Two definitions of a foot in the USA
1. standard ("international") foot = 0.3048 meters
2. U.S. survey foot = 1200/3937 meters = 0.3048006096012 m
<
http://www.vterrain.org/Projections/sp_feet.html>
- Full note with more links below this message body
This topic may have been covered already in BP,
but for a new folk like me, the extract below
was very interesting, including the statement
"... there have been instances where the conversions
were done using the wrong conversion factor
or the computation was otherwise done incorrectly
[by the professionals]..."
So, we need to look closely at the data
we get from anywhere in the USA to see
_if_ we can tell which foot was used or converted.
That last "millimeter" of calculated distance
may be correct to only the last several meters.
This type of multiple definition of a unit/name
is not exclusive to the USA.
There are _many_ other interesting, well written notes
links and resources on geography at
Virtual Terrain Project (VTP)
<
http://www.vterrain.org/index.html>
"The goal of VTP is to foster the creation
of tools for easily constructing any part
of the real world in interactive, 3D digital form.
The VTP software consists of a set of entirely
free and open source applications and libraries."
I've added a link to VTP to BP.
Cheers, 73,
Ron McC.
w2iol@...
Ronald C. McConnell, PhD
WGS-84: N 40º 46' 57.6" +/-0.1"
W 74º 41' 22.1" +/-0.1"
FN20ps.77GU31 +/-
V +5058.3438 H +1504.2531
http://home.earthlink.net/~rcmcc
Don't argue about facts.
- G. Robert Leopold
===============================
State Plane Coordinate Systems and the U.S. Survey Foot
<
http://www.vterrain.org/Projections/sp_feet.html>
There is a unit of measure called the "U.S. Survey Foot."
It is almost exactly equal to a standard foot,
but it's definition is slightly different:
* standard (or "international") foot: .3048 meters
* U.S. survey foot: 1200/3937 meters
In other words, one is defined in relation to the meter
by a decimal expression, the other by a fraction.
Fortunately, the difference is very small,
amounting to less than a few meters over an area
as large as a U.S. state.
However, this small difference can cause problems
in some cases, so it's advisable to always know
which units are being used.
More information about how this relates to the
U.S. State Plane Coordinate System is contained
in this fascinating and informative document:
POLICY OF THE NATIONAL GEODETIC SURVEY CONCERNING
UNITS OF MEASURE FOR THE STATE PLANE COORDINATE SYSTEM OF 1983
January 1991
Key excerpts:
* The State Plane Coordinate System of 1927
was published in a unit of measure identified
as the "U.S. Survey Foot."
By 1959, except for surveying and mapping applications,
the United States had switched to the unit of measure
identified as the "International Foot."
* (In 1983) the U.S. Survey Foot was specified
in 11 states and the International Foot was specified
in 6 states. In all other states the meter
is the only referenced unit of measure in the SPCS 83 legislation.
The remaining 19 states do not yet have any legislation
concerning SPCS 83.
* Because SPCS 83 is presently published by NGS
only in meters, many surveyors, mappers, engineers,
and LIS users are converting these metric values to feet.
[...]
there have been instances where the conversions
were done using the wrong conversion factor
or the computation was otherwise done incorrectly.
More background for those interested:
The "Foot-to-Meter" Conversion
* <
http://www.wsdot.wa.gov/Metrics/ftmtr2.htm>
[USA] State Plane Coordinate System of 1983
* <
http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf>