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

[Bill Thoen]

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!
>
>
>  
>