r.horizon add output of horizon distance (or height in metres) to raster mode

[zst86]

Hi all, I am intersted in modifying r.horizon so that the raster mode can output either the horizon distance, or horizon height in metres above each point/origin in the reference DEM.

The current raster mode output represents horizon height as an angular measurement. This is useful for rockfall risk assessment: locations near the base of a cliff have horizon angles approaching 90°. However, in terms of rockfall risk, it is also very useful to differentiate between a walking track that is located at the base of a small cliff, say 10m high, versus a very tall cliff, say more than 100m high. Both walking tracks may be located at a spot with nearly 90° maximum horizon angle, but the horizon height is very different.

It seems like it would be straightforward to add the horizon distance as a second raster band to the outputs produced in raster mode; alternatively, the horizon height could be caculated as [horizon distance * tan(horizon angle) ] and added as another raster band.

I am having a look at the main.c code, but I am inexperienced and have not figured out how to make such a modification and run it in GRASS GIS. Very much appreciate any help or pointers that anyone may be able to offer.

Thanks,

Zack

[zst86]

Further to this post, I have attempted to use different LLM sites to help modify the main.c code for r.horizon and recompile grass GIS for windows.

This image shows the concept of a cross section through a DEM with a cliff. It is valuable for geological assessment of rockfall risk to understand not just the horizon angle, but also the horizon height. The resulting raster map should then reflect the size of the cliffs.

The attached modified_main.c.txt contains the updated code for main.c that was produced from asking google gemini, chat GPT, and grok.

modified_main.c.txt

I have not yet been able to succesfully compile grass GIS for windows, so currently plodding along with that. I think this would be a very useful feature for geology users of GRASS GIS, to produce a second raster map from r.horizon that reflects horizon height in metres or map units.

[petrasovaa]

Thanks for giving it a try! I think the implementation would probably work, but right now the point mode and raster mode would be somewhat inconsistent. The point mode will give you distance, not height, but your solution will give you raster height, not distance. So I would either prefer a solution that would add height and distance rasters (and add height to the point mode as well) or add a distance raster only.

Can you please make a pull request? See CONTRIBUTING.md.