---
name: "r.topidx"
section: "1grass"
platform: "linux"
package: "grass-doc"
version: "8.4.1-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/r.topidx"
---

# r.topidx(1grass)

Creates a topographic index (wetness index) raster map from an elevation raster map.

## NAME

**r.topidx**  - Creates a topographic index (wetness index) raster map from an elevation raster map.

## KEYWORDS

raster, hydrology, wetness, topographic index

## SYNOPSIS

**r.topidx**\
 **r.topidx --help**\
 **r.topidx** **input**=*name* **output**=*name*  \[--**overwrite**\]  \[--**help**\]  \[--**verbose**\]  \[--**quiet**\]  \[--**ui**\]

### Flags:

- **--overwrite**

  Allow output files to overwrite existing files

- **--help**

  Print usage summary

- **--verbose**

  Verbose module output

- **--quiet**

  Quiet module output

- **--ui**

  Force launching GUI dialog

### Parameters:

- **input**=*name* **\[required\]**

  Name of input elevation raster map

- **output**=*name* **\[required\]**

  Name for output topographic index raster map

## DESCRIPTION

*r.topidx* creates topographic index (wetness index), ln(a/tan(beta)), map from elevation map where

- •

  a: the area of the hillslope per unit contour length that drains through any point,

- •

  tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal).

Input maps may have NULL values. For example, if you have a raster mask set for a watershed (using basin map from *r.water.outlet*), the following command will create a masked elevation map (belev):\

```
r.mapcalc "belev = if(isnull(basin), basin, elev)"
```

*r.stats -Anc* prints out averaged statistics for topographic index.

Lat/long projects are not supported. If data is not projected, please create a new project with a projected coordinate reference system and reproject the data there. Please run *r.topidx* from that project.

## EXAMPLE

Running the command below will generate the topographic index map shown in the figure:\

```
g.region raster=elevation -p
r.topidx input=elevation output=topidx_elevation
```

*Figure: topographic index map derived from elevation map*

## REFERENCES

Cho, H., 2000. GIS Hydrological Modeling System by Using Programming Interface of GRASS. Master’s Thesis, Department of Civil Engineering, Kyungpook National University, Korea.

Moore, I.D., R.B. Grayson, and A.R. Ladson, 1991. Digital Terrain Modeling: A Review of Hydrological, Geomorphological, and Biological Applications. Hydrological Processes 5:3-30.

## SEE ALSO

*r.topmodel,* *r.water.outlet,* *r.mapcalc*\
 Known issues

## AUTHORS

Huidae Cho\
 Hydro Laboratory, Kyungpook National University, South Korea

Based on GRIDATB.FOR by Keith Beven.

## SOURCE CODE

Available at: r.topidx source code (history)

Accessed: Monday Feb 24 17:50:56 2025

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© 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual