[Radiance-general] gendaylit vs gendaymtx

Mon May 15 00:56:38 PDT 2017

Hi Eleonora,

thanks for evaluating this!

Are you sure about the units? 2000W/m2/sr would mean more than 350000 
cd/m2. On the other hand, if the graph would show luminance, then this 
must be just one patch looking to the north? Already for the timestep 
you sent (with low irradiation data) it gives me more than 30000 cd/m2 
close to the sun position (using gendaylit).

Besides the units (I assume you plot cd/m2 for one patch towards the 
north), looking at the graph I think it is necessary to look into that 
deeper to find out the reasons for the deviations. As far as I can see 
this is not caused by the routines we added to gendaylit in order 
prevent problems around sunset/sunrise.

A high r2 does not mean there is no problem- looking at the figure you 
can see a systematic behavior: at high values, gendaymtx delivers higher 
values than gendaylit, for lower ones it is the opposite. The vertical 
line for low gendaylit values could be caused by the sunrise/sunset 
corrections. Especially for the lower values around 2000 there seems 
like a deviation around 40-50% between the two implementations - this is 
definitely too much. Actually the implementation should really deliver 
the same values (except for the sunrise/sunset) - of course 
rounding/variable types and so on could lead to some small deviations, 
but not like this and also not as systematic like this.

cheers

Jan

On 13/05/17 18:16, Eleonora Brembilla wrote:
> 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
>
>
>
>>
>> *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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>>
>>
>> -- 
>> Dr.-Ing.  Jan Wienold
>> Ecole Polytechnique Fédérale de Lausanne (EPFL)
>> EPFL ENAC IA LIPID
>>
>> http://people.epfl.ch/jan.wienold
>> LE 1 111 (Office)
>> Phone    +41 21 69 30849
>>
>>
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>
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-- 
Dr.-Ing.  Jan Wienold
Ecole Polytechnique Fédérale de Lausanne (EPFL)
EPFL ENAC IA LIPID

http://people.epfl.ch/jan.wienold
LE 1 111 (Office)
Phone    +41 21 69 30849

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