[Therion] Turning LIDAR point clouds into cave maps

[Tarquin Wilton-Jones]

Hi Bill,

> A few years ago Apple released a very high-end smartphone which has a 
> LIDAR feature (iPhone 12 Pro).  There are a few more models now which 
> have it.  I have heard of some people using this to create cave maps. 
> There is much I do not understand.
> 
> Has anyone in the group taken the point cloud from an Apple LIDAR scan 
> and turned it into a Therion cave map?  If so, I am very interested in 
> the details of the process.

OK, that's a mountain of a question. But yes they have been turned into 
maps, though I don't know about Therion.

Firstly, iPhone can do LiDAR and photogrammetry at the same time, and 
you can put those together as a mesh with a tonne of post processing (it 
can even do colours because of the photogrammetry!). It can do some of 
this with its own software, but afaik, people prefer to use other 
software for the LiDAR processing. This can be used to create pretty 
accurate 3D views of caves. The loop closures look amazing, but it's 
hard to know whether the post processing is swallowing the errors. There 
is some work going on at the British Cave Surveying Group to get it 
working. Jono was pioneering this.
https://3dcavesurveys.com
https://www.youtube.com/@valaheritage-jonathanleste4205

At the moment, it has no centreline (unless he added that since I last 
looked, but it's not a trivial task), so that currently makes it quite 
incompatible with Therion.

It is also worth noting that the iPhone has a 10 metre range. With 
passages larger than that, it just invents a ceiling or wall, even 
though there isn't one, which can really confuse any software that wants 
to use the result. And no LiDAR scanner copes well with water.

Secondly, with point cloud data, Julian Todd has a completely different 
surveying package used to draw up surveys. You draw them in 3D, using a 
VR headset. It's called TunnelVR. Some people love the experience, it 
feels a lot like a computer game. But it is extremely different from 
Therion - an entire world away, and although it can export paper 
drawings, it is not as mature as Therion in that respect. Julian is very 
enthusiastic about this if you wanted to ask for a demo.

There are others who do post processing of LiDAR data, using white 
spheres placed in the cave to link the scans to each other. These rely 
on very expensive hardware (but this was used as part of the project to 
survey all the world's largest chambers). I am very poor with names, and 
forget who was doing that. Perhaps someone else can remind me.

In all of these cases, a centreline does not exist. You could of course 
use a scanner at every station, and import those as splays. That is more 
data than I would ever want to work with in Therion. It would be 
impossible to see the positions of things like boulders, pitch lips, 
ceiling steps, etc., because the entire thing would be a mass of splay 
lines, and would just be a grey blob. Though you could draw the walls to 
perfection!

Those should be good places to get you started.

Cheers,

Tarquin