[GRASS-user] Changing tracks to vectors in a grid

[John Payne]

I am analyzing some animal tracking data and I have a problem that I am
stumped by.  Imagine that we have a single animal track (created from
satellite data received about once per hour), and that it wanders for many
kilometers.  I would like to overlay a grid on the track (cell size about
500 m x 500m) and record a direction vector for every cell in the grid,
which represents the direction that the animal was traveling when it passed
through that cell.  I tried using v.to.rast with use=dir, which should
create a directional vector in every raster cell that the track passes
through, but it seems that the command does not respect the *order* of the
track.  In other words, it seems that it was just as likely to classify a
particular segment as having a direction of NE as it was to classify the
same segment as SW (180 degrees opposite), and I couldn't see any
consistency to how the directions were chosen, although I may have missed
something.  There are a few other challenges, including that sometimes the
animal re-crossed its own track so that a grid cell may contain more than
one track.  In those cases I would want to sum the vectors (with regular
vector addition).  I want to do a few other things as well, including
finding the center of each grid cell and computing a direction vector that
is orthogonal to some linear landscape feature—I only mention this in case
rasters are not the right approach.   I am new to GRASS and a bit
overwhelmed, and I would be very grateful for any suggestions that would
get me started in the right general direction.

Thanks!
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