Re: [IRCA] 1610 NC TISs. OC on 1610.

["Bob Foxworth" <rfoxwor1@xxxxxxxxxxxxxxx>]

>
> 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
IRCA@xxxxxxxxxxxxxxxx
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: irca@xxxxxxxxxxxxxxxx