, you should delineate your catchment in your preferred GIS application and export a DEM of your catchment as a text file (ascii). Pixels outside the catchment area should be given a distinct value that can be set to NA in R.

For now, the GIS functions of this package are quite limited. The DEM has to be imported as a matrix, which can then be processed by topidx(). Take for instance this minimalistic DEM, saved in a test file called "DEM.txt". Values outside the catchment are given the value -9999 (this can be any other value):

  -9999 -9999 828.9 835.6 -9999
  818.3 826.0 830.7 834.5 836.0
  817.1 824.0 825.2 833.3 836.9
  816.5 820.0 824.1 330.8 -9999
  810.7 815.6 822.2 -9999 -9999

This file can be imported and processed in R with:

  DEM <- read.table("DEM.txt")
  DEM <- as.matrix(DEM)

remove the values outside the catchment:

  DEM[DEM==-9999] <- NA

plot the DEM:

  image(DEM)

Calculate the topographic index. topidx() returns a list with two objects: atb and area. The first is the topographic index (a over tan b) and the second is the cumulative area a. In this case we are interested in atb:

  topindex <- topidx(DEM, resolution=25)$atb

Split the values into a set of classes, for instance for use with topmodel():

  topidxclasses <- make.classes(topindex,16)