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I wanted to calculate and visualize the irradiance values for a plot. Do not know why, but in my copy of QGIS 2.18.5 I am missing appropriate SAGA module in the "Terrain Analysis -> Lightning", so I picked GRASS "r.sun" algorithm.

Results were quite astonishing. It seems that despite of properly geolocated raster upon which the analysis was made, the plot must be located on Venus instead of eastern Poland. Simply it is impossible to receive almost 5 kWh/sq meter a on Jun 21st here.

enter image description here

To double check the numbers, I found standalone copy of SAGA 5.0 and re-run the analysis ("Potential Incoming Solar Radiation" algorithm). This time results were more reliable (raster on the screenshot imported to QGIS for comparison).

enter image description here

Are those two algorithms differ so much?

Has anyone faced the same issue?

Still only testing this functionality.

  1. QGIS version: 2.18.5
  2. GRASS version: 7
  3. SAGA version: 5.0.0.
  4. Input: raster elevation, slope and aspect data (3 separate). SAGA ran on elev raster only. GRASS used all 3.
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  • 2
    I'd post this question on the GRASS-users list lists.osgeo.org/mailman/listinfo/grass-user
    – mankoff
    Nov 25, 2017 at 8:53
  • 2
    Could this Q&A "r.sun delievering unrealistic Values" by @Ulf be of help?
    – Kazuhito
    Nov 26, 2017 at 8:45
  • Thanks @Kazuhito! Now it is more clear why the results look like this. BTW: does the same apply to irradiance calculations in SAGA?
    – proteus
    Nov 26, 2017 at 18:17
  • @mankoff - is there a separate group for SAGA users as well? This matter is getting more interesting through your input and I'd like to find out more about both solutions.
    – proteus
    Nov 26, 2017 at 18:41
  • Could you test the Potential Incoming Solar Radiation function in SAGA 6.4?
    – Kazuhito
    Jun 30, 2018 at 4:09

1 Answer 1

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I do not know much about the background of r.sun and SAGA algorithms. However, cannot be this a problem with interpretation of the units or interpretation of input data?

In case of r.sun this should be daily sum per sq meter. Attaching the screenshot of typical daily values near Krakow from Solargis database, in June approx. 5 kWh/m2/day is just fine. Solargis: Long-term monthly averages of global horizontal irradiation, a location near Krakow, Poland

In case of SAGA units - I do not know. Just a guess - the values might correspond to instant energy. During the clear-sky summer day easily reach around 800 W, even up to 1000 W (=1 kW), presented as instant value.

In both cases, the variability of data in your area is too high, not realistic (at least I do not see any terrain or other features which should cause the shading effects and be responsible for such results).

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  • Thanks for suggestion. Will try to run the analysis again. Funny thing is that when I wanted to validate the results with 25m DEM using same settings, the results were exactly like solargis database indicates...
    – proteus
    Apr 20, 2018 at 9:03
  • It took many months to return to the subject but I've investigated further. Interesting thing is that values are closer to the correct ones only when I run analysis on the raster was transformed to WGS84 CRS instead of WGS 84 UTM 34 as originally I've based upon. Values are still off (in some areas even close to 0), but they in the areas exposed to sunlight, numbers are less out of space. Maybe somebody will figure out what is the cause of this error. I ran out of ideas :)
    – proteus
    Jan 15, 2019 at 15:27

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