thanx
this may explain part of the slight discrepancy i recently observed
between the length of the vawv terminal segment as reported by the
surveyor using state plane coordinates & the same length as computed
by me with a great circle arc calculator
or are 4 hundredths of a foot of aberration in about 80 feet of run
too much to be explained away by that
http://groups.yahoo.com/group/BoundaryPoint/message/15635
but anyway
i hasten to add
it wont really matter in this case or perhaps ever
since i wont be able to measure quite so carefully anyway
being at the mercy of an uncertain altitude change in addition to
the certain latitude & longitude changes
not to mention the unavoidable sag in the tape measure
& a too approximate compass bearing
but it was puzzling & interesting to encounter anyway
--- In
BoundaryPoint@yahoogroups.com, "Ron McConnell" <rcmcc@e...>
wrote:
>
> 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@a...
>
> 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>