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Largo
04-11-2008, 20:58
OK.

I want to make an accurate map of some underwater landmarks. I was thinking of tying a float to each one, surfacing, getting a set of grid coordinates, then moving on to the next landmark. I am a little apprehensive about the GPS device falling into the water, but I can fix that with cable ties. My problem is that something in the back of my mind is telling me that I am doing something stupid.

What am I overlooking?

terrillja
04-11-2008, 21:00
OK.

I want to make an accurate map of some underwater landmarks. I was thinking of tying a float to each one, surfacing, getting a set of grid coordinates, then moving on to the next landmark. I am a little apprehensive about the GPS device falling into the water, but I can fix that with cable ties. My problem is that something in the back of my mind is telling me that I am doing something stupid.

What am I overlooking?
That you can't get the gps precisely over where you are trying to map unless the depth is minimal. The gps will drift some.

fisheater
04-11-2008, 21:11
The floats will drift more (and in different directions and distances).

Why not use a single float to get a decent fix and then use your compass and a measuring line underwater to map out the underwater features?

Largo
04-11-2008, 21:15
My right leg is much stronger than my left (due to injury). I keep finding myself to the left of the object I am trying to swim to. I.E., if I can't find the anchor line, I make a 90 degree turn to the right, swim a little, and viola! There is the anchor line. So, I can't rely on my bearings and kick cycle for laying out a navigation course for others. All I would accomplish is to frustrate people trying to learn underwater navigation.

CompuDude
04-11-2008, 21:26
Just be careful of yo-yoing up and down too much.

navyhmc
04-11-2008, 21:57
The majority of the newer GPS kits are water resistant, meaning they can take a short dip in the water but are not guaranteed to any depth. Make sure your GPS is computer down loadable. I have Magellan Meridian Platinum that I can download to Delorme Topo USA. To add a bit of safety to your GPS, there are several companies that make waterproof bags for GPS's.

Add an anchor to your float, make note of your depth and how much float error you are going to have. This adds a bit of navigation skill to the dive (i.e. if you have 50' of line, a 1 kt current and are at 40' you can expect a 10' down current error to your plot. ) Place your anchor next to your point, surface and add a landmark to your GPS and download it all later.

That's my suggestion

Largo
04-11-2008, 22:01
Thanks, guys. Any more suggestions are welcome.

Technology is great, but you have to keep on top of it, too.

FishFood
04-11-2008, 22:47
Attach the float to a reel, and mark the length of the reel in half foot marks. Compare the length of the reel to your actual depth. Then use the Pythagorean theorem to find the exact amout of float, and use your compass to figure out which way it's floating.

Just a thought

diversteve
04-12-2008, 01:16
I am a little apprehensive about the GPS device falling into the water,
Put it in one of these: GPS Hard Case, GPS Cases, Waterproof GPS Case, GPS Accessory Carrying Case, GPS Card Case (http://www.otterbox.com/products/gps_case/)

navyhmc
04-12-2008, 03:54
Or one of these: http://www.leisurepro.com/Prod/APC104.html

okay, I know it's not ST, but it's all I could find on short notice.

Flatliner
04-12-2008, 08:15
OK, this isn't original BUT here is something I copied from the "other board" because I thought I might want to do it some day. As he points out, it won't get an exact fix but it will get pretty close.

Rob

Underwater GPS:
I'm the above-mentioned person using Magellan GPS underwater. (You may have seen my
recent article in Underwater Magazine). A free eBook is available at:
Scuba Gear and Scuba Diving Equipment - Discount dive gear (Cheap online!) (http://divergps) *(a non-commericial site...nothing for sale).
I use one of 3 possible deploment methods, depending on depth and conditions. Lately, I've been "towing" a clear housing (Otterbox 9000) on the surface...the housing is only pulled below briefly to "mark" a point, or to gain awareness of the boat's position.
Here's the set up... The housing floats on the surface with an attached nose cone (to reduce drag)...this is followed (on the surface)--by about 25 ft. of bouyant poly-line....which is attached to a unique "no drag" dive flag. I simply tug lightly (to get the housing to move forward to the desired position)--then, pull down in a hand-over-hand position...gain data...drop the housing (which returns to the surface).
I've been using different variations of this method for about 6 years with good results.
As stated...the above-mentioned web site will provide further details, and further methods of deployment. *Be sure to see the photo section. **To answer a frequently asked question: It's often asked if this method allows the housing to drift to a distant point (rendering a mark innacurate)...not a problem, in this applicaton...it's where the housing is pulled below (losing signal) that matters. This is easily controlled by the diver. Most divers are not in the water enough to bother, but researchers and the most avid divers can have plenty of fun with this! You can test the basic theory on land: Take a Magellan gps and set up a land simulation. You simply cover the antenna with alluminum foil (to simulate periods of signal loss underwater). In real world applications (after significant periods of signal loss)...it takes about 60 seconds to get a fresh signal. However, in the above described surface-tow method, the wait is avoided, and data is available in mere seconds. Constant signal?---towed antenna??...that's fine if you want to go to the trouble, but I'm trying to use the most basic method possible.

Flatliner
04-12-2008, 08:16
The link appeard to be dead now, I don't have a replacement. Sorry