maxwellfisher

13 August 202114 August 2021

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.

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:

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 !

15 July 202115 July 2021

Ravens Well Part 2

Flythrough of updated model

This is a follow on post from this one started earlier in the year:

Lock Down Projects

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.

16 May 2021

Porth yr Ogof Sump 9

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:

Click to access UBSS_Proc_15_3_259-0.pdf

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.

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.

Survey and model video to follow….

10 May 2021

DIY Helmet Mounted Caving/ Diving Light

There are multiple options for cavers who like diving through sumps to further their caving trips to choose from in the lighting department, the likes of Scurion, Phaeton, Rude Nora and other manufacturers have been making suitable lights for years.

This being the making category of this website you can probably see where this is going !

Sometime in 2018 I discovered the LED driver board (the heart of any modern lamp) for the Phaeton was available for purchase from its designer/ manufacturer in the States which is:

https://www.taskled.com/

I duly ordered a couple of boards, some CREE leds cobs and a suitable switch (tricky to track down and a few month wait for stock). Once these had arrived I soldered the bits together then proceeded to procrastinate for a while whilst trying to figure out how to solder and assemble the 20 pence piece sized board inside a small water proof housing.

This went on to the point where I became more interested in other things and forgot about the parts I had, occasionally coming back to the problem but finding no solution.

My whole cave/ sump diving career has involved the use of a hotch-potch of different hand held lights attached to my helmet, great for redundancy but heavy out the water. For caving trips only I would remove these lights and attach a Petzl MYO which is nice and light in comparison.

I’m not one for caving with super bright lights, its nice to have the option to occasionally use full beam to light up distant parts of passages but for the most part i’m happy to cave on dim settings. During the late summer of 2020 the diggers of Wookey 20 (website in links) had broken through into what they have named ‘The Land of Hope and Glory’ in which was an enticing aven was found and to be climbed by myself and the bolt climbing veteran Tom Chapman.

We duly arrived with equipment and the other proceeded to use their highly powered caving lights to light up the roof of the aven some 30 m above. I set my Petzl Myo to full and could barely see anything !

This lack of power is what reignited my want for a powerful light that was waterproof enough for any diving I had planned and brighter than what I had already. Its main use would be for caving the other side of sumps, centrally head mounted lights give lots of backscatter so are of limited use as a primary light underwater in normal British cave diving conditions.

The parts I had obtained in 2018 were dusted off and I had a fresh look at the problem, mainly the soldering and assembly in such a confined space to keep the overall size and weight of the light down. No futher progress was possible with the idea in my head or in CAD modelling. so I decided to just start making a housing and and to try and assemble it and make it up as I went along.

Strangely, once I had roughed out the housing the ideas started flowing and after a few eureka moments I found a way to assemble it so construction started in earnest.

The body is made of Acetal, with a 5mm Polycarbonate front and an Aluminium heat sink for a rear both sealed by o rings. The battery box houses two 18650 batteries and is made of Acetal.

After having assembled a working version in Autumn 2020 I took it for a few trips, although the light remained dry it worked very nicely in Raven’s well, it didn’t overheat which was one of my concerns using a plastic body and metal backing plate and the combined spot and flood lenses gave a nice pool of light to cave with. A more testing trip a week later in Swildon’s Hole involving free diving to sump 6 however proved it to be less than waterproof. The light worked really well on the trip and owing to the conformal coating I had put on the exposed electrical connections it continued to work even when wet inside but on inspection on the surface it had a fair amount of water inside given the shallow and short nature of the sumps.

The possible source of the leak perplexed me for a while, I kept taking it apart checking things then taking it for a dive only to have it consistently leaking. Whilst descending from the surface with it in my hand so that `I could visually see where the water was coming from showed that it was leaking by the switch which has an o ring seal on.

