[GRASS-user] Low Budget Bathymetry
cbsled at verizon.net
Mon Sep 18 18:53:06 EDT 2006
Start with a "fishfinder", add a GPS and a laptop. On a boat, of course.
I'm using a Humminbird fishfinder, with an attached Humminbird GPS receiver.
The GPS talks to the fishfinder, which in turn outputs NMEA0183 sentences
containing all the required data at a more than adequate level of precision
for my needs, generally about ±3m on position, and less than ±10cm on depth.
Unfortunately, the NMEA0183 protocol has a large number of allowable
sentences, and existing GPS tools such as gpsbabel and gpsd handle only the
position, velocity, and time data in certain NMEA sentences common in the
output from handheld GPS units, and none of the more advanced sentences.
Here is an ideal system:
The incoming sentences are parsed and timestamped on arrival at the on-board
computer by gpsd. Incoming coordinates, ellipsoid and datums are then
translated by the client application as required to match the current GIS
region. The incoming bathymetry is compensated to equal the above MSL
elevation of the bottom using the known water surface elevation as reference.
The position is compensated for best guess change in position between the
bathymetric fix and the last position fix, if there is any time difference
between the two, using the last bearing and velocity fix, and a point is
added to a vector point map or raster DEM map in the GIS, all in nearly real
Is anyone else working on anything like this?
Whitefield, NH USA
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