Mapping a Reef

[mccabejc]

Don't think I've ever seen a handheld computer. You mean something you can hold in one hand?

 

[Don Burke]

Don't think I've ever seen a handheld computer. You mean something you can hold in one hand?

Yep. At one time there was a difference between a handheld computer and a PDA. Since the capabilities of the PDAs have come such a long way, the line is pretty fuzzy these days.

 

[darkmoon3d]

http://www.windmill.co.uk/mapping.html seems to interesting. It allows you to tie data from gps and depth finder (or other devices) into software.

I'm also interested in finding out a low cost quality method to underwater mapping. Would be cool if uw handheld sonar could be used for 3d mapping of objects rather than a simple 2d top-down view of environment.

Here is a link I have that someone sent me from another list I am on.

 

[scubatexastony]

...and a depthfinder and GPS? or a buddy with a total station?

there's a lot of nice software out there for the feild engineer types, or do it the old fashion way and learn coordinate geometry!

nice project anyway.

 

[scubatexastony]

you'd have a shot with these folks....
http://topomaps.usgs.gov/



tony

 

[Diver0001]

I want to make a topographical map of a set of reefs, which are pretty much an extension of a local beach. If that's too much to accomplish, then at least I'd like to be able to draw an outline of the reefs without depth/topo information. I'm trying to figure out the cheapest and best way to do it. I'd like to do it at 0 cost, of course, and might pay some money on equipment only if I can use for other things.

I'm looking for some thoughts on ways to accomplish this. Here's what I've considered so far:

1. Accessed the Terraserver (http://terraserver.microsoft.com/) to find satellite images of the beach, hoping to see an aerial view of the reefs with sufficient definition to define the outline of the reef. Found the exact image I needed, but unfortunately, it didn't have quite enough definition. I guess conditions have to be just right. In any case, here's the photo if anyone is interested. It's the reefs surrounding the splotch of white at the bottom center of the image, where waves are breaking on some parts of the reef:

http://terraserver.microsoft.com/image.aspx?t=1&s=10&x=2130&y=18559&z=11&w=1




Thanks for any input.

I think you have to ask yourself if having a topographical map will solve your navigation problem. Generally speaking if you have a few good landmarks you should be able to find your way around. The level of detail you're looking for isn't probably going to help you dive the site, although it's an interesting exercise in itself.

Looking at the picture the reef has a pretty complex layout and it's not surprising that you get turned around. Maybe another few dives there will help. Also if you know someone who doesn't get lost there then maybe he/she has some tricks for you.

I think if it were me I'd start by verifying the map you have with gps and the fish finder and overlay depth lines on it in something like 5ft increments.....

R..

 

[miketsp]

Ok - if you want to do it for cheap, all you need are 3 people including yourself, two precision compasses, two watches, a set of tide tables, a buoy and your dive computer.
Set up a decent baseline and post an observer at each end of the baseline with a compass and a watch (previously synchronised to your dive computer).
You just swim around the reef holding your computer about 50cm off the bottom and towing the buoy on a short line.
The observers constantly log the bearings of the buoy from the observation points.
There is nothing critical in the process as you only swim slowly.
Then you download your dive computer and start plotting, not forgetting the 50cm correction and to reduce your depth measurements to local reference / sounding datum to compensate for the state of the tide.
If you can replace the compasses by two borrowed theodolites then you can really do some precision work.

I used to to use a similar technique for quick and dirty mapping during off-shore surveys in third world countries before GPSs became common. We would just run around in a launch with a portable echo-sounder while 2 guys tracked us. We always put in a few track crossings to make sure results were consistent.

The alternative if you were by yourself in the boat, was to measure 3 nice fixed points on land and then use a sextant to measure the horizontal angles between them from the boat, noting the angles and putting marks on the echosounder trace. When I got back on shore I used a special plotting device with 3 arms and an angle readout which gave the position on a board with pins stuck on the chart to represent the 3 reference points. In an afternoon's work I could map a large area with a high degree of precision.
The real trick here was to practice using the sextant to read off the two angles as quickly as possible with minimum interval and to choose shore based refences that would avoid small angles.

The finishing touch would be to actually monitor the tidal level and graph it over about 72 hours - starting a day before and continuing to a day after, to make sure your datum correction is OK.