It appears that the DIVA/GH sun position tool and sun vector works different in comparison to other tools.
My Vray results and my grasshopper/Diva predictions were totally off. I looked closer into the matter and what amongst others caught my attention was that the altitude and azimuth values for exactly the same dates, times, locations (via coordinates) between Vray and the Diva sun position tool were up to 15 degrees off from each other. How does this behave in terms of Radiance simulation? I assume that Radiance calculates based on time, etc. input its own sun position? Does DIVA sun position tool and Radiance match?
I made a quick comparison, please see the excel file and my grasshopper definition and location txt file (one has to relink the txt file for locations in gh) because I really would like to understand what went wrong. Does anyone have an idea?
I see the discrepancy in your results and will check them further. As discussed in the user guide - the solar position algorithm used by the GH solar vector component follows a simply formula described in "Solar Engineering of Thermal Processed", 3rd edition by Duffie and Beckman (2006). This has shown good correlation when tested with other solar calculators in past, but I will recheck the code. I do not have the formulas that radiance or vray is using for calculating solar position, but I would expect the variability between all the methods to be much smaller than the ones that are showing up in your definition. Will post an update.
I checked the radiance formula with the radiance online group and they use a different convention for longitude/meridian. I re-ran the calculations and find good agreement with the NOAA online calculator - attached are the numbers. We will be swapping the radiance SPA formula into the solar vector component in the next release (which should be soon).
Comparison spreadsheet attached.
Sorry for the late reply. Thanks for looking into that matter and providing an update soon. I will have a closer look at the excel.
For those who might be interested.
VRay is also close to the NOAA online calculator.