[APG Public List] [APG Members] place names
wilssearch at gmail.com
Tue Oct 26 06:59:43 MDT 2010
Stephen - thank you for pointing out how GPS numbers are coded. Learn something new every day!
On Oct 26, 2010, at 1:29 AM, Stephen Danko wrote:
GPS coordinates can be specified so that they represent either a large general area or a spot the size of a pinpoint. It's all in how many significant figures one lists in the coordinates.
The number of significant figures refers to the number of numerals in the coordinate. 37.79507, -122.40280 has 7 significant figures in the N-S coordinate (the first number) and 8 significant figures in the E-W coordinate (the second number). This is the location of the Transamerica Pyramid in San Francisco. Actually, it is the location of a part of the Transamerica Pyramid. This coordinate has so many significant figures that it points to a very accurate location on the earth. If the coordinate is written as 37.8, -122.4, the area described becomes much larger and includes anything between 37.75 to 37.84, -122.35 to -122.44. Written as 37.8, -122.4, the coordinates don't accurately specify the Transamerica Pyramid, but instead specify about half of San Francisco.
Note that 37.8, -122.4 is not the same as 37.8000, -122.4000. The first set of coordinates gives a general location (the eastern half of San Francisco) and the second set gives a very specific location (the corner of Vallejo and Front Streets in San Francisco).
The latitude longitude system we use today is based on a prime meridian through Greenwich, England. Historically, different places used different prime meridians (including prime meridians in Paris, Washington, D.C., and Rio de Janeiro). For genealogical purposes, all historic systems can be ignored and we can use today's system with a prime meridian through Greenwich because, as Larry pointed out, our goal in using latitude longitude coordinates is to specify where a place is today, using today's system. Using GPS coordinates, Nicole (in a different thread) would have immediately been able to find out where Grossendorf, Germany was, and she would have immediately been able to see that the present name of the place is Władysławowo, Poland.
True latitude/longitude coordinates are not exactly the same as GPS coordinates. This is because GPS uses the International Reference Meridian which is 102.5 meters east of the Prime Meridian through the Royal Observatory in Greenwich. Don't worry about the reason for this difference. Google Earth used GPS coordinates instead of true latitude longitude coordinates, presumably so that GPS systems will match up with Google Earth.
In summary, GPS coordinates can be used to describe a very specific point on the earth or a very general area of the earth depending on the number of significant figures in the coordinates.
Stephen J. Danko
From: Connie Sheets <clsheets1 at yahoo.com>
To: apgpubliclist at apgen.org; apgmembersonlylist at apgen.org
Sent: Mon, October 25, 2010 4:33:55 PM
Subject: Re: [APG Members] [APG Public List] place names
I must admit to only a general knowledge about, and frequent lack of attention to, GPS coordinates. I have been operating under the assumption they describe a specific point on the earth, not a large area like a rural US township of 36 square miles.
I can comprehend how I might want to visit the crossroads approximately one mile southeast of a very small village in Northwest Missouri where my great-great grandfather's house once stood, obtain the GPS coordinates, and record them for posterity with a photograph I have of the house. I also understand how GPS coordinates are useful for locating cemeteries, graves within cemeteries, and other landmarks.
However, if all I know about an ancestor's location is a rural township or county, I would be concerned that I was promoting inaccuracy if I arbitrarily chose (and yes, it would be arbitrary) the center of the township or county.
I will continue to use standard historical place names, with a reference to the modern place name when necessary, for the foreseeable future.
Masters Student, Dept. US Military History
American Military University
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