Detail of switch showing threads not cut up to sealing o ring

Taking it apart and drying it again and inspecting the switch showed that the threads didn’t go all the way to the sealing surface on the switch lip, meaning that when it was screwed down onto the body it couldn’t go down enough to actually engage the o ring and the source of my troubles. These switches are meant to be mounted in a panel using a clearnce hole for the threads and a nut the other side instead of being installed into a threaded body like I had done. What I needed to do was cut a small relief diameter to the top of the threads on the light body so that the switch could be screwed all the way down to compress the O ring and make a seal. I tried this on the light I had made already (v2) but messed it up so version 3 was ‘born’ with a funkier cut away shape and the extra clearance for the threads.

Version 3 from the front with lenses removed

Pressure testing the light on dives and fixing it in between had become a rather exhausting process so I decided to fashion a small pressure pot out of some clear pipe, some end caps I had used on a small dry tube previously, an old bike inner tube valve and a bike pump. Using this setup I was able to cautiously test it to 80 m depth in my back garden.

I am happy to report some months later that after a few diving trips it has been working very well, I have since changed the LED’s to those with a warmer colour temperature, I find this easier on the eyes. Caving with the light on the second dimmest setting gives ample light and even with the occasional bursts to full I am getting many hours before I am having to recharge it.

I have it mounted to my original Petzl Spelios helmet (with the Duo removed) and found that due to the foam in the helmet when inadvertently left in a sump pool the whole lot floats which is a nice benefit, no more worries about dropping and loosing a helmet into the murk.

2 March 20212 March 2021

Dead Reckoning

Estimated track overlaid with scaled image from Google Earth

Having created a dive proof device that can accurately measure and record magnetic bearing, depth and other useful information like temperature, pitch and roll I was keen to see how it might perform as used to plot the route covered underwater by a diver. It was still without distance measurement capability at this point but assuming a fixed speed is maintained then this isn’t an issue, so I chose to strap it to the front of my Aquazepp and do a test to see if this idea was worth further pursuit with the hardware I have.

A device like this has been created before and carries the nickname of the ‘Lazy Boy Mapper’ developed by John Volanthen around 10 years ago and was used to great effect in long and deep caves where normal survey is not practical . Commercially available devices are also available by the like of Seacraft though these stretch into the £1300 region.

http://www.pozoazul-cavediving.org/index.php?option=com_content&view=article&id=16&Itemid=58

Around Christmas 2020 we were emerging from tier 3 lock down but yet to be dropped into tier 4 so a flurry of quarry diving activity took place due to the weather and ban on entering Wales for fun purposes.

I reprogrammed the device to record data continuously which differs to how it was setup to work when used to record lines used for cave diving. Still lacking a form of distance measurement this was intended as a test to see how it would perform and if this idea was worth further pursuit, scootering was done continuously and in one speed so that a fixed estimate of speed could be applied later on.

A rather chilly two laps of Cromhall Quarry were done (in a semi drysuit) at maximum speed. The data is quite encouraging for a first attempt.

The dive begun with a lap around the perimeter before looping through the container and maintaining a fixed bearing across the middle section at a deeper depth, this was followed back on reciprocal where another loop through the same container was done, a reverse lap around the perimeter was completed before finishing with another loop through the same container before exiting.

The ability to scoot through a fixed object like a container provides a reference point to where the data should match up, without this it is difficult to know where you have covered the same ground which allows an assessment of accuracy to be made.

In the image below it can be seen that the start and the end of the dive don’t agree location wise even though i did start and finish the dive in the same place, the rest however appears to have worked very well ! A fixed speed of 60 meters per minute was applied. Speed through the water will vary slightly with subtle changes in diver position throughout the dive so differences are to be expected.

The magnetometer quality factor which should remain fixed on a value of ‘3’ after calibration was changing throughout the dive so this might have been caused by the proximity to the batteries or motor on the scooter so this might be a cause for the drifts seen at the start and end of the dive, it seems to cope with short exposure of large metal objects like scootering through metal containers quite well (it is an intelligent sensor not just a compass so should be able to filter out temporary disturbances in magnetic field).

Future testing and improvements will involve:

Mounting the box on the nose of the scooter, as physically far as possible from the diver, motor and batteries.

A form of speed/ distance measurement will be added.

Calibrating the IMU whilst the box is mounted to the scooter in the hope that it accounts for the field of the scooter.

Align the heading of the box better with the heading of the scooter.

