From: David E. DeLano (david.delano@cox.net)
Date: Fri Jul 19 2002 - 18:13:25 CDT
>The difference may be Datums for the plotting of waypoints accurately.
>WGS84 vs. NAD27 maybe. I tried to find a datum on the maps to no avail.
>For the record, my datum was set to WGS84. If you can find a different
>setting on a Philmont map, I would love to see it. I looked several
>times.
>I was very surprised there was no reference to a datum on the maps.....
The Philmont maps are NAD27. Most USGS maps are NAD27. Most of these maps
haven't been updated in 20-30 years, except for minor updates. If you use a
computer map like TopoUSA, you can get WGS84. The newer versions actually
give you a choice. I keep mine set at NAD27 as not to confuse anyone (e.g.,
the navigator).
>The difference may be Datums for the plotting of waypoints accurately.
>WGS84 vs. NAD27 maybe. I tried to find a datum on the maps to no avail.
>From your description, this is the most likely case.
>So I wonder.........if I go into the setup on my gps and change the datum
to
>NAD27, will the plotted waypoints magically pop into the right locations
>based on the exact grid points that I did locate. I might try this.
John already answered this, but I'll reiterate. This will not work. When
you change your setting, the waypoints will be "converted" to where the
original points would have been in WGS84. They will not have the same
numbers you entered in. This will put you farther and farther off with each
conversion.
>However, that would just confirm that the map is still incorrect as I read
>that WGS84 should be the standard used by default in most of the US.
Yes, WGS84 is the current standard, but the maps haven't been changed. Note
that a new survey has been started sometime in 2000. I haven't heard when
it will be complete.
You also made a statement of how useful the data is. I'll reiterate that
all of the points I entered in were off by less than 50', and that could
easily be attributed to the difference between where the originator marked
the spot, and where I marked the spot. Waypoints that left little doubt,
like the top of Black Mt., were dead on. I suppose it was off a bit too,
but the two dots showed up in TopoUSA on top of each other. I also set a
couple waypoints off the map, with the navigator reading me the data. These
points were also as accurate and any difference could be attributed to how
accurately the navigator was reading the data.
David DeLano
627H2
VC124, Phoenix, AZ
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