BoatBanter.com - WGS84 vs. NAD83

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Having a little problem with my chart display showing my position approx 150
ft west of actual, I checked chart datum (NOAA BSB raster type) and found
it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just for
the heck of it I set the GPS to NAD83. That didn't correct the position
error. From what I understand, WGS84 and NAD83 are practically identical,
so is there any reason not to just leave GPS set to WGS84 (this is in SW
Florida, USA)?

Any advice on what else to check to correct the westerly error would be
appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

--
Eliminate "ns" for email address.

Besides having the correct datum selected in your GPS, check the
longitude/latitude display settings. The difference between
degrees/minutes/seconds and degrees/minutes/tenths may account for the
discrepancy between your GPS and the Chart.

"Len Krauss" wrote in message
...
Having a little problem with my chart display showing my position approx
150
ft west of actual, I checked chart datum (NOAA BSB raster type) and found
it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just for
the heck of it I set the GPS to NAD83. That didn't correct the position
error. From what I understand, WGS84 and NAD83 are practically identical,
so is there any reason not to just leave GPS set to WGS84 (this is in SW
Florida, USA)?

Any advice on what else to check to correct the westerly error would be
appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

--
Eliminate "ns" for email address.

"Len Krauss" wrote in message
...
Having a little problem with my chart display showing my position approx
150
ft west of actual, I checked chart datum (NOAA BSB raster type) and found
it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just for
the heck of it I set the GPS to NAD83. That didn't correct the position
error. From what I understand, WGS84 and NAD83 are practically identical,
so is there any reason not to just leave GPS set to WGS84 (this is in SW
Florida, USA)?

Any advice on what else to check to correct the westerly error would be
appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

My Garmin 182C with the latest Blue Chip puts my boat exactly where it
should be in my marina. Right on the correct finger.
I recently bought Ozi Explorer and a friend gave me a set of older charts on
CD. With this set-up my boat is 150 yards too far North-East. I thought it
might be the datums but no other datum gave better results than WGS4. I
bought the latest paper chart for the area and plotted the L/L shown on the
laptop. Exactly correct on the new chart. So the old chart on the CD was
inaccurate by 150 yards in terms of the location of my marina. I imagine it
is more accurate for navigational hazards but it makes you wonder.

GPS's are great, aren't they?
My GPS 198C put me right in a slip. When tied up, the only difference was
that the GPS indicated I was on the right side when I was centered in a two
boat slip.
The key is to avoid constant heading/speed changes, so the GPS can keep up
with your progress.
Quite a confidence builder.
But, we'll never be without a paper chart on my boat, and the radar's always
on and a lookout posted. Even on nice, sunny days.

"Gordon Wedman" wrote in message
news:WtoIf.1208$n67.820@edtnps89...

"Len Krauss" wrote in message
...
Having a little problem with my chart display showing my position approx
150
ft west of actual, I checked chart datum (NOAA BSB raster type) and
found
it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just
for
the heck of it I set the GPS to NAD83. That didn't correct the position
error. From what I understand, WGS84 and NAD83 are practically
identical,
so is there any reason not to just leave GPS set to WGS84 (this is in SW
Florida, USA)?

Any advice on what else to check to correct the westerly error would be
appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

My Garmin 182C with the latest Blue Chip puts my boat exactly where it
should be in my marina. Right on the correct finger.
I recently bought Ozi Explorer and a friend gave me a set of older charts
on CD. With this set-up my boat is 150 yards too far North-East. I
thought it might be the datums but no other datum gave better results than
WGS4. I bought the latest paper chart for the area and plotted the L/L
shown on the laptop. Exactly correct on the new chart. So the old chart
on the CD was inaccurate by 150 yards in terms of the location of my
marina. I imagine it is more accurate for navigational hazards but it
makes you wonder.

Len Krauss wrote:
Having a little problem with my chart display showing my position
approx 150 ft west of actual, I checked chart datum (NOAA BSB raster
type) and found it was NAD83, whereas my GPS (non-WAAS) had been set
to WGS84. So just for the heck of it I set the GPS to NAD83. That
didn't correct the position error. From what I understand, WGS84 and
NAD83 are practically identical, so is there any reason not to just
leave GPS set to WGS84 (this is in SW Florida, USA)?

Any advice on what else to check to correct the westerly error would
be appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

It's your calibration data that is off by 150ft. Re-calibrate (if your
program allows it!)