14 January 202115 January 2021

Lock Down Projects

Well here we are again, in the throws of another lock down. Travel from your local area is frowned upon, all venues (apart from the sea) are closed for diving so to keep stimulated both physically and mentally I decided to experiment with some underground (local) photogrammetry.

My experience of photogrammetry is limited to mostly following others around underwater whilst they photograph things, a few work projects and some failed underwater attempts in a cave (perhaps a separate post on this later).

For anyone thinking of taking it up I have had very good results with mobile phone cameras, Gopro’s and cheap lights, the process is very simple to perform on a computer so I would encourage people to have a go, the software can be trialed for free so expensive equipment is not necessary.

So back to the lockdown; fortunately I have a site I can access within walking distance of my house in which I can experiment, it is an old water conduit known as ‘Raven’s Well’

Armed with a set of cheap waders from Ebay (its waist deep in some places), a Gopro hero 3+black and two cheap video lights I set off to capture some photos to see how well I could model a part of the site.

I set the Gopro to take a still image every 0.5 seconds, put the lights on full and set off walking slowly around the passages near the entrance with the camera pointing forwards. Care at turns was taken to ensure that lots of overlap was achieved. There is a loop that can be traversed so I walked around to see if the software was able to accurately ‘close the loop’ a fundamental part of survey data assessment.

I went round the loop twice in an anti clock wise direction before heading downstream to the low section before returning to the loop and completing the loop again twice in a clockwise direction, this amounted to 1237 photos, just over ten minutes of photo capture. I have collated them into a short video so the quality and coverage can be seen.

Source Images

This amounted to just over 4 GB of data, the details can be seen be below for the Jpeg images for the photographically minded.

Images can be harvested from video but they lack the metadata that comes from still images so I find this approach easier provided you take enough images first time around, with video you can extract more frames without revisiting the site if required.

Photogrammetry is a computer intensive exercise so before I pressed the ‘Go’ button on the whole set of images I tested a single loops worth to see if what I had captured was going to be worth the wait for processing, this took about two hours to go from raw images to dense cloud, I was happy with the result, it failed to close the loop but had modeled the shape and course of the passages very well, see the below image.

The above image is a plan view of the dense point cloud created from one walk round the loop. The areas circled in red are the same physical areas and should join up however at the area highlighted with a blue line (the first corner) it has failed to adjust for the camera heading change properly which can be seen by the ghost walls, if this piece is manually cut and swung round it allows the areas in red to overlap. I was encouraged enough by this to select all the images and pressed the ‘go button’. After all I had 3 more traverses of the loop and hopefully the addition of more images would help it close properly.

This was a much longer process, which took around two days (Macbook Pro running Windows 7, 64bit, 16 Gb Ram, i7 2.9 Ghz). Waking up to a silent laptop (the fans goes into over drive when its processing) on the second morning I was pleased to see all images had aligned and it had finished so I loaded the dense cloud and started to inspect it. I was very happy with the results, the loop had closed and the passages appeared as they should. The image alignment was run on ‘Medium’ and the Dense Cloud was set to ‘Low’. More detail could be processed at the expense of processing time but for me this is good enough.

The first job once the initial overview had been completed was ‘cleaning’ the water out of the floor, most of the areas have a wet floor and its unsurprising that it struggles to model a constantly moving, colour changing body of water so these points were manually selected and removed. Once this had been completed the mesh and texture were computed, taking just a few hours. Below are some selected views from inside the model, I am working on some sort of video or fly through to be posted when available.

Looking towards entrance down brick roofed tunnel

Future work will involve covering the rest of the site and geo-referencing the data to the real world as an arbitrary scale and alignment is applied straight from the software.

26 December 202026 December 2020

Underwater Testing

Sometimes its one of those days when nothing goes your way (of course you are still happy and healthy but the given goals for the day are unachievable), I set out last week with the aim of thoroughly testing the survey box underwater and having a good ride around on my scooter before the onslaught of a dry Christmas.

