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| Subject: | GISList: Sum: underwater z |
| Date: |
11/05/2001 09:48:04 AM |
| From: |
Koroa, Carolyn |
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Dear Listers,
I received a good number of responses to the question I posed last week
regarding the collection of underwater elevations. As you might imagine,
there were a wide range of suggestions, from plumb bob and hand held GPS,
fathometers, echo sounders and side scan radar, to LIDAR, and no doubt we'll
end up using a combination of most, if not all of these. The suggestions
are summed below.
Thanks to Laura Schmidt of U. of So. Carolina (who responded despite the
fact that the Univ. of Tennessee Volunteers beat the So. Carolina Gamecocks
soundly the weekend before, much to the chagrin of Lou Holtz): Anthony
Quartararo of Spatial Networks, Inc.: Craig Johnson at LSU: Teri Dowdy of
FMSM Engineers in Lexington, KY: Seiichi Nagihara at Texas Tech: John Koles
of Micronics Pty Ltd: Pat Waggaman of Marine Imaging Systems S.A.: Steve
Evans of Kinetic Solutions, LLC: and S.S. Mahal of Mahal International,
Bombay. I really appreciate your suggestions, and I apologize for not
responding to all of you individually.
Original question:
I have a set of about 2000 underwater structures in rivers and streams
spread across seven states for which I need to gather x, y, & z coordinates.
The x and y I know how to get, but what would be a good way to get the
elevation above mean sea level at the top of the structure?
Answers:
1) Have you considered bathymetric LIDAR? It might be cost-prohibitive for a
multi-state project, but you could use the bathymetric data of the river
bottom for other projects.
Laura Schmidt
Resource Coordinator
North Inlet-Winyah Bay NERR
Baruch Marine Field Lab
University of South Carolina
2) The University of South Florida in Tampa conducted a project somewhat
similar and it may be of use to you. There goal was to map the shorelines
of ponds and lakes here in FL, but much of the shoreline is inaccessible due
to vegetation. They devised a method to parallel the shore by a known
offset and take real-time DGPS measurements a small boat (canoe I think).
They then plotted the GPS path and in the GIS, computed the standard offset
to get the "real" shoreline of the water body. This was important for
computing volume of water, etc.
Now, the same general process can be used to gauge below water elevations,
using a real-time DGPS system onboard a boat and have it directly over the
structure you are trying to collect depth for. Once you have that, (which
is sounds like you do) then simply drop a "plumb-bob" or survey's tool, or a
really long telescoping yardstick down the water until it hits the top of
the structure. The precision will not be important unless it is really
important to your application. The DGPS unit measures from a boat (which is
where I presume you got the original x,y ) will have some error built in,
and so without getting "wet" you can drop anchor and reel in your
measurements for the structures. You could then compute the difference
between the measurement and the mean sea level and further corroborate that
with local "land-based" survey elevations. Sounds like a road trip in any
event. Hope this can help, and good luck.
Anthony Quartararo
Spatial NetWorks, Inc.
727.538.0545
ajq3@spatialnetworks.com
www.spatialneworks.com
3) Most commercial divers carry a pneumofathometer (open hose connected to a
gauge on the surface) that allows them to get a depth reading. This reading
shows feet below the surface not mean sea level, so you will have to do the
math. This pneumo hose could be used without a diver if it were connected
to a weight and lowered to the object. You would need to have visibility to
be sure that you were actually touching the submerged item at the point
closest to the surface. Otherwise I would use a diver to make sure you are
getting a correct reading.
Craig Johnson
cjohnson@lsu.edu
4) The enginering firm I work for performs underwater hydrographic
profiling on a regular basis for state and federal agencies. We use a
fathometer coupled
with a GPS unit mounted on the side (or back) of a boat to collect depth and
x-y coordinates simultaneously while traversing the water . We have
developed a custom program in VBA and ArcView that converts the depth to
elevations based on pool and then interpolates the sonar x-y from the GPS
based on the common factor, time. From the resultant point files, we
develop a surface TIN and generate cross-section profiles and contours of
the ground surface. We have performed this methodology in a variety of
situations from river channels, locks and dams and even in shallow ocean and
bay environments.
Teri Dowdy
GIS Programmer/Trainer
FMSM Engineers
Lexington, KY
You probably know that it is fairly easy to measure the depth of an
underwater object relative to the water surface. You can use
fish-finder-like sonar devices to get such measurements. I used to do a lot
of such echo-sounding measurements i
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