Huh?
"Dennis Pogson" wrote in message
...
Len Krauss wrote:
Having a little problem with my chart display showing my position
approx 150 ft west of actual, I checked chart datum (NOAA BSB raster
type) and found it was NAD83, whereas my GPS (non-WAAS) had been set
to WGS84. So just for the heck of it I set the GPS to NAD83. That
didn't correct the position error. From what I understand, WGS84 and
NAD83 are practically identical, so is there any reason not to just
leave GPS set to WGS84 (this is in SW Florida, USA)?

Any advice on what else to check to correct the westerly error would
be appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

It's your calibration data that is off by 150ft. Re-calibrate (if your
program allows it!)

I think you can scan a paper chart or topo map and then upload to some nav
programs. If you do this you have to calibrate the map by telling the
software what L/L is represented by points selected on the chart. I don't
think the original poster is likely to be doing this.
I'm not aware of any calibration being necessary if you up load commercially
produced charts. I believe the calibration data is part of the chart data.

"William Andersen" wrote in message
news:LhUIf.81$xH.38@dukeread03...
Huh?
"Dennis Pogson" wrote in message
...
Len Krauss wrote:
Having a little problem with my chart display showing my position
approx 150 ft west of actual, I checked chart datum (NOAA BSB raster
type) and found it was NAD83, whereas my GPS (non-WAAS) had been set
to WGS84. So just for the heck of it I set the GPS to NAD83. That
didn't correct the position error. From what I understand, WGS84 and
NAD83 are practically identical, so is there any reason not to just
leave GPS set to WGS84 (this is in SW Florida, USA)?

Any advice on what else to check to correct the westerly error would
be appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

It's your calibration data that is off by 150ft. Re-calibrate (if your
program allows it!)

Oh, thanks.
"Gordon Wedman" wrote in message
news:XQ2Jf.1383$Y22.664@clgrps12...
I think you can scan a paper chart or topo map and then upload to some nav
programs. If you do this you have to calibrate the map by telling the
software what L/L is represented by points selected on the chart. I don't
think the original poster is likely to be doing this.
I'm not aware of any calibration being necessary if you up load
commercially produced charts. I believe the calibration data is part of
the chart data.

"William Andersen" wrote in message
news:LhUIf.81$xH.38@dukeread03...
Huh?
"Dennis Pogson" wrote in message
...
Len Krauss wrote:
Having a little problem with my chart display showing my position
approx 150 ft west of actual, I checked chart datum (NOAA BSB raster
type) and found it was NAD83, whereas my GPS (non-WAAS) had been set
to WGS84. So just for the heck of it I set the GPS to NAD83. That
didn't correct the position error. From what I understand, WGS84 and
NAD83 are practically identical, so is there any reason not to just
leave GPS set to WGS84 (this is in SW Florida, USA)?

Any advice on what else to check to correct the westerly error would
be appreciated. (using Capn nav software and Garmin GPS 48).

Thanks,
Len

It's your calibration data that is off by 150ft. Re-calibrate (if your
program allows it!)

William Andersen wrote:
Oh, thanks.
"Gordon Wedman" wrote in message
news:XQ2Jf.1383$Y22.664@clgrps12...
I think you can scan a paper chart or topo map and then upload to
some nav programs. If you do this you have to calibrate the map by
telling the software what L/L is represented by points selected on
the chart. I don't think the original poster is likely to be doing
this.
I'm not aware of any calibration being necessary if you up load
commercially produced charts. I believe the calibration data is
part of the chart data.

Len is keeping very quiet since his original posting. Has he found something
embarrassing, and doesn't wish to tell us?

No, Dennis, nothing embarrassing found. Was just waiting for more input from
everyone.

I was beginning to wonder if magnetic variation could possibly have
something to do with the position discrepancy. My GPS is set to
automatically adjust for it. I've downloaded for my Capn software the latest
magnetic variation data table. I concluded, however, that variation would
only influence magnetic heading displayed, and not lat/lon. In other words,
it has nothing to do with the problem. Does that sound right?

The only other thing I can think of is that chart datum and GPS datum need
to be in agreement. Chart is NAD83 and with GPS set to NAD83, position
error unchanged. Same with GPS set to WGS84. Datum is the only kind of
"calibration" possible, as far as I know.

Len

--
Eliminate "ns" for email address.

Len is keeping very quiet since his original posting. Has he found
something
embarrassing, and doesn't wish to tell us?

Len Krauss wrote:
No, Dennis, nothing embarrassing found. Was just waiting for more
input from everyone.

