[Therion] Turning LIDAR point clouds into cave maps

[kevin dixon]

Mix and match LIDAR and DistoX2 ?

You need to have something sufficiently large enough on your DistoX2
survey stations to register within your point cloud. Preferably at
least two points so that you can rotate the LIDAR point cloud(s) to
match the DistoX2 survey. Mine Surveys using ZEB or equivalent LIDAR
typically do a centreline survey using a theodolite and then add the
ZEB LIDAR clouds using Levelling Staffs at the Centreline survey
stations when doing the ZEB LIDAR survey.

The LIDAR survey can be decimated based upon distance to adjacent
nearest LIDAR points eg. choose a 0.3m point threshold or whatever you
think appropriate for your survey. The resulting manageable clouds can
then be rendered as local theodolite surveys (Range, Inclination,
Bearing) within the DistoX2 Survex/Therion survey, just make sure you
have sufficient registration points between traditional DistoX2 survey
and the LIDAR.

You can do a lot of DistoX2 splays and then try to manually match the
LIDAR point cloud to the DistoX2 splay model. This is the method I
used twelve years ago when I was first experimenting - it is very time
consuming. Now I would suggest you use something that is sufficiently
large enough to register in the LIDAR cloud that can be surveyed with
the DistoX2 eg. a large distinctive rock, formation or other natural
feature or something man-made, tackle bag, container, helmet etc.

There is also potential to use a photogrammetry model to bridge
between DistoX2 survey and LIDAR. Useful software packages to look at
include Meshlab and Meshroom.

Kevin Dixon
York

On Wed, 29 Nov 2023 at 14:09, Tarquin Wilton-Jones via Therion
<therion at speleo.sk> wrote:
>
> 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
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