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| GeoCommunity Mailing List |
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| Subject: | Re: GISList: SUM: Convert township-range-section to coordinates |
| Date: |
11/20/2001 05:18:19 PM |
| From: |
Bill Thoen |
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Bryan Keith wrote:
>
> ...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. ..."
I know what you're up against, and the answer involves some
assumptions and a bit of geometry. The assumptions involve
defining what YOU mean to be the "center of the NW NE
quarter-quarter of sec 15." In real life, sections are not
perfectly square miles: I don't believe there's a single one in
the USA. But most are square... ish. Sometimes sections are
truncated (like at the Colorado borders: some are even split!),
and if there's only half a section or it's triangular (like along
rivers) it's an open question whether there actually IS a
particular quarter.
I once wrote some software to automatically spot oil wells from
footage calls and quarter-quarter section descriptions, and it
handled most any PLSS or Canadian land grid cadastral systems
(they only look similar, though.) Most sections were behaved
well enough to use in an automated process but if a section was
too weird, the program flagged those locations for a human or a
special geocoding routine to handle. Plan for the
program-crashing exceptions: there are some howlers out there!
The first trick is to identify the section lines for your section
as east line, north line, west line and south line. The technique
I used was to find the four most likely corner points and mark
them as line end nodes. If you can't identify four different
points, or the area is too small, punt it to the "weird" bin.
Most sections will be close enough to square to be easily
identified.
Then I would find the section center by calculating the
intersection of diagonal lines running through opposite corner
points. Then I bisect the two outside lines (section borders)
that enclose the quarter I want thus creating two more points.
With these points and the center and the section corner, I now
have the four points that define the quarter. Use the same
algorithm recursively and you can get the center of the quarter
and then the next four points of any quarter-quarter. I used
straight-line bisection because I didn't feel that anything more
fussy was really any more accurate for estimating center points.
However, I did use all line detail when I also had footage calls.
There are other ways, but this is the fastest, and I assume that
the error is not significant given the imprecision of the
location description. In a sense, I'm defining how the centers
are calculated: the exact locations where somebody actually
drills a well may fall elsewhere a bit. If all you have to work
with is a vague location description and no PLSS benchmarks,
witness corners, etc., then you can only be so precise. If
accuracy matters, use GIS --Get It Surveyed.
--
- Bill Thoen
------------------------------------------------------------
GISnet, 1401 Walnut St., Suite C, Boulder, CO 80302
tel: 303-786-9961, fax: 303-443-4856
mailto:bthoen@gisnet.com, http://www.gisnet.com/
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