[GRASS-user] gps trajectories

GRASS GRASS user
1

Dear,
The past days I have been trying to find out the best way to represent
different gps trajectories in GRASS with the dates of the locations.
1. I used R to create SpatialLinesDataFrame and exported it in a shape
file for GRASS.
2. I got stuck in GRASS, because I couldn't access the vertices (i.e.
the points) of the different lines (for each individual trajectory one
line). I wanted to access each point to be able to connect it to a
database with the dates, I need these vertices also for subsequent
operations. [is it really not possible to directly access and
manipulate the vertices in a line?]
3. I tried "v.to.points -v" to extract the vertices, worked fine, I
have now an object with two layers: the points and the individuals.
4. I got stuck since this new object is no longer a line, but a
collection of points, fortunately organized in a hierarchical way
(i.e. nested within individuals)...
Possibly as a consequence, I could no longer easily use
d.vect.thematic to plot the trajectories from each individual.

I have been looking on the web for anything useful on how to use GRASS
for the representation of GPS trajectories, in vain (apart from
functions to read from gps receivers, I have simple csv files for the
trajectories). Therefore, I am curious about the experiences of other
people in this context. And maybe a few tips could help me further on
my way.
Cheers,
Bram

--
Bram Van Moorter
Centre for Conservation Biology
NTNU, Trondheim (Norway)
email: Bram.Van.Moorter@gmail.com
website: http://ase-research.org/moorter

2

Bram wrote:

The past days I have been trying to find out the best way
to represent different gps trajectories in GRASS with the
dates of the locations.
1. I used R to create SpatialLinesDataFrame and exported it
in a shape file for GRASS.
2. I got stuck in GRASS, because I couldn't access the
vertices (i.e. the points) of the different lines (for each
individual trajectory one line). I wanted to access each point
to be able to connect it to a database with the dates, I need
these vertices also for subsequent operations. [is it really
not possible to directly access and manipulate the vertices
in a line?]
3. I tried "v.to.points -v" to extract the vertices, worked
fine, I have now an object with two layers: the points and
the individuals.
4. I got stuck since this new object is no longer a line,
but a collection of points, fortunately organized in a
hierarchical way (i.e. nested within individuals)...
Possibly as a consequence, I could no longer easily use
d.vect.thematic to plot the trajectories from each
individual.

I have been looking on the web for anything useful on how
to use GRASS for the representation of GPS trajectories,
in vain (apart from functions to read from gps receivers,

I have simple csv files for the trajectories). Therefore, I
am curious about the experiences of other people in this
context. And maybe a few tips could help me further on
my way.

you are on the right track and figured out the root of the
problem:

- lines are of one category, and that category links to
  attributes in the database.
- points also have one category each, and those link to the DB
  as well.
- vertices (nodes of the polyline) can not have categories and
  therefore can not have attributes.

this is nothing specific to GRASS. shapefiles, PostGIS, whatever
will have the same issue.

and the solution:

- keep two map layers, one for the line, and one for the points

but converting a track line to points with v.to.points is no
good as the vertex attributes are already lost.

probably the easiest way is to import your points with
v.in.ascii and keep the line you have from R.

if starting from scratch v.in.gpsbabel with the xcsv import
filter would probably be your best bet.

Even if you don't use them, you should have a look at the
v.in.garmin and v.in.gpsbabel modules' help pages, options,
and internal method, as they essentially sove the same problem.

the "usual" method would be to run the script twice, once as
track points and once as track lines. then overplot the points
on top of the line.

One nice thing they'll probably give you which the R shapefile
won't is for each trackline a start/end time and start/end
lat-lon.

as for editing vertices, see v.edit module. (but there are no
attributes to edit). they may be individually moved with v.digit.

Hamish

ps- check out the new "ping" symbol; note grass rotation angle
is theta/math style, ie measured CCW from east. convert from
compass convention with "90-true compass heading".
     http://grass.osgeo.org/wiki/IconSymbols#New