A short line course was laid underwater and surveyed by a friend with a Mnemo survey device (part of which is the inspiration for my own creation). I began taking foresights and backsights with my box (having added two line clips to allow hands free alignment with the line. Unfortunately on logging shot number 13 (unlucky for some) the display somehow went back to front then froze and refused to continue to work. I had planned to survey the course three times for further device assessment and comparison against the Mnemo but was unable to complete one entire loop.

Survey box line clip and count down modifications.

We were planning a dive of around 2 hrs, a bit of surveying then some scooter fun, having had to abandon my surveying after 20 minutes I decided to take my scooter for a run wanting to assess how much battery life I might have available in its top speed. Clipping off the tow cord I took off but every few seconds the motor cut out then restarted, making the already distinctive Aquazepp sound even worse. I quickly decided that it was inconvenient but didn’t seem to be doing any harm so stopped next to a known landmark then took off for a lap on the turning of a fresh minute of the dive time on my computer.

It took 20 minutes to complete a lap which is known to be about 1100 m at the depth is was taken at. Arriving back at the line course I decided that I was against any further scootering but would surface quickly, reboot the survey box then head back to the line course to complete my goals.

Surfacing went fine, kept kit on, isolated my oxygen supply, quick minute with a handy screw driver to open the power port on the survey, rebooted and re-calibrated it, all fine so closed it back up. Mask on, oxygen back on… PSSSSSSSSSSSSSSSsss the hose had popped off of my Kiss MAV 🙁

Kiss MAV thread failure

I thought it might have just come undone to start with but after a few tries it was clear the threads had gone and that was the end of the rebreather diving for the day, I had left my scooter underwater so went back in on my open circuit bailout to retrieve it.

To top the day off there was a power cut so no hot drinks or food were available 🙁

The list of annoyances goes as:

i) Frozen survey box display and Arduino after 13 shots (hopefully a random occurrence)

ii) Scooter not running smoothly (investigation ongoing)

iii) Kiss valve thread failure (its quite old and known to happen so…)

On arriving home and sorting out gear I was still able to download the 13 shots I had taken before the device froze, after the addition of the line clips I’m happy with the few foresight and backsights I was able to take, the first belay was metal as well as one in the middle so those are expected to be bad but the rest agree quite well I think, further testing to be done to confirm.

So taking away some positives…. I was able to complete a ‘fast lap’ on the scooter and after charging the batteries have discovered that 24 minutes of use used 2500 mAh from each battery giving about 160 minutes of burn time at top speed. The data I was able to collect with the box is encouraging and it seems to be able to record foresights and backsights to within a few degrees away from large metal objects. The Kiss valve failed on the surface not underwater. Further testing planned….

10 December 2020

Rebreather Oxygen Sensor Isolation

Sharing the output from one oxygen sensor to two or more monitoring systems on a rebreather is a popular way of increasing redundancy (and complexity).

This does not cover cell failure issues or corrosion problems but if done correctly allows monitoring redundancy to be added in a shared cell monitoring system.

A few commercial options are available from the likes of Revo, Narked at 90 and Ocean Opportunity. Some include a single resistor, some use two resistors I thought an experiment was in order to check what the difference might be between the single and double resistor approach.

I used an old home made PP02 monitor I made some years ago courtesy of:

http://www.ppo2.com/Displays_1.htm

It uses a very sensitive DPM and has three digits after the decimal place unlike most modern dive computers (the dancing digit).

Three circuits were tested, the first with no isolation to prove the need for it, the second with a single resistor on each split and the third with two resistors on each split (one on the +ve, one on the -ve) the results can be seen in the video below:

Rebreather Oxygen Sensor Isolation test video

All tests were done in air close to sea level, results may differ with increased PP02

The display was re-calibrated each time a set of resistors was added.

The best redundancy comes in the form of 3 cells and 3 separate monitor systems like the original Kiss Classic displays.

28 October 20203 November 2020

Small Waterproof Boxes

Dive proof containers suitable for taking small items through sumps

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.

30 September 202030 September 2020

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.

Front end with clearance for laser beam, rear hole to align with reference point

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.

Using a plastic mitre to keep heading constant for flat shots

Non metallic objects used for angled shots wood held together by dowels not metal !

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