[GRASSLIST:3541] Re: Geophysical/Potential field modules for
hamish_nospam at yahoo.com
Sat May 29 01:25:52 EDT 2004
> This is part of what is needed, but not all of it. What happens in
> geophysical survey is that someone walks up and down transects within
> a grid; the data recorder ticks off the readings at each point; it may
> let the operator record the transect. As so...
> 1.1 2.5 3.1 4.5 5.1
> 1.2 2.4 3.2 4.4 5.2
> 1.3 2.3 3.3 4.3 5.3
> 1.4 2.2 3.4 4.2 5.4
> 1.4 2.1 3.5 4.1 5.5
> The data recorder outputs a data file more or less like this.....
> 1 time date value
> 2 time date value
> 3 time date value
> These need to be translated into xy coordinates for each point given
> the (x) spacing between transects and the (y) spacing between data
> reading along transects.
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]
[*] 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..
Note you can deal with no-data points (NULL) in the GRASS 5.0/5.3 sites
format by using a numerical value of 'nan'. e.g.:
easting|northing|#1 %nan @"date/time"
This probably won't work with most sites modules, but it works with more
than you might think as 'nan' is a valid floating point number on many
systems. e.g. 'd.site.labels attr=double' doesn't mind it.
> I do think it would be handy for other reasons to add a -g flag to
I think so too; done.
also fixed a bug (G_find_cell() was stripping the mapset off the map name).
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