Having had a brief chat with someone last year regarding using a Disto underwater and having just read Duncans reply to my post in the the previous topic my interest in how to achive a successful undewater survey has been prodded.

Obviously a housing of some type is needed, what are people using? What are the constraints if any of using a Disto in this way. I was actually quite suprised when told a Disto would work underwater.

My reason for asking is that we have a couple of short sumps to survey this summer and were wondering the best way to do it.

Incidentally these sumps and most of the others in the system we are working in were surveyed in the 80's but unfortunately as is quite often the case the raw data has been lost, this would have been quite useful now.

Now for the plug!. If you have any data new or old get it in the Cave Registy Data Archive, as the years roll by this will become a valuable resource for future generations. You can even have it hidden from public view if you really want but the main thing is that it is saved in a central location.

http://cave-registry.org.uk/

Details are in Gavin Newman's excellent new film: "Tunnel"

Basically we tested a DistoX "underwater" by submerging it in the vegetable compartment from Gavin's fridge and shooting legs just below the surface of the canal at the Wookey Hole resurgence.  In favourable conditions we could achieve a range of around 5 m - the lengths were wrong but you could correct this by multiplying by the refractive index of water at 635-650 nm.

Gavin machined a housing out of aluminium with windows for the laser and display with one waterproof button and pressure tested it a depth equivalent 400 m.  A test with two divers - one carrying a target and the other operating the DistoX looked promising but we discovered that this was unwieldy and the distance measurements were frequently erroneous (though I have made it work over very short legs in good conditions).

In order to get accurate distance data we laboriously laid tape measures along the dive line and installed Ping-Pong ball floats to act as targets.  The diver worked his way upstream doing several shots per leg - initially Gavin wrote each result down but latterly I found storing it on the DistoX worked and would mark the end of each leg by one or two shots straight up or down so that it would be apparent on the download.  The depth and distance were recorded though we also found that the inclination recorded by the DistoX was reliable - i.e. the depth change between stations calculated from the inclination and slope length match that registered by the diver's depth gauge readings.

In short we used the DistoX as an accurate underwater compass with the added benefit that we didn't have to write the bearings down.

There are more elaborate devices out there: https://www.facebook.com/ArianesLine/videos/1142389219163835/ or http://www.hydrographic-catalogue.com/articles/using-underwater-lidar-for-cave-mapping-223

Thanks for explaining that Duncan, very interesting, lots of food for thought.  :-\

Swallowneck said:

Thanks for explaining that Duncan, very interesting, lots of food for thought.  :-\

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I should have added this link to bespoke underwater cave surveying device: http://arianesline.azurewebsites.net/Home/Mnemo which is tracked along the diver's guideline to measure it.

There is also John Volanthen's "lazy boy" mapper which was used in Pozo Azul and also to survey Sump 3 in Gough's Cave - this device records depth and bearing plus distance via a propeller.

Also  I  should mention that I've been using a handheld SONAR wand to measure passage dimensions underwater.

A lot depends on how accurate you want your survey to be. The standard CDG method evolved to enable divers to survey underwater using instruments routinely carried and to collect data in as short a time as possible. The accuracy of this method varies enormously; the main variable being the individual diver's skill. With experience it's possible to get within 2% misclosure.

The main errors with the above method are directional, as a diving compass is difficult to align accurately. A more reliable method was devised in the Dales which improves accuracy by having the compass screwed onto an A4 survey slate, allowing it to be aligned more precisely along the diving line. Using a surveying tape with a peg on the end allows a solo diver to produce far more accurate underwater surveys but it's slower than the basic CDG method and extra equipment must be carried. (The through route from Keld Head to Kingsdale Master Cave was done this way and managed 0.7% misclosure.) The method is described in detail in CDG Newsletters, in an article in the CREG magazine "Compass Points" and (without checking directly) I think in the CDG Manual.

A problem with using electronic devices relying on light beams underwater is that visibility is often a limiting factor (as Duncan has explained very well above). But using a Disto as an electronic compass to survey along the line seems a good idea, as it should mitigate against human error and be faster.

You say your sumps are "short". Could you lay a tape measure straight through and take bearings whilst above water from each end, eliminating the need to survey underwater? For best accuracy you'd need to measure the position of the deepest point and record the depth there - the latter is easy to do accurately with electronic depth gauges / computers. Doing this ensures your plan view isn't longer than it should be. (This "cheat" method can also work if you need one belay underwater - but any further belays would then necessitate underwater readings being taken other than the first and final legs, which can still be done from the surface.)

Devices which work using propellors obviously don't suffer from visibility problems at all but are not quite as accurate in collecting leg lengths as using Disto distance sightings or a tape measure.

The best possible answer to your question might be to consider the pros and cons of all methods, decide just how accurate you want your survey to be and choose the technique which is most appropriate for your own situation.

I have created my own device inspired by others using an Arduino and other simple components readily available in our current times.

Yet to test it underwater but it like the Nmemo is designed to be run along the line whilst the button controls the logging of data to the onboard SD card.


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If it proves successful it could of course be miniturised

That looks neat Max - I'll look forward to hearing how you get on with it. 

Pitlamp said:

You say your sumps are "short". Could you lay a tape measure straight through and take bearings whilst above water from each end, eliminating the need to survey underwater? For best accuracy you'd need to measure the position of the deepest point and record the depth there - the latter is easy to do accurately with electronic depth gauges / computers. Doing this ensures your plan view isn't longer than it should be. (This "cheat" method can also work if you need one belay underwater - but any further belays would then necessitate underwater readings being taken other than the first and final legs, which can still be done from the surface.)

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I was thinking along the same lines (no pun intended) I think will be enough accuracy for what we need. I shall ask my surveyor!

Pitlamp said:

You say your sumps are "short".

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This is exactly what we did for Ogof Capel (before the sumps were bypassed).  A tape measure was laid through Sump 1 with a shot from either end to the elbow (which we measured the depth and distance to).  Sump 2 was shallow, straight and short enough to just run a tape through it.

You say your sumps are "short". Could you lay a tape measure straight through and take bearings whilst above water from each end, eliminating the need to survey underwater? For best accuracy you'd need to measure the position of the deepest point and record the depth there - the latter is easy to do accurately with electronic depth gauges / computers. Doing this ensures your plan view isn't longer than it should be. (This "cheat" method can also work if you need one belay underwater - but any further belays would then necessitate underwater readings being taken other than the first and final legs, which can still be done from the surface.)

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This is just what we intended to do as we have a line of sight in and out of the sump which is approx 3m deep x 4m long. To make it a bit more awkward the sump has a cross rift which we need to measure and to cap it all off our friendly Transilvanian diver found another small passage leading off the end of this rift last year so a disto in a housing combined with a tape does seem a good idea to get a better survey.

Thanks for all the feedback, when we eventually get around to doing it I shall report back.