On Wednesday 30 August 2006 18:14, Henry House wrote:
I have a set of surveyed-section boundaries* for northern California
from the United States Public Land Survey System and would like to use
GRASS to find geometric fractions of these roughly square sections.
For example, I would like to divide a particular section into equal
quarters by bisecting it east-west and north-south and then save the
northwest quarter for further analysis. Can anyone suggest a method to
do this acurately?
* Used for legal descriptions of parcels given in terms of townships and
ranges.
Hi Henry,
I think that the approach suggested by Hamish will work, but only when using a
projection where the PLSS sections area oriented _exactly_ NS and EW. For
example, looking at the PLSS data in the AEA projection that we use for many
things, the sections are _not_ oriented as such:
http://169.237.35.250/~dylan/temp/plss1.png
However, there is an 'angle' operator in the v.mkgrid module, which may be
able to account for this- although I cannot think of an _automated_ approach
to deriving this angle.
I have extracted a single section, marked red in the above image, like this:
v.extract in=pls out=pls1 list=80800
where 80800 is the cat, or feature id, of the section of interest. Zooming to
the extents of this section:
g.region vect=pls1
http://169.237.35.250/~dylan/temp/plss2.png
looking at the options for v.mkgrid:
Parameters:
map name of vector map
grid number of ROWS and COLUMNS in grid
position Where to place the grid:
region - current region
coor - use 'coor' and 'box' options
options: region,coor
default: region
coor lower left EASTING and NORTHING coordinates of map
box WIDTH and HEIGHT of boxes in grid
angle angle of rotation (in degrees counter-clockwise)
default: 0
.... it looks like we might be able to define the grid in terms of a
lower-left point, and the width and height of grid cells, and an angle of
rotation.
this will require some math, but might easily be accomplished with some right
triangle-based trig.
fitting the region to _exactly_ that of the bounding box of our extracted
section:
g.region `v.info -g pls1 | awk ' {gsub("north","n"); gsub("south","s") ;
gsub("east","e") ; gsub("west","w"); gsub("top","t") ; gsub("bottom","b")}
$1 !~ /b|t/ {print $0}' | tr "\n" " "` -p
extracting vertices of selected section: can do this with v.out.ascii and awk,
or with v.to.points
will post some more ideas later.
good luck!
cheers,
Dylan
--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341
