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Archiver > TMG > 2011-02 > 1297704310


From: "John Cardinal" <>
Subject: Re: [TMG] Latitude and Longitude Conversion
Date: Mon, 14 Feb 2011 12:25:10 -0500
In-Reply-To: <4D595ABD.5070504@gmail.com>


Rick Van Dusen wrote:
> Unfortunately, Lat/Long readings are not as precise as we'd like
> (which is ironic, given that they're SO much better than what we had
> only a few years ago).
>
> Eric has noted that at 41 degrees N, one second is about 20m. If
> TMG is precise only to integer seconds, that's only within 20m, or
> plus/minus 10m, 30 ft.
>
> And Richard has noted that most civilian GPS units are really only
> precise to about plus/minus 15m.
>
> We may not be talking about mound to dugout, but even 30-50 ft in a
> cemetery can be a long way. (I have ancestors in at least one
> cemetery that is not 50 ft across.)
>
> From many stories I've heard, having a Lat/Long just for the
> cemetery itself would have been a huge help.
>
> So it seems that we just have to learn to be content with being
> able to locate any point on earth within plus/minus 10-15m; and the
> conversion to/from integer seconds turns out to be no more limiting
> than the rest of the technology.

Rick,

We can ignore the accuracy of GPS for the first level discussion of this
topic: if I determine that I want to store a LatLong of
42.346351,-71.097615, then I don't want software deciding that it's OK to
record it as the equivalent of 42.346111,-71.0975.

Next, while accuracy of GPS devices is an important topic, it's also
important to realize that coordinates don't always come from GPS devices. As
noted by multiple people, GPS devices are accurate to within 15 to 20
meters, but coordinates taken from online mapping services are subject to
different limitations. For areas with high definition satellite imagery,
it's quite possible to record locations more accurately than 15m/50ft. The
exact physical location will not match the LatLong coordinates exactly, but
when creating a map, the marker can designate a spot to within about 10sq
ft. For that reason, and to keep the marker in the same place on output as
you recorded on input, it's important to record the exact LatLong value as
reported by the online mapping service. In practice, you can truncate the
values to six decimal places as that is accurate to within less than a foot
and it's highly unlikely you have (or need) that level of accuracy. At five
decimal places, the distance increases to about 3.5ft; given that decimal
places don't cost anything, I stick with six.

I am not claiming that a LatLong chosen from a satellite map is accurate to
five or six decimal places in the physical sense, but if you want the
markers on online maps to reflect the spot you designated, record the values
to six decimal places and don't lose any precision due to format
conversions.

John


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