Illuminance pre-post process

Hi, why we are supposed to in 8760 hourly data in illuminance data, in one step, we delete the first item to have 8759, and in the second step, we delete the last item to have 8759 data, and at the end, we average between them? - as @chris mentioned in Tutorial Videos- with this process, the value of each item goes to roughly half.
what is the reason for this work?

@zaanaa ,

I think I know what you are asking about, though a more specific example or a link would be helpful.

Illuminance data in EPW files is a little weird in that the illuminance value for a given time (let’s say 12:00) is an average illuminance for the time period over the last hour (from 11:00 to 12:00). So the illuminance at 12:00 in the EPW file is more like the actual illuminance at 11:30.

In order to have data that is aligned with a sun vector exactly at noon, I would need to average together the EPW value at 12:00 and 1:00 (actually closer to 11:30 to 12:30).

I hope that explains why we have to do all of this extra math. It’s just because someone over 25 years ago decided that this was how they wanted to structure EPW files.