Including Original "Paul H. Letters" Copyright © 1996-2017 Paul V. Heinrich - All rights reserved.



Saturday, 23 February 2013

Township and Range - Public Land Survey System (PLSS)

Township and Range - Public Land Survey System (PLSS)

In “[Rockhounds] Township and Range - Public Land
Survey System (PLSS)” at
http://lists.drizzle.com/pipermail/rockhounds_lists.drizzle.com/2013-February/000623.html ,
Lanny R wrote;

“The two systems are two different systems, designed
for two different purposes. The PLSS was designed to
describe a piece of land; the GPS system and its
coordinate system was designed to locate a point.
Describing a piece of land with a coordinate system
is cumbersome.”

Also GPS can become complicated even when a person
only uses latitude and longitude. This is because there has
been changes in the North American Datum. For example,
presumably GPS systems currently use either the North
American Datum of 1983 (NAD83) or its close cousin
World Geodetic System 1984 (WGS84) to calculate latitude
and longitude. This might create a problem because a
number of paper USGS Maps use the North American
Datum of 1927 (NAD27). In the western United States, the
difference in location between same latitude and longitude
coordinates in NAD83 and NAD27 can be from 40 to 100 m
on the ground as illustrated in “File:Datum Shift Between
NAD27 and NAD83” at
http://en.wikipedia.org/wiki/File:Datum_Shift_Between_NAD27_and_NAD83.png

Also, see “North American Datum” at;
http://en.wikipedia.org/wiki/North_American_Datum

NAD27: What Is It and Why You Should Care by Dane E. Ericksen,
P.E., Hammett & Edison, Inc., Consulting Engineers. 1994 SBE
National Convention and World Media Expo
http://h-e.com/sites/h-e.com/files/tech_docs/de_sbe94.pdf

Fortunately, DGRs (digital versions) of topographic maps
are normally, but not always, georeferenced to NAD83 and
latitude and longitude from a GPS unit should be the same
spot on the map. Still to be precise, a person still needs to
determine whether the GPS unit they are using, the maps
and aerial imagery that they are using; and the coordinates
that they have been given are based on either NAD83,
NAD27, or WGS84. Also, when someone publishes points
using latitude and longitude, they need to specify the datum
(NAD83, NAD27, or WGS84) used by the GPS unit, maps etc.
used to determine the coordinates. Finally, to further confuse
people, a lot of states have their mapping and coordinates in
latitude and longitude in local datums known as State Plane
Coordinate Systems, which are typically based upon NAD27.
Some states have different State Plane Coordinate Systems
for different parts of a states. For example Texas has four
separate state plane coordinate systems and Louisiana has
two of them.

Having worked with archaeologists, I prefer the UTM
coordinate system over latitude and longitude. For
more details, go see “Universal Transverse Mercator
coordinate system” at
http://en.wikipedia.org/wiki/UTM_coordinate_system

and “Transverse Mercator projection” at
http://en.wikipedia.org/wiki/Transverse_Mercator_projection

Other online information.

Projections, Coordinate Systems and Datums explained
http://geospatial.osu.edu/conference/proceedings/workshops/conner.pdf

Datums and Coordinate Systems
http://www.fs.fed.us/database/gps/aboutgps/documents/04_datum.ppt

Projection, Datum, and Map Scale - New Mexico Tech
http://www.nmt.edu/~gjones/Lecture4-5.ppt

Yours,

Paul H.