[GRASS-user] Low Budget Bathymetry

Carl Brown 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?

Carl Brown
Whitefield, NH USA

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