We are working on a topo survey of a drainage channel and have an interesting challenge. Nearly all of it has dry areas where we can set control and traverse down the middle of the channel, but there is roughly 400' between two dams that consistently holds about 2' of water. The slopes of both sides of the channel are too aggressive and wooded to set up a gun on and the tree canopy kills any chance of operating a GPS unit, so the only alternative is to set a couple traverse points in the water down the middle of the channel. Since the channel has about two feet of water in it and the floor of the channel is limestone, I can't think of any way we are going to be able to set something solid enough to set-up on. Any ideas??ÿ
You might try setting nails in the face of surrounding trees and use a reflectorless total station to do resections and carry you control down the channel.
Traverse through tripod only with no point set, maybe bring some sand bags to weight the legs. If you will need to return set some control on the bank to resect from.
We used to brace with rebar then stick one piece across the triangle with a sand bag wired to it, makes a mighty stable tripod in the wind.
Set a rebar in the channel. Us an extension rebar/pipe to drive it. Or, a hammer drill to make a pilot hole., to set it in. Let it stick up 1" or 1-1/2". Find with your boot.
Make a 6" dia pvc pipe, with plexiglass site window. Push it down into the water, so the op can see the rebar.
"Flag" it with 15' long nylon construction string. Set them in places, that "line up" with visual reference items.
To find it, line up with references, flag on bank, etc. Walk around til you find your string.
Make TBS's. Those are Temporary Back Sites. Nail in tree, with reflector tape.
It's pretty academic, but don't skip any steps. 2 TBS'S per setup, to make sure you have control, for the life of the project.
Send me some $.
😉
Thank you!
Nate
Send me some $.
I've tried that, plus subtle requests to the green and yellow teams for equipment sponsorship, and a boat.
Agree on resections being the way to go. Nails or reflective targets on/in trees will work great.
But if it's only 400 feet between dams, why not set up two points on each dam, spaced as far apart as you can get them? It's not like it's a long distance for a total station to measure.
Then set up wherever you please down in the basin and perform a resection (preferably three-point) to any of those, making sure you use at least one point on each dam. With proper procedures that's plenty good enough for a drainage channel.
We do a lot of total station work with no point set under the tripod. The coordinate value is assigned to the center of the tribrach plate. I use HI for gun above tribrach (0.196m) and prism (0.083 m). No measure ups required, which gives better accuracy in heights. Never understood why a lot of surveyors find it necessary to mark every single point. And put three stakes around them as well. I have posted pics?ÿ before where we traversed down a shallow river using a canoe and two kayaks. GPS on the same setup where possible. At the end of the day we would set nails in trees to resect from the next day.?ÿ?ÿ
Here is a typical setup in the river (shallow).?ÿ
We did have one incident where we could hear thunder in the distance, but no rain on us. Suddenly, I heard a roar, and a big wave of water came down the river. I radioed the FS and BS guys, put everything in the canoe, and rode the flash flood for about a 1/2 mile. In about 20 minutes it was back to normal. I initially thought they had opened the gates on the dam for some reason, but it turned out there was a torrential downpour on a tributary and it came roaring downstream with no warning.?ÿ ?ÿ
Another reason that we tried to stay out in the water is that we were warned by multiple people that there were a lot of copperheads in that area, and I did see three copperheads, one eating a fish and a also a water moccasin swimming along.?ÿ?ÿ
And here is a map of the traverse (red are GPS observations on traverse points).?ÿ
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for cross sections
