[GRASS-user] Working with USA Public Land Survey System

[Clifford J Mugnier]

Henry,

Hopefully you are not proposing to do this in the context of preparing any
"legal descriptions" for the conveyance of real property in the state of
California.

If that in fact IS the case, then that sort of activity is a criminal act
under California Law.  Only a Registered, Licensed Land Surveyor can do
that, and then only on the ground.  It cannot be done "in the office."

Be careful what you ask for ...

Clifford J. Mugnier, C.P., C.M.S.
National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing
and
Chief of Geodesy,
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
CEBA 3223A
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic]
Voice and Facsimile:  (225) 578-4474 [Research]
Honorary Life Member of the
Louisiana Society of Professional Surveyors
======================================================
http://www.asprs.org/resources/GRIDS/
http://appl003.lsu.edu/eng/ceeweb.nsf/$Content/Mugnier
======================================================




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