Tuesday, April 15, 2014

UTC Setting and Chronolux...

Preamble: questions about how UTC setting affects insolation duration and shadows in an active model keep coming. That's right it is not an obvious question, so finally I decided to write some more or less detailed explanations in this regard...

As everyone has already mentioned geo-location dialog has changed a couple of SketchUp's versions ago. In older versions it was possible to set UTC manually during setting a custom geo-location:

Now UTC setting has moved into Shadow Settings palette and SketchUp calculates UTC automatically just from current geo-location's longitude (looks like it just adds/subtracts an hour per each 15 degrees of longitude). But officially valid UTC may differ from such calculated UTC by as much as 2 hours:

Anyway, UTC does not affect actual geometry of sun's trajectory at a given day of a year. That's why actually UTC setting does not affect measured duration of insolation at a given day. Automatically calculated UTC causes time scale in which time shift between local noon and solar noon is not greater than half an hour. The mentioned shift equals zero in case if longitude of geo-located model is divisible by 15 without a reminder.

In case if it is necessary to have official local time on insolation calculation results, UTC has to be set to an officially valid value.
Note that both sunrise and sunset time shift after changing of UTC setting. That's why it is necessary to re-adjust Time Settings of Chronolux in case if UTC setting was changed.

Note that also it is necessary to re-adjust time settings even after changing of geo-location of an active model in order to refresh time scale.
Unfortunately this requirement does not present in Help System. The ideal usage scenario is to set geo-location and UTC of a sketchup model once at the very beginning before insolation testing and change not mentioned settings later.

It is also necessary to mention that Chronolux utilizes SketchUp's internal "real world shadow engine". In fact Chronolux just takes coordinates of the sun at a given time and date from 'shadow_info' object provided by SketchUp's API. And here is a really interesting and useful article which has detailed explanations of SketchUp's shadows mechanism's principles: Creating Real World Shadows.
Useful hint: pay attention to Time paragraph of mentioned article and in case if you got time zones confusion you may find out that you're not alone.

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