[APG Public List] mapping and research
john at jytangledweb.org
Mon Nov 1 20:50:25 MDT 2010
On 11/1/2010 9:46 PM, DonnDevine at aol.com wrote:
> For all the reasons offered earlier in this and the preceding threads
> "Digital Locations" and "place names," I agree that numerical reference
> systems are a useful way to describe the location of a geographic
> feature independent of changing boundaries and landscapes. As such, it
> is an item of information (evidence, when relevant to an issue) that
> needs a citation to its source--a sextant reading, GPS instrument, map
> measurement, GoogleMaps solution, or an original source cited elsewhere.
> I do have a problem with "digital location" as an alternative to "GPS
> location". "Digital location," like the more descriptive "geodetic
> location," is a generic term applicable to different numerical reference
I introduced it as a generic term on purpose. I propose that it
could be the generic name applied to any in its family of
source reference types. Then the researcher would specify precisely
which system they used, GPS, UTM, etc., which is important to note,
if not implicitly conveyed by the style of coordinates reported.
But it is a convenient generic umbrella way to classify all the types.
Just like Church Records, Census Records, have subtypes, so would
> systems for precisely designating points on the ground. Besides GPS
> (which is not a rectangular grid, and as noted earlier, differs slightly
> from latitude and longitude measured from the traditional meridian
> through the Greenwich Observatory, being based on a line 5.31 seconds of
> arc to the east, or 102.5 meters at the latitude of the observatory),
> there are a number of other digital systems for designating geographic
> locations by numerical reference to a rectangular grid on a projected
> flat surface, known as plane coordinate systems. The most common is the
> UTM or Universal Transverse Mercator grid, with locations designated by
> distance in meters on x and y axes from an origin point in each of its
> 18 east-west zones. Other systems, commonly use in land surveying, are
> the older legally-adopted state plane coordinate systems, with locations
> designated in feet north and east from the origin of each state's grid.
> Note that the GPS designates a location on the most recent calculation
> of the earth's spheroidal surface in relation to its axis and equator. A
> GPS designation as a reference to a particular land location is subject
> to change as the continental plates shift with tectonic drift. The
> precision of the GPS system will now allow these changes to be observed
> and measured over the years (and the aiming points for intercontinental
> missiles adjusted accordingly).
> Donn Devine, CG, CGL
> Wilmington DE
> CG, Certified Genealogist, CGL, and Certified Genealogical Lecturer are
> service marks of the Board for Certification of Genealogists, used under
> license by board certificants after periodic evaluation, and the board
> name is registered in the US Patent & Trademark Office.
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