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| Subject: | RE: GISList: SUM: Convert township-range-section to coordinates |
| Date: |
11/20/2001 10:06:41 AM |
| From: |
Andrew Hunter |
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Hi
It is possible to determine the lat/long of 1/4 section centroids, however
you need a number of pieces of information in order to do it. Up here in
Canada we have the Township System, which is essentially the same as the
PLSS.
In order to be able to convert between the cadastral referencing system and
lat/long or UTM you need to know to know some basic information. These
cadastral systems were created by setting out a prime meridian and then
laying out Range lines at 6-mile intervals. Each Range was then split into
Townships 6 miles wide. Each township is then divided into 36 sections. In
Alberta these are number starting at the bottom right corner in row prime
order (I think it may differ in some parts of the states). Each section is
then split in 4 to create 1/4 sections. 1/4 sections can also be split in
four to get to the Legal Subdivision level, and so on.
In Alberta Ranges are numbered from 1 at the border with Montana to 126 at
the border of Northwest Territories. Townships are numbered from 1 westwards
from each prime meridian.
So if you know your prime meridian and the latitude of your initial baseline
you can then calculate the lat/long of each corner, or centroid, of a
cadastral polygon down to any level of accuracy you wish, basically once you
get to section level just keep dividing by 4.
Some things to be aware of:
1. The numbering direction of Ranges and Townships and Sections may be
different from up here in Canada, I'm not sure.
2. The location of baselines. As the world is somewhat spherical,
adjustments need to be made to the rectangular cadastral system so that
distortions can be minimized. These adjustments are made at baselines.
Essentially the township width of 6 miles is reestablished.
Anyway I hope that helps somewhat.
Regards
Andrew Hunter
-----Original Message-----
From: gislist-return-3594-ahunter=ucalgary.ca@geocomm.com
[mailto:gislist-return-3594-ahunter=ucalgary.ca@geocomm.com\On Behalf Of
Michael Gallagher
Sent: Monday, November 19, 2001 22:55
To: bryan@geomega.com: 'gislist@geocomm.com': 'Cynthia Feirman': 'Jonathan
Bailey'
Subject: Re: GISList: SUM: Convert township-range-section to coordinates
on 11/19/01 4:49 PM, Bryan Keith at bryan@geomega.com wrote:
> Well, this isn't much of a SUM since I still don't have an answer.
Perhaps
> I wasn't completely clear in my initial post (included below). I had a
> number of suggestions and hints including how to calculate a polygon
> centroid, how convert long-lat to UTM, how to get the bounding coordinates
> of a polygon (none of which I was having trouble with). However, no one
> offered a technique to approximate the center of a quarter section if that
> section isn't roughly square. It's really a problem of not knowing what
is
> meant by "northwest quarter of the northeast quarter of section 15" when
> section 15 looks more like a triangle than a square. Theoretically
> sections should be square, but there are many exceptions. I'm not
fretting
> about it too much at this point since I still don't have the data that was
> supposed to make this an "urgent" problem two weeks ago. I'll post a real
> summary to this list if I find out anything new.
>
It may well be that the two systems are independent and cannot be converted.
Try talking to a surveyor, there must be surveyor listservs. There may be
information in the original plat that will give you geodetic control points,
but that still won't allow a conversion(except for the points themselves).
The PLSS was not consistent to the degree needed to convert to lat/long or
UTM. And it is unique to each state (I think) as well. It had a lot of
variation and later correction was a political process as much as a
technical one. In Wisconsin the iron deposits in the north resulted in large
magnetic errors and some vary odd township shapes.
You might be able to use the same points on a known base and your provided
files to to provide a Lat/Long reference. Projection distortion should not
be an issue given the typical scale township-range is used.
This would involve finding the same coverage in a different dataset that has
identifiable matching points in both (Road intersections, Town centers,
etc,) If you can scale and distort (hopefully just a little) the two files
until they match you could get your reference system. I do this sometimes in
my small scale mapping and can get files to match well within the precision
of the map, usually in about five minutes. Unprojected data should work ok.
Good luck
Mike Gallagher
Midwest Educational Graphics
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