Well its been a little longer than I anticipated updating this page with what I have been working on but rest assured this isn’t due to a lack of activity more a lack of internet at home.
Numerous projects have been going on both on the surface and underground but I thought I would start with some updated data from Ravens Well. For Christmas I treated myself to a small drone (the DJI Mini2) mainly so I could add some surface features to the underground surveys I have been working on though flying it around aimlessly is equally fun !
The learning curve I found to be very easy and by lunch on Christmas day I had produced a model of my house. After a bit more practice and experimentation I was ready to capture the images required to create a surface model of the area above Ravens Well around the three lamps junction of Bristol, ever curious each time I walk/ cycle/ drive over the top as to where the tunnels are exactly beneath this would provide the answer more accurately than just overlaying the underground model/ survey roughly in Google Earth as the Axbridge Caving Group have done.
Ravens well and surface model of the three lamps junction
The flight took about 20 minutes well within the drones ability and Metashape processed the images very efficiently. I used RTK GPS to measure the positions of some prominent features on the ground mainly road marking which were clear and well defined in the point cloud. These were then matched up in CloudCompare resulting in a reasonably accurate surface model of the area. I once again scrambled down to the entrance to the tunnels and to my surprise found that I had fixed RTK status next to the entrance so I marked a temporary point and quickly surveyed from the GPS point to a point marked inside so that the drone data and underground data could be tied to the same reference and overlaid.
Surface point being measured with RTK GPS
Same point in model with tunnels visible underneath
Overall a successful addition to projects, easy to do and gives a better understanding of the relationship between above and underground features.
Three lamps Junction area with tunnel underneath (long shadows from surface features caused by low winter sun)
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.
DistoX2 in underwater housing
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).
Box ready for testing
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.
Button making on the lathe
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 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.
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.
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.
Coded Metshape Target
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.
Plan view of the Distox2 data in Survex
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.
Video Overview of Model
Low resolution Sketchfab upload (Due to free account limits)
A few friends have been working at Rickford Rising removing large boulders and making progress downwards from where the previous protagonist left off some years ago. I have always fancied a dive here but had never got round to it for various reasons. It usually has good visibility so I thought I would attempt to create an accurate photogrammetry model of the site as a snap shot of progress at this point in time, and hope to go back and update the model as progress towards the Burrington Master Cave is made !
A video has been put together by the digging team and gives a rough overview of the site:
I made four short dives on my visit, facing head first slowly filming the descent from surface to dig face allowing sufficient time for the visibility to clear each time and a survey dive (after having planted the yellow builders square on the first dive).
The builders square was used to provide a scale reference (the lipped edge is 12″ or 0.31m long) and also to provide a fixed straight edge along which I could repeatedly align the edge of my survey box with to make foresights and backsights against to allow the model to be orientated correctly. Accurate depth of the square was also measured, 6.8 m to the yellow surface on the day but this will vary with flow and water level effects.
End of the dig and planted builders square (krab is ali)
I have setup the survey box so that it logs data continuously, this allows the diver to retreat whilst measurements are taken minimising any magnetic effects of steel cylinders or other dive gear, this is apparent in the good agreement between foresights and backsights taken, they are no worse than what I have measured on land away from ferrous materials. The upper part of the dig is festooned with scaffold and other metal things.
Survey results
The survey data was input to Survex and corrected for local magnetic deviation, a resulting bearing of 161° to was calculated for the lipped edge of the plastic square.
The model was processed without issue and was manually scaled, rotated aligned and translated with the survey data measured and the entrance location provided by the cave registry in CloudCompare.
The video from one of the dives can be seen here, attempting to slowly film the waters surface from underneath, down the rift to the current dig face.
A flythrough of the model is available to view below:
The image above shows progress so far, colour coded by area
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.
Historical comparison (UBSS on left)Plan view, blue line where visible is the DistoX2 centreline data Side view, blue line where visible is the DistoX2 centreline data UCWC looking downstream from Sump 8 (textured model) Cobley crawl looking downstream into Sump 8 (textured model) High level muddy tube (textured model) Sump 9 looking upstream (textured model)
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.
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:
In the mean time I have added some survey data with a Distox2 to align the model to giving it scale in meters and alignment to magnetic North. The results were quite pleasing from a surveyors point of view aligning to with 0.2m for the 4 points used.
Alignment was achieved using a 7 parameter transformation in CloudCompare meaning the model is now more than just a visual representation. Work is ongoing to complete a model for all passages at this site.
A trackplot for the flythrough was created and added as an overlay to the video to give an indication of location at any given time, this was done in DashWare, an excellent free video editing program.
Porth yr Ogof is a cave situated in the Brecon Beacons in South Wales frequented by divers as well as dry cavers, I believe it has the largest cave entrance in Wales.
The large bedding plane entrance to the main cave
The naming convention is rather confusing in that to reach sump 9 you start in sump 3 at the Tradesman’s entrance, dive through sumps: 3,2 and 1, before exiting the water for a short section of passage (Upper Cave Water Chamber), before diving sump 8, crawling for a short while before diving sump 9, the subject of the post. It is also possible to start in sump 1 entering via the Top Entrance negating about 150 m worth of diving by missing sumps 2 and 3, information and an old survey can be found here:
Late in 2020 I visited Parker Series which is the dry section of the cave guarded by sump 9, after diving through it struck me how ideal a place it was to attempt to create a model using photogrammetry. The water is usually clear and the passage is mostly formed as a small tube meaning cheap lights and cameras would make a reasonable job of it.
Source Images from first attempt
I set off on a second trip with my Gopro Hero 3+ and Ebay video lights and moving very slowly captured stills every 0.5 seconds using the interval function. It took around 300 images to cover the sump each way. The Ebay video lights I used do have an odd colouration where they overlap but this doesn’t bother me, it might bother photography purists but my main intention is to measures the size, shape and direction of the passage and for this true colour representation doesn’t matter.
Typical source image, taken about midway through the sump
Loading the 300 images into the software and processing the images yielded a nice accurately aligned point cloud of the sump, I didn’t cover the three side passages on this occasion so these are missing but the smooth curves that the passage follows can be clearly seen, the UBSS survey lacks detail in comparison.
Plan view of model
Whilst a nice representation of the sump has been created it bears no orientation or dimensional reference to the actual passage. I visited again with the intention of covering the side passages and adding a means of aligning the data to magnetic north, absolute depth and scaling it so that distance and size was more accurately represented.
To do this I took a plastic builders square (with weight attached) and my survey device back to the sump, I dived a short way into the sump before placing it on the floor and returning to the start for my camera. I then took photos through the passage as normal this time covering the builders square on the floor. I continued to the first side passage and tried to cover as much as possible of it. Once this had been done I placed my survey box on the raised edge of the square to be used as a heading reference and took 2 for-sights and 2 back-sights. I had programmed a timer so that I had time to retreat a few metres for each shot so as to minimise any magnetic distortion from my diving equipment being close to the magnetometer. The fore and back sights agree quite well so it was worth doing this.
Plastic builders square with survey box used to measure heading and depth.
I repeated the process towards the end of the sump so that I had two heading references (one to align to and one to check against), the depths of the centers of the yellow square were measured to align the depths to and the edge of the yellow square was used to estimate scale from. Once out of the cave the photos were then processed to generate a new point cloud containing the yellow squares which were then rotated, translated and scaled to the ‘correct’ figures.
