[Radiance-general] gendaylit vs gendaymtx

[Greg Ward]

Hi Eleonora,

The gendaymtx command and gendaylit are separate implementations based on the Perez model with no code in common.  The gendaylit program was developed first, long ago by Jean-Jacques Delaunay, then more recently expanded, updated and improved by Jan Wienold and Wendelin Sprenger.  In contrast, the gendaymtx program was derived a few years ago from C code provided by Ian Ashdown.  The two programs have been spot-checked against each other, but as far as I know, no one has done a thorough comparison of either the methods or the outputs to ensure they behave the same.  In fact, there is some room for interpretation in Perez's model, and numerous places where exceptional values must be caught and dealt with.  Finally, I don't think all the options of gendaylit are included in gendaymtx.

If you can be more specific by checking when, where and by how much your output vectors differ, this would help.  Or at the least, attach the relevant part(s) of your London weather file so we can check ourselves.

Cheers,
-Greg

> From: Eleonora Brembilla <E.Brembilla at lboro.ac.uk>
> Date: May 12, 2017 8:16:18 AM PDT
> 
> Hi everyone!
> 
> I have a doubt on how gendaymtx works. Or better, what’s the difference between the Perez model implementation in gendaymtx and in gendaylit?
> 
> I was comparing the output of these two methods, for each hour of the year:
> gendaymtx -m 1 GBR_London.wea > London.dmx
> gendaylit 1 4 9.5 -a 51.15 -o 0.18 -m -0 -W 178 76 | genskyvec -m 1 >  london_0104h9_rad.skv
> 
> And I was expecting to get an identical result, but instead the two data series are somehow different. The data I used for the gendaylit -W option are taken from the same .wea file I used for gendaymtx.
> 
> Sorry if you discussed about this previously, I searched a bit in the mailing list but I didn’t find a precise answer.
> 
> Thanks a lot!
> Eleonora
> 
> 
> 
> Eleonora Brembilla, MSc
> PhD Student
> 
> Building Energy Research Group
> Daylight and CBDM
> School of Civil & Building Engineering
> Loughborough University
> LE11 3TU
> UK
> 
> @EleBrembilla
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