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

[Richard Greenwood]

On 8/30/06, Henry House <hajhouse at houseag.com> 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.
>
> --
> Henry House
> +1 530 753 3361 ext. 13
> Please don't send me HTML mail! My mail system frequently rejects it.
> The unintelligible text that may follow is a digital signature.
> See <http://hajhouse.org/pgp> to find out how to use it.
> My OpenPGP key: <http://hajhouse.org/hajhouse.asc>.

I can not offer any technical solutions to your questions, but as a
Professional Land Surveyor, I do want to stave off any potential
misconceptions about the US Public Land Survey System (PLSS). It is
not, and was never intended to be, a regular Cartesian coordinate grid
like e.g. UTM and using processing algorithms that assume it is a
regular grid will usually lead to faulty results.

The PLSS was developed more than 200 years ago to mark points on the
ground and distribute real property. It is not a geodetic reference
system. The PLSS accommodates measurement errors (that were larger 200
years ago than they are now!) along with "errors" due to convergence
of meridians, and distributes those errors in places that can not be
mathematically modeled. Legally, the only way to divide a section
involves extensive field measurements.

The US Bureau of Land Management (BLM) has a digital product called
the Geographic Coordinate Database (GCDB) that if some areas includes
divisions of sections. This is still not a legal division of the
section, but attempts to follow the legal principles (clear as mud,
huh?). I suggest that you might Google on "California GCDB"  for a
while before investing too much time a purely mathematical approach.

Regards,
Rich

-- 
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com