[APG Public List] mapping and research

DonnDevine at aol.com DonnDevine at aol.com
Mon Nov 1 19:46:09 MDT 2010


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  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  

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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