[Radiance-general] gendaylit vs gendaymtx

Sat May 13 09:16:23 PDT 2017

Thanks Greg and thanks Jan.

Jan, I had a look at your presentation and tried a comparison with the -i 60 option in gendaylit, but that did not improve much the correlation with gendaymtx.

Over the whole year the differences are really not that big, the coefficient of correlation for the London wea file is 0.97. Only cause I was expecting identical results, I was worried I had left something out in the gendaylit command options, that might have been important.

I attached a scatter plot with the results for the whole year, and a temporal map with the R coefficient over the sky patches for each hour. Some of the ‘worst' correlations (still minor) are for high sun angles.

The wea I used was derived from the epw available from the Energy Plus website, using epw2wea (https://energyplus.net/weather-location/europe_wmo_region_6/GBR/GBR_London.Gatwick.037760_IWEC).

Thanks again
Eleonora

[cid:d219e77c-265c-4a1f-a811-42725a6893e9 at eurprd04.prod.outlook.com]

From: Jan Wienold <jan.wienold at epfl.ch<mailto:jan.wienold at epfl.ch>>
Subject: Re: [Radiance-general] gendaylit vs gendaymtx
Date: 12 May 2017 17:25:19 BST
To: <radiance-general at radiance-online.org<mailto:radiance-general at radiance-online.org>>

Hi Eleonora,

I had a presentation at the radiance workshop in Boulder (uff I think this was in 2013) - there you can find how gendaylit treats "problematic" cases (close to sunrise and sunset).
I assume (and hope), that the differences are only for these hours. The core code of gendaylit was checked several times against the publication from R. Perez and was not changed in the last 20years.

best,
Jan

On 05/12/2017 05:25 PM, Greg Ward wrote:
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<mailto: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<https://twitter.com/EleBrembilla>

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Ecole Polytechnique Fédérale de Lausanne (EPFL)
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