Last year I experimented with some photogrammetry in the confines of the Daren Cilau entrance crawl, the section known as the vice to be more specific, details can be found here:
Spurred on by the success in this short bit of passage I convinced myself that it would be a worthwhile project to try and accurately model the entire crawl from the surface to the final rescue box. One could simply video the entire crawl in single trip and use the video to produce a model but it is important to me to make this model accurate and to do this it needs to be surveyed so that the resulting model and can aligned to the survey data. For assessment of accuracy I surveyed both in and out so the loop closure could be examined and any poor data identified and sorted. To make this task more manageable I have broken the cave up into sections between the rescue boxes, these also make handy survey markers that aren’t likely to change location in between trips.
To date I have completed three trips; a trip consists of caving to the rescue box at the far end of the section with a small bag, surveying out from the box to the previous one then surveying back in. Caving slowly out with the Gopro and light trying to get steady footage before turning at the previous box and filming back in to the starting point. I then pack up the gear and cave back out. This involves lots of awkward caving for those familiar with the cave.
Home made ‘filming stick’
The first two of the trips have been successful and a decent model has been created from the surface (box 0) to box 2. On the third trip I found that my Gopro had been switched on prematurely and battery warning was already on before I started but this wasn’t discovered until after complementing the survey so I had a rather rushed trip filming back from box 3 to box 2 before the battery ran out. After some failed attempts at saving the data from the third trip this means I will have to repeat it as I was moving too fast for the image alignment to be successful.
Plan view of disto data from entrance (B0) to box 3 (B3) coloured by section/ date surveyed.
So far my attempts at underground photogrammetry have been limited to easy places where walking steadily through a passage poses little to no issue, this where caves are involved is somewhat limiting due to the nature of cave passage formation.
I set myself the challenge of attempting to model something smaller and less easily walkable than what I have done so far to see how collecting the video might be and how well the software would cope with what will likely be non ideal footage for reconstruction.
The entrance passage to Daren Cilau struck me as a good place to try, more particularly the section known as the Vice; A narrow section in which cavers must turn onto one side and keep their bodies high to prevent being jammed into the narrow trench in the floor. It doesn’t pose a particular difficulty to myself in terms of the passage vs my body size and fairly large people can get themselves through but I thought it would make an interesting experiment, the main issue being capturing reasonable video with one hand whilst negotiating the section of cave. It is an iconic cave in British Caving and probably has never been measured before in such detail.
With the above goals in mind I set off one day over to Wales, my plan was to cave the short distance into to The Vice, mark some (min of 4) stations on some prominent features before, through and after The Vice, survey these stations with a distoX2, then cave through this small section a few times whilst trying to take steady video. Using the marked stations the resultant model could then be scaled and aligned correctly. These stations were marked with Tippex which was carefully removed after use.
Source video of one of the outwards trips
A single dive torch was used attached to my Gopro on a cheap ebay selfie stick to keep weight and bulk down.
In practice this went fairly smoothly, the Vice was passed a number of times: In: Marking stations Out: Survey with Distox2 In and out: filming In and out: filming again In and out: removing Tippex stations
On arriving home I set about processing the images and compiling the survey data. It has become my normal practice to film both directions into and out of a passage, if the alignment is to be considered accurate then both in and out passes should match up (plus any additional passes). This is closing the loop which allows relative accuracy to be assessed and also gives a different perspective on features if its successful.
Separate models for each in and out pass were created first, unfortunately each one giving a slightly different representation of the passage when attempts at aligning them in Cloud Compare were made. I then made a model combining both a single in and out set of images, this was successful in that there only appeared to be a single cave passage which showed that the software was able to combine both sets of images semi correctly. However when this point cloud was matched with the survey data although a low RMS (0.05 m)value was reported the survey data appeared to shoot through the walls of the cave which I know not to be true. I then made a new attempt using all in and all out passes and matched the reference points with the survey data again, this time I got a low RMS (0.03 m) value and the survey data appeared as it should within the bounds of the walls, a good result which I am happy with. This goes to show the importance of some quality control and verification against other sources of data. The images below detail these results.
Single inward pass fitted to survey data
The above image shows the model created from a single inward set of images, Metashape reported all images were aligned and after 7 parameter transformation to the survey points in Cloud Compare a low RMS value of around 0.05 m was reported, however on inspection the survey data (blue line) goes through the walls so something isn’t right. The deviation occurs through the narrowest section where steady filming was most difficult. The image below is the result of the 7 param. transformation.
7 parameter shift results for the single inward model
The below images show the same process as above but using the model derived from a combination of all 4 in and out passes.
All in and out passes fitted to survey data (the roof cut off to allow the data to be seen)
The results for the 7 parameter shift can be seen below:
7 parameter shift results
I am very happy with the results of this, not only has a model of the cave been constructed but it also appears accurate when aligned to distox2 data. The model plus a few extras to aid with scale have been uploaded to Sketchfab here:
