Prismless through water?

My VX, which is a scanner and a reflectorless robot, can scan through water. While I don't know how well, it did bounce through the water and reflect off of the hard surface on the back side. We were scanning a dam that had breached and the holes in the concrete wall full of water (on the bottom side of the spillway), as shown in my RealWorx software, did generate points on the bottom of the pool that I had to "trim" out of the scan for the surface model.

However, we made NO attempt to measure these points in with a prism so I don't know how accurate they were (I didn't need them anyway).

In regards to this, I can't help but be reminded of "Little Beaver" in that "you gotta shoot em where they ain't and hit em where they is" (or refraction of the light in the water and I don't know how much or how little that plays in this scenario).

Good luck

I can think of three issues:
1. The attenuation of water will depend on the wavelength of EDM, so you may get a weak return

2. The index of refraction/speed of light in water makes an error in the measured distance of perhaps 1/3 the distance of the water portion of the path.

3. The index of refraction change at the surface of the water will badly distort angles (the bent stick illusion).

Regardless of whether or not it "works," the answer will be wrong. How wrong? Beats me!

The refractive index of air is about 1.0003, whereas the refractive index of water, is about 1.33. Do the math, and you'll see what I mean.

break out the waders...
Loyal

edit...and what Bill said!

I would guess that you may get a result but wouldn't trust the value. Would seem that the wavelength through water verses air would change the distance which would distort the Z value collected. However, it could be that with enough trial and error, maybe you could change the parts per million or temperature to get a working value. Just a thought.

Obviously, a value that equals the elevation of the water would be... well... not the bottom.

This is a small beaver pond with a mucky bottom. There is the question "What is the bottom, since I'll probably sink up to my knees in muck.

I'm all for low-tech where it's going to be an issue. Can't argue with sounding it with a window weight and a tape and shooting the water with the prism.

Sounds fun, but precarious for the man in the boat (no pun).

🙂

Don't do it! Get a small boat that has little draft and float it. Then you may need to figure out the muck depth (if this is for a construction project) and that requires locations and a rod that you can shoot the top of the muck and then push into the muck until you hit something solid and locate that. That's the way I've always done it. If it's just for water volume calculations then it’s going to be a bit of a guess in a mucky pond-almost an art form. But prismless: not a chance.

Prism on top of a level rod so you can get the height of the prism...if'n you don't have rtk gps that is...and a john boat, or any other flat bottom skiff...

This way you can get the top of the muck as well as the depth of the muck to hard bottom. Working with the silt in a hydro job is a pain in the .....

> Don't do it! Get a small boat that has little draft and float it.

Agree 100%. Just shove the rod down and rely on your judgement of the resistance to measure whatever depth you need. Shoot in the location from shore.

Sometimes the old fashioned ways are the best. Just ask the beavers who made the pond.

Back in the late 1980s-early 1990s, NOAA was doing aerial photogrammetry in littoral waters (that were clear). The VAX/VMS software, "GIANT" that I later commercialized to run on a PC had the option to "correct for water refraction." NOAA even got Kodak to come up with a color emulsion for aerial film that would enhance the ability to penetrate (clear) salt water up to 10 meters deep.

Now they use LIDAR for all of that stuff.

Do what I do: knock a couple of holes in the dam and drain it. I did a large boundary earlier this year and had to knock down a dam almost a mile from the site just to traverse the thing.

Yes...drain the dam.

in scan mode how many points per minute does the VX shoot?

> Do what I do: knock a couple of holes in the dam and drain it.

Approach that one with caution. Beavers have rights too, sometimes even more than us mere mortals, and there are agencies watching.

All though, I do agree with that common sense approach. I suppose it would depend on the definition of "small beaver pond", which only cerolli can determine.

Subject: Go Figure

This is a copy of an actual letter sent to Ryan DeVries, from the
Michigan Department of Environmental Quality, State of Michigan. Wait
till you read this guy's response - but read the entire letter before
you get to the response.

Mr. Ryan DeVries
2088 Dagget
Pierson, MI 49339
SUBJECT: DEQ File No. 97-59-0023; T11N; R10W, Sec. 20;

Site Location: Montcalm County

Dear Mr. DeVries:

It has come to the attention of the Department of Environmental Quality
that there has been recent unauthorized activity on the above referenced
parcel of property. You have been certified as the legal landowner
and/or contractor who did the following unauthorized activity:

Construction and maintenance of two wood debris dams across the outlet
stream of Spring Pond.

A permit must be issued prior to the start of this type of activity. A
review of the Department's files shows that no permits have been issued.

Therefore, the Department has determined that this activity is in
violation of Part 301, Inland Lakes and Streams, of the Natural Resource
and Environmental Protection Act, Act 451 of the Public Acts of 1994,
being sections 324.30101 to 324.30113 of the Michigan Compiled Laws
annotated.

The Department has been informed that one or both of the dams partially
failed during a recent rain event, causing debris and flooding at
downstream locations. We find that dams of this nature are inherently
hazardous and cannot be permitted.

The Department therefore orders you to cease and desist all activities
at this location, and to restore the stream to a free-flow condition by
removing all wood and brush forming the dams from the stream channel.
All restoration work shall be completed no later than January 31, 2002.

Please notify this office when the restoration has been completed so
that a follow-up site inspection may be scheduled by our staff. Failure
to comply with this request or any further unauthorized activity on the
site may result in this case being referred for elevated enforcement
action.

We anticipate and would appreciate your full cooperation in this matter.
Please feel free to contact me at this office if you have any questions.

Sincerely,
David L. Price
District Representative
Land and Water Management Division
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RESPONSE:

Dear Mr. Price,

Re: DEQ File No. 97-59-0023; T11N; R10W, Sec. 20;
Montcalm County

Reference your certified letter dated 12/17/2000 has been referred to me
to respond to. First of all, Mr. Ryan De Vries is not the legal
landowner and/or contractor at 2088 Dagget, Pierson, Michigan.

I am the legal owner and a couple of beavers are in the (State
unauthorized) process of constructing and maintaining two wood "debris"
dams across the outlet stream of my Spring Pond.

While I did not pay for, authorize, nor supervise their dam project, I
think they would be highly offended that you call their skillful use of
natural building materials "debris." I would like to challenge your
department to attempt to emulate their dam project any time and/or any
place you choose. I believe I can safely state there is no way you could
ever match their dam skills, their dam resourcefulness, their dam
ingenuity, their dam persistence, their dam determination and/or their
dam work ethic.

As to your request, I do not think the beavers are aware that they must
first fill out a dam permit prior to the start of this type of dam
activity. My first dam question to you is:
(1) Are you trying to discriminate against my Spring Pond Beavers? or,
(2) do you require all beavers throughout this State to conform to said
dam request?

If you are not discriminating against these particular beavers, through
the Freedom of Information Act I request completed copies of all those
other applicable beaver dam permits that have been issued. Perhaps we
will see if there really is a dam violation of P! art 301, Inland Lakes
and Streams, of the Natural Resource and Environmental Protection Act,
Act 451 of the Public Acts of 1994, being sections 324.3010,1 to
324.30113 of the Michigan Compiled Laws, annotated. I have several
concerns. My first concern is aren't the beavers entitled to legal
representation?

The Spring Pond Beavers are financially destitute and are unable to pay
for said representation - so the State will have to provide them with a
lawyer.

The Department's dam concern that either one or both of the dams failed
during a recent rain event causing flooding is proof that this is a
natural occurrence, which the Department is required to protect. In
other words, we should leave the Spring Pond Beavers alone rather than
harrass them and call their dam names. If you want the stream "restored"
to a dam free-flow condition - please contact the beavers - but if you
are going to arrest them they obviously did not pay any attention to
your dam letter (being unable to read English).

In my humble ! opinion, the Spring Pond Beavers have a right to build
their unauthorized dams as long as the sky is blue, the grass is green
and water flows downstream. They have more dam right than I do to live
and enjoy Spring Pond. If the Department of Natural Resources and
Environmental Protection lives up to its name, it should protect the
natural resources
(Beavers) and the environment (Beavers' Dams).

So, as far as the beavers and I are concerned, this dam case can be
referred for more elevated enforcement action right now. Why wait until
1/31/2002 The Spring Pond Beavers may be under the dam ice then, and
there will be no way for you or your dam staff to contact/harass them
then.

In conclusion, I would like to bring to your attention a real
environmental quality (health) problem in the area. It is the bears.
Bears are actually defecating in our woods. I definitely believe you
should be persecuting the defecating bears and leave the beavers alone.

If you are going to investigate the beaver dam, watch your step! (The
bears are not careful where they dump!)

Being unable to comply with your dam request, and being unable to
contact you on your answering machine, I am sending this response to
your office via another government organization - the USPS. Maybe,
someday, it will get there.

Sincerely,
Stephen L. Tvedten
The University of Texas at: Austin
Office Community Relations/Accounting unit
P.O. Box 7367
Austin, TX 78713

Don, you got that one right. I actually have a copy of that letter somewhere on some floppy disk. It was widely circulated back then.

Having lived and worked within 20 miles of that location for about 20 yrs, I can attest to both the overly anal agencies and the professionally written quality response. That supposedly is a true story, but not sure how it ever panned out. It is only one example of the many bureaucratic nightmares people must deal with.

When it came to beaver dams & swamps, I'd always wait until winter when things froze up. Never a problem.

Dam Beavers! (and the rest of the story)

I have tried this before, it bounced off of the surface and got distances from the distant bank. Didn't work at all.