Calculating the topographical index - Revision history

Wbuytaert at 09:04, 9 November 2009

2009-11-09T09:04:20Z

Wbuytaert at 01:03, 11 January 2009

2009-01-11T01:03:24Z

Wbuytaert at 13:08, 26 June 2008

2008-06-26T13:08:56Z

Wbuytaert at 16:37, 30 May 2008

2008-05-30T16:37:35Z

Wbuytaert at 04:19, 5 May 2008

2008-05-05T04:19:10Z

New page

[[Category:Projects]]
[[Category:Hydrology in R]]

===The topographic index===

Some GIS programs can calculate the topographic index, but the 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)
topindex <- topidx(DEM, resolution=25)
topidxclasses <- topidxclasses(topindex,16)

← Older revision		Revision as of 09:04, 9 November 2009
Line 1:		Line 1:
−	~~[[Category:Projects]]~~
−	~~[[Category:Hydrology in R]]~~

−	~~===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)~~

@@ Line 27: / Line 27: @@
     image(DEM)
-calculate the topographic index:
+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)
+    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)

@@ Line 4: / Line 4: @@
 ===The topographic index===
-Some GIS programs can calculate the topographic index, but the 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.
+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):