Alastair(Offline)

Hi All,

Just a quick update on our practise day at Vobster. In the end 5 of us turned up to run through survey techniques.

Dive 1 we had two teams. Team 1 was Clare & David - who layed a line course which David then surveyed. Joe played sheepdog in team 2 while Marcus and I surveyed.

On dive 2 we again had two teams and I spent my time switching between the teams while shooting video. Team 1 was Joe and David who surveyed the line but this time took a measuring tape to see what improvement of accuracy we got compared to the survey on the first dive. Joe ran the survey while David used the tape. Team 2 was Clare (survey) and Marcus (tape).

I felt we learnt several lessons which will help on the Unknown Sailing Ship dives later this year. First off was that the cave survey sheets while good had boxes to allow you to indicate the size of the cave passage. We might be better off using a slightly different format to more easily identify wreck features.

Secondly we seemed to find that any point where we created an intersection with another line with a line arrow we were best off treating that as a new station on the survey. It would lead to less confusion when it came to mapping.

David’s work knotting line at 1m increments was very successful and did seem to offer us enough accuracy combined with estimates to let us produce a solid map. If needed though we can take the tape measures on a dive. A couple of the others commented on having their hands full so I think the idea of the compass mod I posted about the other day would be useful.

Clare and I spent both dives on the RB80’s which was good opportunity to keep building the hours on the units.

Anyway here’s a quick cut of some of the video. It’s the first one I’ve done other than some skill demos so don’t be too critical!

YouTube - DIR-UK Survey Practise April 2008

Cheers
Al

__________________
Helium is our friend - GI3

neilh (Online)

That's interesting but not too surprising I guess - do the NAS have anything similar to the cave survey sheets at all? I would have thought someone out there would have done something like this?

Joe Hesketh (Online)

NAS do, but this is typically a large A4 sheet pasted onto a chopping board . Also, their detailed survey techniques involve using tape to take multiple measurements of individual points on each object. Each point is given a 3D position by measuring it's distance from at least two known fixed points on the wreck, which is impractical for our project.

Fred's sheets are fine, after all what we're doing is much more akin to cave survey than what the NAS generally do. We just happen to be laying it over a wreck site rather than through a passage. Al's really only talking about taking out some of the boxes and making others bigger, that's all.

Joe

neilh (Online)

If you used one with a handle then couldn't you rig a leash/boltsnap and job's a good 'un?

Fair enough, but can I ask why the NAS approach is impractical in this case? I know very little about this stuff and find it interesting, so I'm curious to hear why you're having to adapt the techniques.

David Martin(Offline)

We brought a couple of boards along just in case last autumn. Funnily enough, even with the boltsnaps, they stayed on the boat (and not even the boat we dived off, but the boat left in the harbour).


The NAS method is very accurate, but also very time consuming. It also relies on being able to add permaent markers/tags as control points (which remain in place for the duration of the project (ie years) and temporary markers to each data point being surveyed. The method also works best (ie pretty much requires) the two divers to be at opposite ends of the tape measure. In the sort of vis we have, this would impose a maxium limit of about 5m for a measurement. Each point should, under the NAS method be measured from 4 other points (3 are required for 3D data, the 4th being for redundancy/error checking). The area we laid line accross last year was approx 65m long by 40m wide, we'll be extending that this year. We just couldn't cover the site with the NAS method in the available time, even it was possible to hammer permanent markers into the solid seabed for control points.

With the distance/bearing method, we'll should get reasonable data, which combined with video/stills is more useful than a very accurate survey of a small portion of the site.

David

neilh (Online)

Thanks David, interesting stuff - presumably this is a problem that NAS would also encounter, especially around the UK. Are they interested in your findings and techniques on this project or is it more the case that typical NAS projects have enough funding and time available for it not to matter?

LauNar (Online)

We tested a pretty quick but efficient method for getting a good idea about the layout of a wreck site last summer. It only works for fairly flat sites, though. We only tried this once on a short weekend project, but the results were quite promising:

1. Lay lines or tape measures across the site, in the same direction, so that the distance between lines is about 1-3m, depending on visibility. We only used two lines, but the wreck area was smallish. A bigger site might need more lines.

2. Mark the lines with arrows every 2m, numbered or otherwise easily identified markers would be nice but not necessary.

3. Photograph swimming above (1-2m) a line with wide angle (100degree non-fisheye is pretty good for this), shooting at a downward angle (about 45deg). You can't really take too many photos here, a photo/arrow is minimum, but a photo between each arrow won't hurt. Ideally you'll see the left and right lines at the edges of the photos.

