[GRASSLIST:3543] Re: Geophysical/Potential field modules for GRASS?

[Michael Barton]

On May 28, 2004, at 10:25 PM, Hamish wrote:

>
> Sounds like a job for Matlab/Octave to me .. We don't get to lay nice
> grids over our study sites at sea (or manage to get the ship to go in a
> straight line anyway), so what I've done in the past is to log the NMEA
> output from a GPS into a laptop while our instruments spit out
> "1,time,date,value" style data which gets recorded into another 
> file[*].
> A Matlab postprocessing script does a linear interpolation of position
> from the NMEA log for each data point, and directly writes a GRASS 
> sites
> file. Any spatial interpolations of the data can then be done from
> there in the GIS. [It's not as hard as it might sound]
Hamish

This seems like a very nice setup for your case. For the geophysical 
survey I've had been involved with (admittedly limited), the scale is 
too small for GPS to be of use unless high resolution differential 
correction is applied (data points along transects separated by 
centimeters and transects separated by a few meters). The grid is set 
up in advance, however (a luxury you don't have at sea). So the xy 
coordinate values have to assigned by 'dead reckoning' based on the 
predefined transect grid. I haven't used Matlab or Octave, but I 
suppose they could do this kind of transformation. It's pretty 
straightforward in a spreadsheet. However, if this is the common way 
this kind of work is done for magnetometry, resistivity, conductivity, 
and GPR, then it would feasible to design some kind of pre-processing 
module. What I don't know is the extent to which the ascii output from 
the relevant data recorders used by different manufacturers is 
sufficiently standardized so as to plug into such a module. If everyone 
records 'new transect' and 'data point' in very different ways, this 
could be difficult as you suggest.

>
> [*] all clocks sycronized to GPS time before hand of course
>
> If you already have a grid set out, you can skip all the GPS stuff & 
> the
> script is all the easier.
>
> I don't see how this could be implemented as a generic GIS module.
> Each job is custom..
>
>

Michael
____________________
C. Michael Barton, Professor
School of Human Origins, Cultures, & Societies
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