How to measure a guideline precisely?

[hxtk]

I'm making a map of a local dive site, and I'd like it to be with more precision than I can reasonably get by counting kicks. I've got a primary reel with knots tied at regular intervals for measuring distance, but there isn't an abundance of available tie-offs. Most of the permanent guideline here uses PVC pipes driven into the silt as tie-off points, and in most cases there is not enough space to simply share the tie-off point with the permanent guideline.

The local topography is such that for much of the site I could just not make tie-offs and simply measure out radially from my primary tie-off without running the risk of unexpected line traps, but that doesn't really feel right. Maybe that's my best option, though? Also, that assumes I never go more than 320ft between two suitable tie-offs, but at the scale of this map I'm not sure that's a realistic assumption, and it would be rather hard for me to justify getting an exploration reel just for this project.

 

[tbone1004]

how far between the silt stakes? Easiest way to survey is to use a big survey reel. They're usually less than $30 for a 300ft tape measure. Grab one of those and survey between the silt stakes for your map then you can plot it on the computer to get the rest of the distances. If you have to go farther than that, you get a buddy and you use tug signals on the tape. So you reel it out until it's at the end, mark that spot on the ground with something, mark that spot on your survey, then measure from that spot going out. A 1-2lb colored weight works well for bottom marking
https://www.amazon.com/Komelon-6633...=1540327839&sr=8-3&keywords=open+tape+measure

 

[kafkaland]

how far between the silt stakes? Easiest way to survey is to use a big survey reel. They're usually less than $30 for a 300ft tape measure. Grab one of those and survey between the silt stakes for your map then you can plot it on the computer to get the rest of the distances. If you have to go farther than that, you get a buddy and you use tug signals on the tape. So you reel it out until it's at the end, mark that spot on the ground with something, mark that spot on your survey, then measure from that spot going out. A 1-2lb colored weight works well for bottom marking
https://www.amazon.com/Komelon-6633...=1540327839&sr=8-3&keywords=open+tape+measure

There are a couple of options. One is the aforementioned measuring tape. I personally like it, but it has a bit of a bad rap among cave surveyors for reasons that aren't quite clear to me. An alternative for measuring distances on an established line is to make a precisely measure a short piece of string, perhaps 2ft. long, and then using it to measure the length of the permanent line by moving it along the line in a hand-over-hand fashion, and counting how many times the measuring string fits between tie-offs. Or, if you want to go high-tech, there is a measuring device by the name of MNemo, which you can clamp on and move along the line, and it automatically records distance, heading, and depth. If you know a regular cave explorer or surveyor, you might be able to borrow one. For surveys off the existing lines, your best bet is probably still the measuring tape or knotted line.

 

[Diver0001]

Since it's a large site I would tend to apply a totally different technique to it.

For my Divemaster we were asked to map a site to 80 meters on either side of the entrance to a depth of 15m at a site with average visibility of about 3m. Since the bottom sloped off slowly the 15 meter line was nearly 300m from the coast at some points.

We initially started mapping the site using a reel with knots in it. Early on we realized that we needed a lot more line so we started mapping using X number of reel lengths.....

When the coin fell was after making a few dives and seeing nothing and having calculated that we would need to make 20-25 more dives to map the entire site like this. We realized that most of the site was going to be featureless sand, so I went back to a previous day's data points and re-swam it again using a grid that I drew on a slate and just mapping depths at given intervals of time (each minute) while trying to swim at a constant rate and direction. The conclusion was that this was just as accurate as measuring everything by using a line because of the large size of the site and margin of error using either technique. The difference was that by not fiddling around with a line we could cut about 15 dives off of our plan.

So we finished the assignment using the grid on a slate and just noted times and depths, putting it all together on the computer later.

We were right to assume that most of the site was featureless but where was one significant feature in the area, which we went back to later and made more detailed notes about. This took 1 more dive. The resulting map covered an area of roughly 160m/300m and was pretty good by my reckoning. We started poorly but managed to find a way to map it out fairly efficiently without using any guide lines. Just a grid and bottom timer and the ability to swim at a fairly constant speed.

Of course the instructor claimed that giving us this insight was exactly the point of the exercise but I think he just didn't realize how far off the shoreline the 15m depth was.

Incidentally, while we thought we were being very clever by developing more efficient techniques as we went along another team mapped the site from a boat using a fish finder in one afternoon and only made 1 dive to check out the only interesting feature..... so I kind of learned 2 lessons from that....

R..

 

[txgoose]

Incidentally, while we thought we were being very clever by developing more efficient techniques as we went along another team mapped the site from a boat using a fish finder in one afternoon and only made 1 dive to check out the only interesting feature..... so I kind of learned 2 lessons from that....

R..

Spirit of the law vs. letter of the law? I'd say that you did it well.

 

[tbone1004]

oh, also of rather important note, please remember that nylon stretches considerably when wet so if you are trying to get very accurate measurements, you need something that doesn't stretch.