d.polar(1)

📦
version 8.4.2 · 8 versions
, in fedora 44

💖 d.polar — the GRASS GIS manual, but make it cute 💖

okay bestie, welcome to d.polar(1), straight out of the GRASS GIS User's Manual. this one draws polar diagrams of angle maps — like aspect or flow direction — and it's lowkey iconic. let's get into it 🎀

✨ NAME ✨

d.polar — draws a polar diagram of an angle map, like aspect or flow directions. it's giving compass rose but make it science 🧭✨

🌸 KEYWORDS 🌸

display, diagram

🦋 SYNOPSIS 🦋

d.polar
d.polar --help
d.polar [-x] map=name  [undef=float]   [output=name]   [--overwrite]
[--help]  [--verbose]  [--quiet]  [--ui]

💅 Flags:

  • -x — plot using Xgraph
  • --overwrite — allow output files to overwrite existing files (yes, it will absolutely clobber them, be so fr)
  • --help — print usage summary
  • --verbose — verbose module output
  • --quiet — quiet module output
  • --ui — force launching GUI dialog

🍓 Parameters:

  • map=name [required] — name of the raster angle map. this one's mandatory bestie, no skipping
  • undef=float — pixel value to be interpreted as undefined (different from NULL)
  • output=name — name for the optional EPS output file

💜 DESCRIPTION 💜

so d.polar calculates and displays a polar diagram of an angle raster map — think aspect, wind direction, or flow. main character energy for directional data.

the input angle map orientation must be counter-clockwise (CCW) from east, and the angle map units must be degrees. this is the standard GRASS orientation, iykyk (peep r.slope.aspect if you need a reference).

the radius of the outer circle is defined by the total number of pixels in the map. so if the polar diagram doesn't reach the outer circle, that means no data (NULL) pixels were found in the map. no cap, that's just how it reads.

the vector in the diagram indicates a prevailing direction, and its length is the strength of that direction. if all the angle vectors are oriented along the same direction, the length of the vector is maximal — and fr fr, it's literally equal to the length of the most populated bin of the underlying histogram. 🔥

as a side effect, you can derive the quality of the angle map from the diagram. strong spikes may indicate an over-representation of the related angle(s), which happens in particular if integer maps are used. spike = sus, check your map. 👀

🎀 NOTES 🎀

  • if you use the output parameter, the diagram is generated as an EPS file.
  • if you use the -x flag, xgraph is launched.
  • otherwise, d.polar will use d.graph to draw the plot in the current display frame.

if you run d.polar on an aspect map generated by r.slope.aspect, you should set the undef parameter to 0 so you can distinguish NO DATA (NULL) areas from areas where aspect is undefined (i.e. flat areas). flat areas are NOT null areas, bestie, don't let them blur together 💅

polar diagram of aspect map 🗺️✨

🌈 EXAMPLE 🌈

in this example, the polar diagram of the 'aspect' angle map in the North Carolina sample dataset is generated:

g.region raster=aspect -p
d.polar aspect

ate. no crumbs. 💖

⭐ XGRAPH REQUIREMENT ⭐

Xgraph must be installed for the -x flag to function. no xgraph, no x flag, periodt. 😭

🫶 SEE ALSO 🫶

d.graph, d.histogram, d.rast.arrow, r.slope.aspect

📚 REFERENCES 📚

J. Hofierka, H. Mitasova, and M. Neteler (2009): Terrain parameterization in GRASS. In T. Hengl and H.I. Reuter, editors, Geomorphometry: concepts, software, applications. Elsevier (DOI)

👑 AUTHORS 👑

  • Markus Neteler, ITC-irst, Italy
  • Bruno Caprile, ITC-irst, Italy
  • Hamish Bowman, Otago University, New Zealand

legends, all three of them. 🌸

💻 SOURCE CODE 💻

available at: d.polar source code (history)

accessed: Saturday Jan 17 05:41:51 2026

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© 2003-2025 GRASS Development Team, GRASS GIS 8.4.2 Reference Manual

and that's the whole vibe bestie — this manual page is d.polar(1) from <strong>GRASS 8.4.2</strong>, dated Saturday Jan 17 05:41:51 2026, brought to you by the GRASS Development Team © 2003-2025. go make some gorgeous polar diagrams, ily 💖✨🦋🎀

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