DAYSIM Hourly Illuminance Data - Interval average or spot check?

Hello,

My question is regarding the DAYSIM illuminance output file (.ill) data. In particular, say I run a simulation at 60-min interval and get the following ILL file output snippet:

1 5 0.500 0 0 0 
1 5 1.500 0 0 0 
1 5 2.500 0 0 0 
1 5 3.500 0 0 0 
1 5 4.500 0 0 0
1 5 5.500 0 0 0
1 5 6.500 0 0 0
1 5 7.500 63 74 87
1 5 8.500 816 961 1159
1 5 9.500 1365 1608 1933

I am curious to know if the sensor point illuminance values are actually the hourly mean (i.e. 9.5 is the average from 9 to 10) or whether these values are "spot check" readings taken at the half-hour mark. I have yet to find any literature that specifies this, so any clarification would be greatly appreciated.

Thanks!

Zach.

Tags: average, daysim, interval, timestep

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It's both, in a way.  The sun + sky brightness are based on data measured over the full 60-min time period (since this is what the EPW file provides).  The sun position, on the other hand, is taken at the half-hour mark -- or, for sunrise/sunset hours, the midpoint of the sun-up period.  In reality, though, the sun position isn't so precise either, because the direct contribution is the result of interpolating between the 4 nearest (fixed) sun positions.  

Jon

Thanks Jon! That does clear things up a bit.

As a followup question to your explanation: If hourly timesteps are used, does DAYSIM run just one set of calculations for the hour, or does it run it in 5-min timesteps (smallest possible interval) and then output the average of the 12 timesteps for each hour? Looking at the source code, I think it's the former but I want to be certain.

Thanks again for your time

Cheers,

Zach.

Jon Sargent said:

It's both, in a way.  The sun + sky brightness are based on data measured over the full 60-min time period (since this is what the EPW file provides).  The sun position, on the other hand, is taken at the half-hour mark -- or, for sunrise/sunset hours, the midpoint of the sun-up period.  In reality, though, the sun position isn't so precise either, because the direct contribution is the result of interpolating between the 4 nearest (fixed) sun positions.  

Jon

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