jeffrey.siegel@activecenter.com
At 05:53 PM 5/14/05 -0400, you wrote:

Jeffrey,
Outdoor Navigator Chart 17372 dated Sept. 2003, corrected to Dec 2003,
inset for "The Summit" clearly states that the datum of the chart is NAD 1927.
Outdoor Navigator we all agree does not provide any method for changing (or
correcting for the datum?). If it is a fact that the chart inset is in NAD
1927, then we must assume that if the position reports from a GPS are in
WGS84 datum there will be an error of some small magnitude due to the datum
being different.

I have just conducted a test using your chart described above, and a Garmin
48. I set the GPS in simulation mode, put the boat at 56 40.000 North and
133 44.000 West, speed ZERO. This was using WGS84. This is a cross hair
location exactly on one of the lat/long lines on the chart.
I then changed ONLY the datum, used in the Garmin to NAD 1927 Alaska. The
new position is 0.025 minutes north and 0.095 minutes east of the original
position, a location about 90 percent of the way over to the red marker
number "24". These differences are reasonably in line with the changes to
be seen in charts with similar issues in Washington state. I would estimate
that in raw distance this is about 50 yards north and about 100 yards east.

Is it your claim that the Outdoor Nav chart inset has been corrected to
WGS84 before being calibrated? If so, then we need to know that is the
case. As it is I would have to go physically back to that chart location
and run the test to see if your chart used with a GPS using WGS84 displays
the boat at the cross hair location or 50 to 100 yards somewhere else.
If it is not the case then the best way to chart plot that location using
that inset is to change the datum as I have described.

By the way, when we went through this inset area using our paper chart, we
had the GPS in NAD 1927 datum and the locations appeared to be right on the
money and would have been off by the amount I mentioned above if we had
used WGS84.

In either event, the user community is entitled to a clear understanding of
what is going on.
Either the chart inset as displayed and clearly says that the datum is NAD
1927 and the electronic chart is in that 1927 datum

or

maptech has recalibrated the inset and we are entitled to know that, in
which case is maptech making a warranty? that what is on the face of the
chart is incorrect and that we can navigate using WGS84 with safety? in an
area that requires precision navigation on the order of less than 30 yards.
After all the fallback if the chart plotter software does not work, we
should be able to do our own plotting by hand on the face of the PDA. But
of course then we would have to be in the "right" datum, or else.

This is a not a complicated problem, nor does it involve any tricky
mathematics and a lot of math jargon. If you can't explain this in simple
english then I have right to assume you don't understand the problem.
Frankly, I can't imagine that you can't explain this in simple English.
Your turn...

Regards,
Mike

Capt. Mike Maurice
Tualatin(Portland), Oregon

"Ron Rogers" rcrogers6@kennett.net
At 11:58 AM 5/15/05 -0400, you wrote:

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