A mountain of a question! That is for sure. I have been talking by
email with a friend who claims to have done dozens of cave maps using
the Apple LiDAR system. So far he has not gone into detail on how it
was done. I asked for some samples of the maps he made. No Therion for
him - he uses Xara and Compass.
Update - While writing this, he sent me some sample files and a bit more
detail. His description:
"My process is create the "LIDARonly" version. Then layer on the
cartography...then create the standard cave format...strip off the LIDAR
(image without Lidar in the file name)."
Looking at the sample files, it looks like he extracts some images from
the point cloud. One of them is an overhead view and others are cross
sections and profiles. He then uses these JPG images as the drawing
background to produce a traditional cave map. The LiDAR scan
essentially replaces the in-cave sketching. I suspect there is some
custom software to extract the images from the point cloud.
Another of my caving friends was doing photogrammetry over ten years
ago. He used a regular digital camera and processed the photos through
https://www.agisoft.com/ PhotoScan. It was amazing, and totally useless
for cave survey except perhaps as a way to document formations.
Yet another of my caving friends does survey in a slightly different
way. In the cave he sets survey stations and records centerline data,
just like everyone else does. But then he takes his GoPro and shoots a
video of everything he just did. Later - out of the cave - he uses the
centerline data and the GoPro video to make a traditional sketch. That
is where Dennis stops. He does not go on to turn his sketch into a map.
He hands it off to cartographers to take care of that.
The stuff at 3DCaveSurveys is way cool - but it is not a cave map
despite claims to the contrary. I found another web site which has a 3D
visualization of a cave made with the Apple lidar system.
https://www.tetongravity.com/story/gear-tech/digitalisation-of-caves-proceeds-using-lidar-on-new-iphones
I downloaded the Torhola GLB file and was able to view it in MeshLab.
Way cool stuff! He calls it a "model, not a map, and I agree with that
nomenclature.
I remember some years ago hearing about some mapping projects where the
survey stations were marked with small balls in a specific color. The
processing software recognized that color and provided a way to connect
those points to known x,y,z locations - a centerline. It was
fantastically expensive in both money and computer resources.
In a way, we do sort of the same thing with Therion. The centerline
data is processed to produce a set of x,y,z coordinates. The survey
stations are flagged in that data set. Then when the sketch is drawn
out, we insert points of type "survey station" which are also flagged.
The sketch can be considered as a sort of two dimensional point cloud.
Therion knows how to match up the survey station points in the sketch
with the survey stations in the x,y,z coordinate set. From there it can
morph everything around.
So far the 3D presentations I have seen of caves would be useful
auxiliary material for a cave map, but they are no substitute for
traditional maps.
===============
Bill Gee
On 11/29/23 08:09, Tarquin Wilton-Jones via Therion 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
_______________________________________________
Therion mailing list
Therion@speleo.sk
https://mailman.speleo.sk/listinfo/therion
_______________________________________________
Therion mailing list
Therion@speleo.sk
https://mailman.speleo.sk/listinfo/therion
