[GRASSLIST:9772] Re: access boundary coordinates from square vector

[Dylan Beaudette]

On Monday 09 January 2006 12:29 am, Radim Blazek wrote:
> On 1/9/06, Dylan Beaudette <dylan.beaudette at gmail.com> wrote:
> > Greetings,
> >
> > I am trying to access the four coordinates (nodes) that define the
> > boundary of a square vector, via the C API. so far i am having no luck.
> >
> > Any tips on where to get started?
>
> A bounding box of the whole vector or points of one area feature?
>
> Radim


perhaps a better description of the problem is in order.

In some parts of the US, there is a legal system of describing parcels of land 
called PLSS. In its time (the last century) it worked reasonably well, but 
now it is mostly a pain to use. I am trying to create a module for grass that 
will convert a PLSS location into a geographic location, based on the plsnet 
vector layer describing PLSS grid. 

This vector is composed of a network of four-sided polygons, with an attribute 
associated with each polygon indicating part of the PLSS code that defines 
its location. However, the complete PLSS code describes an area 1/16th that 
of the total area, in a fashion like:

NE 1/4 of NE 1/4 of NE 1/4 of Section 1

Where section 1 is a 1x1mile polygon. This part of the PLSS code can be 
interpreted as ' the northeast quater of the northeast quarter of the 
northeast quarter of section 1". Very strange indeed...

Since I would like to convert a description like this into a geographic 
coordinate, I need to subdivide the "section" polygons contained in the 
plsnet vector file. I have started work on an algorithm that will do this, 
however I need some way to access the 4 nodes which define the vertices of 
each four-sides polygon. 

I have started with the following code to loop through the points in each line 
of a line or boundary vector type:

---------------------
/* loop through each line in the dataset */
  Vect_open_old(&In, in_opt->answer, "");

    Points = Vect_new_line_struct();
    Cats = Vect_new_cats_struct();
	
	ltype = Vect_read_line(&In, Points, Cats, line);
	nlines = Vect_get_num_lines(&In);
	
	printf("%i lines\n",nlines);

	for (line = 1; line <= nlines; line++) {

		/* loop through each point in a line */
		for (j = 0; j < Points->n_points; j++) {
			printf("%f %f    %i\n", Points->x[j], Points->y[j], j);

		}		/* end looping through point in a line */


		printf("\n");


	 
	}			/* end looping thru lines */

----------------------------------------

the output from this modue is as follows:

v.plss in=pls_test type=boundary
5 lines
-2071410.623504 514294.371112    0
-2070032.693579 513872.241006    1
-2069957.750231 513849.160982    2

-2071410.623504 514294.371112    0
-2070032.693579 513872.241006    1
-2069957.750231 513849.160982    2

-2071410.623504 514294.371112    0
-2070032.693579 513872.241006    1
-2069957.750231 513849.160982    2

-2071410.623504 514294.371112    0
-2070032.693579 513872.241006    1
-2069957.750231 513849.160982    2

-2071410.623504 514294.371112    0
-2070032.693579 513872.241006    1
-2069957.750231 513849.160982    2

.... this doesn't quite match the output from v.out.ascii:

B  2
 -2070987.20745953 515885.1103599
 -2069456.34247776 515791.24632579
B  2
 -2069456.34247776 515791.24632579
 -2069957.75023142 513849.16098207
B  2
 -2070987.20745953 515885.1103599
 -2071410.62350447 514294.37111217
B  3
 -2071410.62350447 514294.37111217
 -2070032.6935785 513872.24100595
 -2069957.75023142 513849.16098207
C  1 1
 -2070412.47906158 514949.62378849
 1     1144


Ideally, I would like to get a point data type with the NW, NE, SW, and SE x,y 
locations...

Any ideas?

Thanks in advance!



-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341