r.topidx(1)

💧 r.topidx — the GRASS GIS manual, but make it cute

okay bestie, welcome to the GRASS GIS User's Manual page for r.topidx ✨ it's a 1 (section one, we're fancy like that) and it's about to give you a topographic index, no cap 🦋

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🌸 NAME 🌸

r.topidx — makes a topographic index (aka the wetness index) raster map, built from an elevation raster map. that's it, that's the whole thing 💅

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🎀 KEYWORDS 🎀

raster, hydrology, wetness, topographic index — yes she has range 👑

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✨ SYNOPSIS ✨

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

🫶 Flags:

• --overwrite — let the output files just overwrite existing files. bold move, respect 🫡
• --help — prints the usage summary, for when you're lost (we've all been there 🙈)
• --verbose — verbose module output, she's chatty and we love that
• --quiet — quiet module output. shhh 🤫
• --ui — force launches the GUI dialog, for the visual learners 💖

🦋 Parameters:

• input=name — [required] name of the input elevation raster map. mandatory. non-negotiable. periodt.
• output=name — [required] name for the output topographic index raster map. also mandatory bestie 💅

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💜 DESCRIPTION 💜

okay so r.topidx creates a topographic index (the wetness index), specifically ln(a/tan(beta)), as a map, derived from an 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, which is (delta vertical) / (delta horizontal) 📐

Input maps are allowed to have NULL values, no drama. so like, if you've got a raster mask set for a watershed (using a basin map from r.water.outlet), this command right here will create a masked elevation map called belev:

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

also, r.stats -Anc prints out averaged statistics for topographic index. cute, useful, we stan ⭐

🚨 lat/long projects are NOT supported. if your data isn't projected, please create a new project with a projected coordinate reference system and reproject the data into it, then run r.topidx from that project. no shortcuts, bestie 😭

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🌈 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 🗺️✨ (it's giving beautiful, honestly)

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📚 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.

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👀 SEE ALSO 👀

r.topmodel, r.water.outlet, r.mapcalc

also: Known issues — go look, iykyk 🫶

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💖 AUTHORS 💖

Huidae Cho Hydro Laboratory, Kyungpook National University, South Korea 🇰🇷

Based on GRIDATB.FOR by Keith Beven. iconic lineage fr fr 🌟

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🧚 SOURCE CODE 🧚

Available at: r.topidx source code (history)

Accessed: Saturday Jan 17 05:41:04 2026

Main index | Raster index | Topics index | Keywords index | Graphical index | Full index

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okay that's the whole page, you're officially hydrated (hydrologically speaking 💧). this was the GRASS GIS 8.4.2 Reference Manual, © 2003-2025 GRASS Development Team, running on GRASS 8.4.2 — and yeah, accessed Saturday Jan 17 05:41:04 2026, so consider yourself timestamped bestie 🎀✨ slay, go make some topographic indices 💅🦋