[GRASSLIST:795] Re: help needed in digitizing a contour map

[Moritz Lennert]

Amit Keney said:
> hi everyone
>
>      I am a newbie at grass and I want to digitize a contour map and
> view the landscape in nviz.
>
> actually it is a hand drawn contour map of the sea bed and i have
> already got it scanned ( in parts , since the map is too big)
>
> I imported the parts as a raster image . I also got the various parts
> patched and georeferenced in grass.
> now I want to digitize the contour lines and then view ( how the land i
> mean the sea bed really looks like) with the nviz tool.

You have two choices:

1) transform your raster lines into vector lines by using r.thin and
r.line (see
http://www.geog.uni-hannover.de/grass/gdp/grass5tutor/HTML/c1450.html)

2) use your raster map as a backdrop in v.digit (Customize -> Select a
Backdrop CELL map) (see
http://www.geog.uni-hannover.de/grass/gdp/grass5tutor/HTML/c921.html#AEN1009
for digitizing of contours)

The choice between the two depends on the quality of the scanned contours
(are they easily recognized by r.line) and the number of contours (if not
too many digitizing by hand with backdrop might be the most efficient).

Once you've created a vector contour map you have to create a digital
elevation model. See
http://www.geog.uni-hannover.de/grass/gdp/grass5tutor/HTML/c1478.html on
that. The easiest will probably be
http://www.geog.uni-hannover.de/grass/gdp/html_grass5/html/v.surf.rst.html.

> also how can i get the digitized values of all the contours in ascii form
> actually i want it in the form of a 3 column output
>
>   lon  lat  contour_depth
>

A contour is a line, so it normally doesn't have one pair of lat-long
coordinates associated with it. Do you want a collection of points with
elevation data ? Then you might want to look at
http://grass.itc.it/gdp/html_grass5/html/v.to.sites.html.

Moritz


>
> is that possible in grass
>
>