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From: Mike Maurice (no email)
Date: Sun May 15 2005 - 13:09:42 EDT
"Ron Rogers" <>
At 11:58 AM 5/15/05 -0400, you wrote:
>I think that we need an empirically valid comparison. To do that, I believe
>you would need your Garmin set to 1927 and to leave your PDA with
>Jeff's/Maptech's software untouched. Then go to a coastal location that is
>unlikely to have changed since 1927. Perhaps a lighthouse or a rock
>promontory and compare the two readouts. If they agree, fine. If they do
>not, which one is accurately reflecting your known, stationary location?
Ron,
You don't understand the problem. You have the idea almost but not quite
right.
The issue is a chart or it's inset which is still in NAD 1927 datum. If it
is then it is absolutely necessary to use the correct datum with it when
using the GPS to plot on that chart or inset, see Bowditch.
You may recall that a couple of years ago there were a few complaints from
folks going in and out of Ft. Bragg Calif., that their plotting was off by
about 100 yards and showed them on the beach. That was most likely due to
the inset chart being NAD 1927 where the main chart was in WGS84 and their
GPS was in WG84 at the time. The solution then and still is for any chart
with a datum other than WGS84, to change the GPS to reflect the correct datum.
You meant to say above that it is not the "coastal location that is
unlikely to have changed since 1927", but that the chart datum has not
changed for that location(the key word here is "unlikely", which should be
removed). There may not be anyplace, although that itself is statistically
unlikely, within the lower 48 where the datum has not been corrected to be
on WGS84. But I have good proof that there is at least one place in SE
Alaska where the datum is still 1927 and the locations between datums are
different, period. Unless an electronic chart maker corrects a chart by
hand that has a datum other than WGS84 and changes it to reflect the
correction to WGS84 then leaving it uncorrected will result in a datum
error UNLESS the GPS is changed to reflect the datum in use when the chart
was drawn. However, if this electronic correction is done, then the face of
the displayed chart needs to have a note describing this change.
There is no other legitimate way to do this as any other way is more error
prone.
Regards,
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
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