Some Notes on the Public Land Survey System (PLSS)
A substantial number of published references to fossil sites give locations using the Public Land Survey System (PLSS) coordinates or Section, Township, Range, (Basepoint). In Canada, a similar system is used called the Dominion Land Survey (DLS).
There are several problems with PLSS/DLS locations.
- It is very easy to make transcription errors in writing these locations down or copying them. Some locations have been hopelessly garbled before I got them. I surely mangled some on my own.
- The locations consist of a section, range, township, and basepoint("meridian"), but the basepoint is seldom carried along when the location is published. It is not uncommon to omit part of the range and township designation as well. Often it is possible to infer the basepoint, and often to guess at missing N/S and E/W designators on Range and Township. But not always.
- The PLSS and DLS are survey systems, not mapping systems. What that means is that the survey tries to follow a methodical process in putting down survey markers. And it usually succeeds. But if an error is made because of haste, difficult topography, bad luck, or alcohol, the misplaced survey marker usually is not moved when it is found to be misplaced. Thus, although most sections are tidy one mile squares within 36 square mile townships, a few are irregular polygons of various sorts. There is no practical way for a modest computer program to handle misplaced survey markers or odd shaped sections resulting when two surveys run by different surveyors meet and their cumulative errors have to be dealt with.
- PLSS/DLS positions result from starting at an astronomically(?) determined basepoint that may be slightly inaccurate, then walking the countryside with measuring tools, marking off distances. Occasional adjustments are made for the geometric problems of mapping rectilinear grids onto a spherical surface. Under the best of circumstances, there will be modest errors converting these positions to Latitude and Longitude. Add to that the fact that the PLSS/DLS, USGS Lat-Long mapping and GPSes probably use different earth models, and yet another model is probably used in converting PLSS/DLS to Latitude-Longitude. This is a recipie for ending up a ten or twenty minute walk from where the fossils are. ... And that is when things go well.
- Some sources omit "unnecessary" information and are likely to present a position as 2-25-2w. The problem is that not everyone is consistent about the order of presentation for PLSS data. The most common format is Section, Township, Range. But the popular TRS2LL program uses Township-Range-Section order. Other variations are possible and quite likely are in use.
- The general nature of PLSS/DLS is as follows:
- All locations within a given survey zone are relative to a parallel of latitude known as the "Baseline", and a meridian of Longitude known as the "Prime Meridian". Each survey has its own Basline and Prime Meridian. I infer that a survey starts by determining the intersection of the Prime Meridian and Baseline using astronomical measurements (sextant and chronometer) and marking that spot.
- Six mile by six mile 'Townships' are then marked off relative to the Baseline and Prime Meridian. They are designated as Township 1 North/South, 2 North/South, ... Range 1 East/West, 2 East/West, ... etc.
- The Township itself is divided into 36 sections numbered in a bizzare manner -- Boustrophedonically. In the US, the North-Easterly most section is 1. Sections then count west to 6. Section 7 is the section South of section 6. Sections 8-12 are then numbered West to East. Sections 13-18 are below 7-12 and count from East to West. 19-24 run West to East, 25-30 East to West, and finally 31-36 are across the bottom of the township numbered West to East. The corners and midpoints of the Sections are identified by markers. The DLS uses a similar numbering scheme except that Section 1 is in the SouthEast and section numbers count West and North rather than West and South as in the US. Some pre-PLSS surveys, in Ohio for example, appear to number sections in other ways than PLSS -- for example incrementing N to S rather than E to W. Since many section numbers in these townships may be omitted on topo maps and section boundaries are often ambiguous it's quite likely that I've put some sites in the wrong section. I suspect that the numbering is related to the order in which sections are surveyed, but that's just a guess.
- I currently think that in the Canadian DLS -- unlike the PLSS -- the basepoint is the SouthEastern Corner of the survey area so that all townships are North and West of the basepoint. That would mean that there are no DLS locations with South or East modifiers. But that's just a guess and could be wrong.
- Sections except those on the Northern and Western boundary are -- conceptually at least -- exactly 1 mile by one mile. On the Northern boundary sections 1-6 may be slightly off from one mile due to accumulated surveying errors within the township and the fact the Earth is not a perfect sphere. These errors are quite small. Similarly, the Western-most sections are slightly compressed because the Western boundary of the township runs due North-South placing it on a Meridian of longitude. The meridians converge as one approaches the pole. Again, the adjustments are quite small and the resulting sections are close enough to a square mile not to annoy landowners.
- There is one further problem. If no other adjustment was made the convergance of meridians would make townships 6 miles wide in Oklahoma substantially less than 6 miles wide in the Dakotas. If an adjustment were made so that townships were always 6 miles wide on their Southern boundary, North-South roads along PLSS land boundaries would jog a bit every six miles. Instead, Township widths are adjusted for meridian convergance every 24 or 36 miles resulting in fewer, but larger jogs (and more grief for PLSS to Lat Lon conversion software).
Copyright 2016, Donald Kenney (Donald.Kenney@GMail.com). Unless otherwise stated, permission is hereby granted to use any materials on these pages under the V2.5 Creative Commons License
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