Every time I use this trick, it occurs to me how useful it is. In this case, the task was to transfer an elevation from the water surface of a lake for which the pool elevation as measured with a gauge at the dam was logged every 15 minutes. The lake is a flood control lake that is now about 50 ft. below the supposed normal operating level and the transfer was to a benchmark about 40 ft. above the water surface elevation.
Even on a relatively still day, small waves and endless ripples create lots of rapid variations in water surface at a staff standing in the water, making it impossible to read with much accuracy. To buffer that effect, the trick was to
a) make a miniature stilling well from a small plastic bucket (I used a 1 quart plastic paint can) by poking enough small holes in the side of it that it filled fairly rapidly when standing in the shallows, rim above the water surface,
b) set it in the shallows approximately level,
c) set up the prism pole with prism and target with the point resting on the bottom of the plastic bucket (I used a prism pole tripod to hold the pole in place),
d) once he plastic bucket has filled to a level that shows no significant changes resulting from small waves and ripples in the lake surrounding it, use a pocket tape to measure the height of the water surface inside the bucket above the bottom of the bucket that the prism pole is resting on. Measure the height right beside the pole and, better yet, on both sides of it to verify that the bottom is close enough to level that the reading is the same.
e) subtract that measurement from the value of the target height and the height transfer can be made with a total station using the corrected target height as if the pole were resting magically on the mean water surface.
Naturally, there are some caveats. For one, on a long lake the release rate at the dam should be small enough that the slope of the water surface from the nearest gauge to the point of interest should be negligible.
Here are the results of the transfer from a lake near Austin expressed as the elevation of the project benchmark as separately calculated from each. These were made as two repeated measurements on two days more than a week apart, with the lake level differing by more than 0.25 ft. average between the two days. The height differences were via total station over 250 ft. distances from instrument to both BS and FS targets.
[pre]
12/04/2012
671.102 ft. 1
671.102 2
12/13/2012
671.104 ft. 1
671.092 2
-----------------
671.100 ft. mean
[/pre]
This isn't a trick, it's a technique! 😉
I like it; thanks for posting this.
Another way to do it
At the beginning of the site visit:
Is to take a 1"x2" x30" hub, and drive it into the ground, in about 2 feet of water, and then place a 5 gallon bucket over it, without a bottom. Carefully drive the hub, until the top is flush with the water. Call the dam, and note the dam elevation, and the site elevation, and the rate of flow at the dam. Let it sit, while you do other things,
Then Repeat, before you leave, several hours later, only this time note the depth, of the stick, or how much it is sticking up. This will spread the measurement out a little. I have gotten up to about 0.05' of difference this way. I think mostly it was due to wind.
Gives more of a MEAN measurement.
Also, this way, you can set the tip of the prism pole, on top of the hub.
There are many ways to skin a cat.
Remember, we are not looking for repeatability, but closeness to the truth here. Site elevations can be critical. More data is better. If it is critical, spread out the measurements for a couple of days, and confidence and accuracy go up. Especially if the wind direction changes.
More is better.
Nate
Another way to do it
> Remember, we are not looking for repeatability, but closeness to the truth here.
If the pool level is changing by more than 0.01 over fifteen minutes, that usually means that the release at the dam is significant. That probably also means there's a backwater curve on the lake surface upstream from the dam (if the reservoir is following a stream or river). I don't think I'd want to be using the water surface under those conditions.
On the other hand, for small changes in pool level with negligible water surface slope, all a person needs to do is just remeasure the height of the water in the plastic bucket I described. As the pool level changes, so does the height. I didn't mention it above, but simply remeasuring the water height before and after the level transfer gives a measure of the lake surface constancy.
I began using this technique with a 5-gallon plastic bucket and have discovered that a much smaller container works just fine as long as the waves aren't so large that it isn't constantly under water. An occasional wave overtopping the bucket is harmless since the water will drain out and the surface inside will return to equilibrium.
Another way to do it
If the distance from the gauge station is large, the orthometric correction could be significant. The water (if not close to the geoid height) stands at a constant dynamic height, not a constant orthometric height.
Multiple readings spaced out in time are important. On a lake you should consider flow rate, wind, and seiche Link1 Link 2. Tides are possibly negligible unless you are on the Great Lakes.
Another way to do it
> If the distance from the gauge station is large, the orthometric correction could be significant. The water (if not close to the geoid height) stands at a constant dynamic height, not a constant orthometric height.
>
> Multiple readings spaced out in time are important. On a lake you should consider flow rate, wind, and seiche.
This technique works best when :
a) the release rate from the dam is known to be negligible,
b) the pool level indicated by gauges at the dam is available at times times quite close to that of the level transfer from the lake at the project site,
c) the distance from the dam is relatively small (not tens of miles), and
d) there are no strong winds that might affect the lake surface.
What first interested me in using the water surface of the lake for level transfer was reading an account of a US Geological Survey leveling party that ran a line of levels to the water surface on one side of the lake and then drove around and continued from the level of the water surface on the other side. There was no description of the technique used by the surveyors to do the transfer to and from the actual water surface, so that started me thinking about ways to accomplish it.
Another way to do it
a 1x2 is a guard stake in California.
I worked with a Surveyor from Illinois. He kept calling it a hub or a flat. I tried to change his ways but couldn't succeed ;-).
I've never tested this, but I've always wondered about digging a hole a few feet from the water's edge deep enough for water to fill the hole, waiting for equilibrium and measuring from there. The times we've needed to carry an elevation from the lake we work around, the precision requirements have not been exacting, so we've just worked from the water surface at a protected crevice or corner in the shoreline.
Another way to do it
A simple concept that I would normally not think of, interesting methods....working in a shipyard to level units(example 60x40x50 prefabbed portions of a ship)we used a "water level" a 1/2" clear plastic/vinyl tube 50'-60' long each end attached to a stand. The tube was filled with water and a few drops of bakery coloring...holding a known elevation with one stand and checking various spots with the other by measuring down to the deck from the height of the water in the tube.We also used a regular level.
Always find your posts interesting.
> I've never tested this, but I've always wondered about digging a hole a few feet from the water's edge deep enough for water to fill the hole, waiting for equilibrium and measuring from there.
Well, before you try that, I'd suggest giving the 1 qt. plastic bucket with prism pole sitting on the bottom a test run. It works even along rocky shorelines where digging a hole would be a major undertaking.
Another way to do it
And that's how they did it in Ancient Egypt:
"For larger scale leveling, it is speculated that they would use channels or wooden troughs filled with water, though none of these have survived."
From a really interesting read at http://www.fig.net/pub/cairo/papers/wshs_02/wshs02_02_paulson.pdf
Another way to do it
> From a really interesting read at http://www.fig.net/pub/cairo/papers/wshs_02/wshs02_02_paulson.pdfbr >
Thanks! I did find that interesting. I have a book called "How the Great Pyramid was Built", by Craig B. Smith. It is excellent.
Another way to do it
> Saw a cool animation of dynamic drawdown, that one of our hydro modeling consultants recently gave.
That migh be a cool presentation at an LSAW meeting; do you have a contact?
🙂
Dugger
Another way to do it
> South Puget Chapter?
That's the one...:-D
Another way to do it
Thanks Andy...look forward to reading this.
Another way to do it
Hey gschrock, Sounds interesting and yes easy to make but gives more reliable results....especially more so than line levels.
Another way to do it
Hello gschrock,
I used water levels only in a "construction survey" manner...not in the field. If you look at the link you will see a "prefabbed unit" of a ship prior to being joined. That unit is then attached to other units already "fitted" and in final position, see other pictures. At that stage we would orientate the unit x,y,z and make ready for final scribe/cut/join.
https://www.gdbiw.com/Shipbuilding.html
I also used a water level when I built my house, extremely reliable
What are you referring to "laying out agricultural fields in India" ?
Saw a special once where a village in India made bio-gas from cow manure which powered generators and produced electricity...simple but effective
rlshound
Another way to do it
Awesome...like to see that!