Just a quick post to say that the underwater Disto didn’t work quite as intended, I made a number of trips to Sump 9 in Porth Yr Ogof this summer with the aim of surveying it accurately. The water is usually crystal clear on entry to this sump however the laser was not able to read the short distances I had hoped it would. For shots that should have been 1 to 2 m in length it was just returning valves of around 0.15 m every time, the bearings will be useful but I need to return and measure the lengths traditionally.
Underwater DistoX2 Housing
I will start by saying that this idea has been copied from other cavers/ divers, this isn’t the the first time someone has wanted to use a Distox2 underwater. My reason for wanting a waterproof Distox2 is allow me to use the same methodology for underwater photogrammetry geo-referencing that I use for above water photogrammetry geo-referencing to preserve accuracy through sumps. Lasers do have limitations underwater, light is attenuated by water quite readily especially red light or red lasers. so the range is going to be very limited even in very clear waters.
The main body of the housing is made from Acetal, a small cut out sealed with an o ring allows the laser to emit and receive through the front of the box and is covered by 5 mm thick perspex. The lid is 12 mm thick polycarbonate and houses two brass buttons sealed with o rings to actuate the on/ off buttons. The springs have been taken from a Gopro housing and are the most ferrous part, making the bearing swing by 0.2° when they are swept close to the Distox2, all screws are brass. A standoff is fitted to the rear to extend the rear reference point to make it easier to align to the survey station. The gland sticking out the side doesn’t have a function other than to seal the hole present in the side ( the boxes previous life was a waterproof box for an Arduino Mega).
Due to the difference in refractive index of water and air the distance readings taken underwater are no longer correct when compared to the same measurements taken in air. The refractive index of air is around 1.00 and approximately 1.33 in the water I would be using the device in.
To prove this I made a test in my bath, I put two pencil marks at either ends of my bath, with the Distox2 in the housing I made a measurement with the marks and device underwater then again with the water drained, I also measured with a tape measure for a sanity check.
The tape read 1.415 m, the dry Distox2 in the box read 1.31 m and the wet shot read 1.70 m.
The distance from the rear of the disto to the reference extension stick needs to be added which is an extra distance of 0.109 m, this added to 1.31 m gives 1.419 m which is very close to the taped measurement.
Converting the underwater shots distance requires the offset from the front of the device to the rear to be subtracted first as this is a fixed offset added by the Disto and doesn’t need to be scaled from water to air. There is also around 0.02 m of air in front of the disto before the laser passes through the housing lense and into the water and this doesn’t need to be scaled either, i’m purposefully ignoring the 5 mm of perspex as this is very small compared to the distance of the shots being taken and probably only accounts for a few millimeters of difference.
So starting with the wet shot distance of 1.70 m we need to subtract 0.134 m which is the length of the disto plus the air gap in front. This gives 1.566 m, which we need to scale by the refractive index of water 1.33, which gives 1.177 m. To this we need to add the length of the disto plus air gap and the extension distance so 1.177 + 0.134 + 0.109 + 1.420 m which is very close to the taped distance and the dry measured distance – Result !
The distoX2 manual available here:
Click to access DistoX2_UserManual.pdf
It states that the wavelength of the laser is 635 nm, using this calculator here we can confirm the refractive index of water is around 1.33xx.
https://www.staff.tugraz.at/manfred.kriechbaum/xitami/java/H2Orindex.html
The next phase is some pressure/ wet testing of the housing without the Disto installed in case it leaks then I can begin to put it into use, sump 9 in Porth Yr Ogof is the first place I have in mind for it so that data collected in Parker Series can be accurately aligned to the rest of the cave.
Daren Entrance Crawl (again…and again … and again…)
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.
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.
The distances between boxes is as follows:
Box 0 to Box 1: 79 m
Box 1 to Box 2: 41 m
Box 2 to Box 3: 51 m
Pictures, model and videos to follow……………..
Ravens Well Part 3
Its has taken some time (a bit longer then planned) but i’m pleased to say that the entirety of Ravens Well has been surveyed and photographed resulting in a complete and accurate (to the disto data) model. The same techniques and equipment that were previously mentioned have been used, some sections proved tricky for the software to understand so I have introduced the coded Metashape markers to provide visual and spatial reference, this seems to have solved the issues in certain sections of passages where the alignment was struggling.
There are some ferrous areas inside as the disto data shows some larger than expected loop closures in certain areas despite careful practice. Metal girders are visible in the far downstream area which is the bit which shows the greatest variation when overlaying my data with the previous survey performed by Axbridge Caving Group.
I’m working on a video walk through of the whole site like the previous version so I will upload this when its complete, here is a quick video overview for now, below is low resolution model uploaded to Sketchfab which you can explore with your mouse/ phone.
More Porth….
Over the summer months this year (2021) I continued to visit, survey and video various parts of the cave upstream of Upper Cave Water Chamber. The results are very pleasing because not only is the level of detail being captured far in excess of any survey that has been conducted before (at this site) but this detailed data (the models produced by photogrammetry) is also matching well with the Distox2 data which gives real world scale and orientation meaning its not just pretty pictures which I prefer to avoid.
This area of the cave lends itself very well to my process as each section is short and clearly divided by short sumps giving nice workable areas to focus on. The below image shows a comparison with the UBSS survey in plan view.
Where to go next ? I would like to improve the alignment of Sump 9 relative to the rest of the model as so far this has just been best fitted to the other data and I would prefer to link it directly. After this then adding the passages in Parker Series is the next logical step but perhaps this will have to wait until next summer.
I have uploaded the models of the three dry sections to Sketchfab, its low resolution due to the limits on the free account.
The Vice
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.
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.
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.
The below images show the same process as above but using the model derived from a combination of all 4 in and out passes.
The results for the 7 parameter shift can be seen below:
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:
‘The Vice’ Ogof Daren Cilau by cave-dive-make on Sketchfab
What I would like to do is attempt to model the entire entrance crawl in this manner…. it might take a while but watch this space !
Small Waterproof Boxes
Keeping small delicate items dry whilst transporting them through sumps can be done using a variety of containers.
Common small items that might be useful are: Spare batteries for lighting, disto’s, small cameras, or maybe some cigarettes and a lighter !
Inside ones drysuit might be the easiest option (if worn) though lithium batteries inside a suit under pressure might not be best idea and the item(s) may become uncomfortable during the dive due to migration and suit squeeze.
I have a few small containers I have successfully used to carry items through shallow sumps, pictured above are two boxes from Inglesport, an old UK400 torch body and a small home made drytube.
The smaller Inglesport box has enough room for a DistoX2 (if foam is removed) or an Olympus TG4 (waterproof itself to 20m) but not much else. This has survived many a dive to 25 m without leaking.
The larger Inglesport box I have has more room inside than the yellow one, I can fit the disto, an old android phone used for surveying, tipex and a few other small items quite easily inside, this has also done a few dives to 25 m without issue.
For deeper dives I prefer to use my old UK400 torch body, i’m not sure of the exact maximum depth rating of these torches but this one in particular has been to 50 m without issues many times in its previous life as a torch and would likely go deeper, there is the larger version the UK 800 if more space is needed. Its downside is the odd shape though the handle could be cut off to make it smaller if required.
The other item in the photo at the top is a small dry tube I made, it is untested below 25 m and has a larger capacity than the boxes or torch, I used it Mexico to take some dry clothes (merino wool leggings and trouser just about fit in it) through a sump to sleep in whilst camping. Both lids seal with barrel o rings and a large snoopy loop holds the lids in place, once underwater the pressure holds the lids in place.
Small containers like these do have a small amount of bouyancy but this can be offset with rocks or just ignored.
DistoX2 Calibration Jig
An internet search will bring up a number of devices people have created to help when calibrating a DistoX2, mostly these seem to require the use of a 3d printer or a trip to the local plumbing store. I have made a rotary cradle out of some scrap plastic material that allows the the heading of the Distox2 to be maintained whilst rolling the unit to collect the required shots for calibration to be performed.
It is two discs (cut on a lathe whilst sandwiched and bolted together so they are exactly the same size) which clamp around the outside of the Disto body. The front has a large hole for the laser and the rear has a small hole in which the rear reference point is to be aligned with.
Before use the front end (whilst maintaining the position of the reference point to the rear) must be moved so that the laser exit is inline with the centre of the circle of plastic. This was done via trial and error, the jig and Disto was pointed at a wall around 4 m away and rotated until the laser described as small a circle as possible (nearly a dot) on the wall.
Once satisfied I then collected the 56 shots required for calibration. I prefer doing this in my garden, i’m yet to get better results using targets on a cave wall.
Once the shots have been collected they are grouped then analysed in Topodroid, i’m very happy with the results and would definitely use this jig the next time I calibrate the disto
Underwater Survey Device Assessment
One of the previous posts on this website details the device I have assembled in the hope to speed up underwater cave surveying and at the same time make it more accurate than using the traditional divers compass, depth gauge and slate.
Using the Adafruit BN0055 ‘9 DoF IMU’ inside a waterproof housing as the tilt compensated compass should give a reasonable degree of accuracy but just how accurate is it going to be ?
To find out I ran some tests using a DistoX2 for comparison.
A small wooden jig was constructed that allowed easy foresight and backsight alignment of the home built device and the DistoX2 so that comparable shots could be easily collected.
The sizing of the recess in the wood is such that when the box is rotated for the foresight/ backsights and pushed up against the right and left hand edges the sensor of the BN0055 is in approximate alignment with the plastic pegs used to align the DistoX2, the BN0055 is mounted around 90° out from the long axis of the box so its raw reported bearings is around 90° different.
Forty comparable foresight and backsight shots were taken with both devices and the data entered into a spreadsheet. The first task was determining the average difference between the DistoX2 data and the box data (I should come up with a decent name for this device…) The average difference between the two was 89.56°.
The Raw data from the box was then corrected by 89.56° and re-compared to the DistoX2 data. Average difference to Distox2 and Standard deviation values were calculated.
The foresight and backsight differences were also calculated to give a quality check for the shots as the jig wasn’t moved until foresight/ backsights were taken with both devices. The DistoX2 foresight/ backsight differences were far smaller than those calculated for my home made device.
These tests were conducted on a flat surface so further tilted tests will be done to assess this aspect, overall I am happy with the results so far, I have been able to buy an off the shelf sensor and without any complicated calibrations or maths have a sensor that is able to report magnetic bearing to within a few degrees of a DistoX2.
Assuming the tilted performance isn’t much worse then any large errors underwater will come from the ferrous metal equipment carried by the diver (or in the sump) and the ability of the diver to align the device with the dive line which is another challenge itself which needs thinking about.