[postgis-users] working with PLSS: state-plane survey system coordinates

[Dylan Beaudette]

Thanks for the tips Bill:

Working through some example data on paper I was able to come up with a couple 
simple algorithms to accomplish this task. I have a working python script 
which will lookup sections from a PostGIS table, quarter them, and write them 
back to a new table with some new attributes. I will post the code as soon as 
it is cleaned up a little.

attached is some sample output for a single township: sections have a unique 
color, quarter-sections are labeled as such.

However, I haven't quite come up with a good approach to throwing out 
triangle-like sections: other than a simple check for 4 'corners' : NE NW SE 
SW ... although this seems to work most of the time, a more robust approach 
might be better.

Cheers,

Dylan

On Thursday 21 September 2006 16:31, Bill Thoen wrote:
> The algorithm is in two parts. First you need to find the section
> corners. In most cases this is pretty easy, but there are irregular
> sections where, for example, a river cuts off a corner. When that
> happens then it's anybody's guess where exactly the north, east, west
> and south lines actually are. (If you had the surveyor's notes that
> would be handy, but usually you have to make do with the data you have.)
> But those are the gnarly exceptions. I deal with this by locating the
> points in the section boundary that most approximate a rectangle, and
> reject any sections that look more like a triangle than a rectangle.
>
> The second part of the algorithm is to quarter the section. To do this,
> find the midpoints of the section lines (use straight lines between the
> corners unless you really like extra work), connect the opposite sides
> with lines, and where they intersect is the section center. Now you have
> all the points necessary to define a quarter section, so create a
> polygon from that for the desired quarter section, and if you need to
> quarter it again, just call the above routine recursively until you
> reach the quarter and level you need.
>
> Finally, if you need to locate something within a quarter or quarter
> quarter section and you have footage calls (e.g. 330' from north line,
> 278' from east line, or 330 fnl 278 fel or 330 N 278 E), then you'd
> better check the surveyor practices for your state. In general these are
> due directions (go due south from the north line and west from the east
> line) but the exact details may vary from state to state. Anyway to
> locate the point, just translate the appropriate lines in the right
> directions and the intersection is your spot.
>
> - Bill Thoen
>
> Dylan Beaudette wrote:
> >Hi everyone,
> >
> >Here is a rather odd question:
> >
> >say i have a vector description of 'sections' as defined in the PLSS
> >(http://nationalatlas.gov/articles/boundaries/a_plss.html), and I would
> > like associate legal definitions with a real-world coordinates.
> >
> >starting with "T10, R10, Section 10, mount diablo base meridian" , it
> > would be a simple matter to lookup the matching section polygon, locate
> > its centroid, and dump the x,y coordinates.
> >
> >however, it is common practice to define a location in terms of 1/4 of 1/4
> >section, i.e. :
> >
> >"NE 1/4 of NE 1/4 of Section 10, T10, R10"
> >
> >the problem then arises: how can i approximately subdivide the section
> > polygon (as it is usually not a square) such that : the polygon is
> > bisected along each axis twice:
> >
> >+------+------+
> >
> >|         |    | * |
> >|
> >|         +--+--|
> >
> >+------+------+
> >
> >
> >
> >+------+------+
> >
> >note that the NE 1/4 of the NE 1/4 of the section is marked with a * .
> > also note that the section will not always be oriented parallel to the x
> > and y axis as in the example above.
> >
> >therefore the question is : is there a generic way to bisect a polygon
> >resembling a square (an approximation is ok too) within an x,y plane ?
> >
> >ideas?
> >
> >thanks!

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