4. Organize the photos so that they can be viewed quickly after each other.

The result is something like this and this. It gives you a nice "diver's perspective" view of the site, and is quite quick to perform. It has some advantages over video:
-more detail
-a lot smaller file size
-easier to move back and forth
-very few video housings have wide enough ports/lenses

Obviously the lines will also help with drawings and mosaic photography, but those methods usually require somewhat more bottom time and a lot more postprocessing to get presentable results.

__________________
//LN
--
My UW photos

vid(Offline)

Many NAS projects are much shallower than this, so the sophiticated survey technique becomes "less" of a problem, relatively.
NAS certainly don't "have enough funding and time available for it not to matter", and I'm sure they'd be interested in how the survey is going.
I think it might be worth asking if this could be considered under the "adopt a wreck" program.

vid

stevechesh(Offline)

I read the post so i had a look on the NAS website about this scheme and alarm bells starting ringing.....

quote from here: NAS how to adopt a wreck site
-----------------------------------------------------------------------
The 'Adopt a wreck' programme and awards scheme is part of a wider 'Dive with a purpose' initiative by the NAS to encourage divers and others to appreciate and preserve our underwater heritage. The hope is that groups will look at a site more closely, and take on a minimum level of stewardship, monitoring how the site changes over time.
-----------------------------------------------------------------------

I can't for the life of me understand why this particular wreck needs "adopting". It's been researched for 20 odd years, and every dive on it has been for the purpose of identifying it. What does a minimum level of stewardship mean? Is it the start of controlling dive sites, as has happened on so many occasions previously? There is a document doing the rounds called Benchmarking Competancy. This involves the NAS and English Heritage (EH). A word of caution here - EH have relied upon, taken and used info provided by divers for there own gains, then shut down dive sites, claiming the work as there own many times. Once these people get involved, you'll have a nightmare trying to dive this or other similar sites like it - and there are many.

My advice would be to steer well clear of these organisations - certainly for the time being at least - and carry on doing your own research. I've researched many wrecks over the past few years, and this one in particular for the last 2 and there are a few names in the frame.

No survey will determine the name of this wreck in my opinion. It will be determined by the cargo alone, or a lucky find like a builders plate. That said, i don't think a survey is a bad idea, but if it was my project, i wouldn't get too hung up on the in depth measurement details. You've already got some pretty important info. The approx length and a beam measurement - However, no sailing ship in the world had a 40 metre beam - You'd be looking at really no more than 10 metre beam maximum, but realistically, anything between 4 and 8 metres would be about right for a sailing ship of this time.

I've told a few of the guys involved in this project particular items to look for and lift. These items frequently have manufacturers markings and dates on them that will help with the ID. I know these items are on the ship, because i've seen them, but didn't have the equipment to lift them myself at the time. All my lift bags had already been sent up.

I've already had two trips canned to this particular wreck this year, but i'll get out and have a look again soon. The stern section that has collapsed is the area to look at in my opinion. It needs a good dig down into the shale to see whats under there, as the bits and bobs on the top of the wreck aren't revealing too much at the moment.... Start digging and lifting and you'll be rewarded.......

David Martin(Offline)

I'd agree that a survey won't tell us what the wreck is. However, creating a map of the site allows us to record the other features, helps to navigate round the area, explain the site to new team members, and provide a focal point for a display/presentation of the project work.


The extent of the site we looked at was about 40m accross - this includes certain items of cargo and rigging to the East of the main wreckage. The smaller wreckage extends well beyond that.

I'm not too hung up on exact depths. However, although the height above the sea bed won't really tell us that much in a lot of cases, it would be quite significant if trying to plot data points on distance measurements alone - the wreck sticks up about 3m or so - which is significant in the context of measurements accross a 4-10m beam (I would think that ignoring it would lead to much greater inaccuracies than chosing the less accurate method).

The NAS did seem quite interested in what we were planning on doing and also in discussing how we felt (even last winter before really starting on the site) how we might need to adapt some of the techniques discussed. I think though, that the NAS is generally going to want to encourage people towards accurate survey techniques - after all it's what they're about. We have a limited time on the site (due to relatively short bottom times imposed by conditions and relatively few trips per year imposed by weather, costs etc) a more simple survey in conjunction with other activities seems a more appropriate use of this time. I think time is a more limiting factor on our dives than on many other sites the NAS might work on, both in terms of bottom time per dive and also number of dives per year. One is also clearly slower when swimming/turning etc if in doubles plus 3 stages than if just in a single.

David