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I am wondering if I can calculate the sunhours for a specific coordinate pair with r.sun on GRASS GIS. I want to give a coordinate pair as input and calculate how many sunhours there are on a specific day.

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  • You should use r.sunhours instead. – xunilk Jan 24 at 16:51
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You have to use r.sunhours instead. I am not very familiar with the command in the GRASS environment but a graphical interface of it can be found in Processing Tools of QGIS 3 (it uses GRASS 7). In this case, you can produce a raster for whatever specific coordinate pair (by the way, you only need latitude; not coordinate pair) on a specific day.

For instance, in following image I have a shapefile world borders where it can be observed r.sunhours command filtered in Processing Tools browser of QGIS 3.

enter image description here

Double click in command name for opening following dialog; where it was setting values for day 2021-01-24 (number day equal 24), g.region of world_borders, region cellsize 0.1 degrees and open output only for 'Sunshine Hours'.

enter image description here

After clicking in Run, it was obtained Sunshine Hours map for day 2021-01-24 where it can also be observed that hours days only depend of latitude.

enter image description here

For this arbitrary point in USA (-92.83933054393307 35.402592979079515), "Sunshine Hours" is 10.11789 (see following image). By using this alternative method, obtained value was 10.216379 hours. There is a difference of (10.21637-10.11789)*60 = 5.90925 minutes.

enter image description here

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