When to use a Ground Plane for Radiance simulations

Hi LBT community,

I’m a Radiance beginner and hoping that @mikkel or another expert can help me with a Radiance question. Is there any guidance on when you should or shouldn’t manually set up a ground plane as a HB face for daylight / radiation simulations?

I had a look here and on the Radiance forum, which has helped, but I’d really appreciate an expert’s take.

Two useful resources I spotted were this, which suggests a ground plane should be used (although you need to be careful with Radiance parameters if you do).

And also this Radiance tutorial, which suggests the the ground is set up as a glow material, making me think I shouldn’t manually set up a ground plane.

The specific study I’m doing right now is for a façade radiation study for a high rise office in central London (but also interested how the ground plane should be used for annual daylight studies for low rise buildings with no context).

I’ve run simulations with and without a HB face for a ground plane. The annual result when including a ground plane is higher, marginally for the more south facing facades, and by a greater amount for the NW facade.

Here’s the model and script (with ground plane included):
SimpleModel.3dm (649.2 KB)
Solar Radiation Comparison Ground Plane.gh (1.4 MB)

Would also be good to know how HB deals with ground light source by default.

I’m happy to move this over to the Radiance forum if that’s more suited, but thought others here would benefit.

Thanks in advance!

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Hey @charlie.brooker ,

This is a good question and I’m sure @mikkel can tell you a lot more than I can but I’ll just give you the short answer.

Using an explicit geometry for the ground can make your Radiance simulations more accurate, especially if you know the reflectance of the ground materials and you have geometry that shades the ground next to the region you are studying. However, specifying a ground geometry is usually not required in order to achieve a reasonably accurate simulation because Honeybee always uses an emissive ground hemisphere that essentially mimics a 0.2 reflectance ground surface. Granted, you won’t be capturing a lot of the complexities of light bounces off the ground and walls when you rely on this emissive hemisphere. But it’s “good enough” for a lot of cases.

One thing you mentioned that I’ll give some extra warning about is making sure you select the correct ambient resolution (-ar) when you use such a large ground surface like the one in your screenshot. Personally, I would recommend using a much smaller ground surface since the size of the bounding box around your Radiance geometry can affect the size of the geometric details that you can resolve with the Radiance Ray tracing. There’s a component called HB Ambient Resolution, which can give you the recommended -ar for a given Honeybee model and the dimension of the smallest detail that you need to resolve.


Thanks @chris, that’s a great explanation. I’ll do a bit more research into -ar to build my knowledge.

I definitely need to start adding the HB ambient resolution component to my scripts. I imagine the energy models with large context buildings we’ve also been using as daylight models could be suffering from not incorporating it.

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