> > My QTH is 34 47 03 N and 77 23 15 W. >
> Of interest to me, plugging my location into Google or Microsoft > Streets program, there's a "slight" error in locating my QTH. My QTH was > determined using a couple of Garmin GPS units. Perhaps Google and > Microsoft are deferring accuracy due to this being a military area? > > Mike Another interesting question .... it looks as if you stated your location in FCC style coordinates of deg-min-sec. As I figure it, the resolution accuracy (not the position accuracy) of such a reading is very roughly 100 feet, that is, if 34-47-03 changed to 34-47-04, you would have changed ~ 100 feet. Of course this will be different in the N-S direction, than in the E-W direction, as typically the area bounded by any adjacent second lines is a rectangle which varies by how far N or S one travels, as the E-W separation converges at the poles.. My Magellan MAP330M which is 5 years old, can give a reading in its highest resolution mode of DD MM.MMM, or such as 28 deg 03.456 min. This implies a resolution accuracy of roughly six feet, which is somewhat greater than one would normally trust [even with SA turned off] due to various factors such as h t and v dilution of precision, caused by ionospheric instability etc. My own thought would be to rely on what the Garmin is telling you, especially if the readings are repeatable from day to day. I'd think a variation of 20 to 30 feet is pretty good. On some days it will seem to be better than that, others, not. I would always use the GPS at the highest display resolution, even if the values don't always track exactly, so as to know the amount of error you're dealing with. I think DD MM SS is a bit more granular than you need. It's probably harder to repeatably find the points where the sec. reading changes as one travels. Then the question is why the map info is off (you didn't say by how much, and if the two sources are each off by the same amount and direction). That, I think, is important to understanding the reason for the error. It may be benign, i.e. old map coordinates in some old database being used to import into new programs. I would think the "FCC style" of DD MM SS is plenty good enough for their field inspectors** to be able to locate transmitter sites, so they probably have little need to try and upgrade their database format. I can't (yet) imagine any military influence on publishing of map coordinates for adjacent civilian areas, which I believe is your situation. **do I overestimate today's RI ?? Maybe in Russia where it is common to publish maps with deliberately skewed data. Unfortunately, one sees coordinates published in any of three formats, the two I have above, and then the DD.DDDDD format (fractions of a degree) which I suppose are most useful for programs that do distance measurement. It's not always obvious which of the first two are being used by someone, unless the smallest (rightmost) value is > 60, you can assume it is not seconds, but fractions of a minute. If I see a coordinate in DD MM SS I have to manually convert it to DD MM.MM if I want to create a waypoint in my Magellan. Perhaps the latest ones have fixed this problem? - Bob _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