I was beginning to wonder if magnetic variation could possibly have
something to do with the position discrepancy. My GPS is set to
automatically adjust for it. I've downloaded for my Capn software the
latest magnetic variation data table. I concluded, however, that
variation would only influence magnetic heading displayed, and not
lat/lon. In other words, it has nothing to do with the problem. Does
that sound right?

Yes.

The only other thing I can think of is that chart datum and GPS datum
need to be in agreement. Chart is NAD83 and with GPS set to NAD83,
position error unchanged. Same with GPS set to WGS84. Datum is the
only kind of "calibration" possible, as far as I know.

Len

As far as I am aware, Garmins download all data from the satellites in
WGS84, and setting any other datum in the set-up program only changes the
numbers of Lat./Long. in the display.

If this is correct, the sentences being passed to your charting program are
in WGS84, and the chart datum is set to NAD83. However, since you maintain
that at that location, the two datums are virtually the same, there should
be little, if any discrepancy in the displayed position.

I am not familiar with the Cap'n software, but in Ozi, you have the facility
to change the datum of the displayed chart, i.e. the Lat/Lon overlay, and to
change it back again at will. Asssuming you change it to a datum which the
chart was not calibrated to, and there is a position difference between the
two datums, all that happens is that the positions of buoys and other marks
changes relative to the basic graphic. I usually place these to 2 decimals
only, since Reeds and other almanacs only gives the positions to 2 decimal
places (in UK).

Since the Lat/Lon shows up immediately I move the mouse cursor over a buoy,
if the third decimal place is anything other than a zero, I know that
something has changed, and usually this is the datum of the map/chart.

The many facilities for self-calibration etc. in Ozi enable the experienced
user to work out if something is not just as it should be, but with other
software it may not be possible to dig deep into the program and suss things
out. Datums, calibration, and projections are the usual culprits, plus of
course an assurance that the basic graphic is dead accurate (not always the
case, particularly with NASA satellite photographic downloads).

My guess is that the problem lies with the basic graphic, but with so many
potential variables, it's one hell of a job finding out. You could of course
take a precise reading on a buoy on the chart, and compare this to the
position shown in your almanac, just as a check.

Do you have another chart of the same area? If not, tell me precisely where
you are and I'll send you one, even if you are at the South Pole! How's that
for an offer?

BTW, for the record, I too use a GPS48, but have recently acquired a GPSMAP
60CS. Let me assure you that whatever else is wrong, it ain't your GPS! My 2
instruments are usually within .002 of longitude of each other, which I
make to be about 7 feet! (at 56 deg N.!)

Dennis.

Thanks for the offer Dennis.

First chance I get I'll get my paper chart and get a get lat/lons on a
couple long established fixed points, then see what my GPS lat/lons are for
them. My computer charts are recently downloaded NOAA BSB raster type. The
error I observed is in the position of my boat as shown on the my PC chart
display -- 150 ft to the west (and on land) when it is in fact floating in a
canal. It may well be that the GPS lat/lon is correct, and that the error
lies with the graphic, as you say. I have a backup GPS 48, so can also check
that they're in agreement.

I opened the suspect chart file in WordPad had had a look at header info. I
don't know what all those parmeters mean, but the chart datum shown there is
correct.

Capn software has no features I'm aware of for tweaking/calibrating. After I
check out everything, and can speak about specifics, I'll give the vendor a
call.

Len
--
Eliminate "ns" for email address.
"Dennis Pogson" wrote in message
...
Len Krauss wrote:
No, Dennis, nothing embarrassing found. Was just waiting for more
input from everyone.

I was beginning to wonder if magnetic variation could possibly have
something to do with the position discrepancy. My GPS is set to
automatically adjust for it. I've downloaded for my Capn software the
latest magnetic variation data table. I concluded, however, that
variation would only influence magnetic heading displayed, and not
lat/lon. In other words, it has nothing to do with the problem. Does
that sound right?

Yes.

The only other thing I can think of is that chart datum and GPS datum
need to be in agreement. Chart is NAD83 and with GPS set to NAD83,
position error unchanged. Same with GPS set to WGS84. Datum is the
only kind of "calibration" possible, as far as I know.

Len

As far as I am aware, Garmins download all data from the satellites in
WGS84, and setting any other datum in the set-up program only changes the
numbers of Lat./Long. in the display.

If this is correct, the sentences being passed to your charting program
are
in WGS84, and the chart datum is set to NAD83. However, since you maintain
that at that location, the two datums are virtually the same, there should
be little, if any discrepancy in the displayed position.

I am not familiar with the Cap'n software, but in Ozi, you have the
facility
to change the datum of the displayed chart, i.e. the Lat/Lon overlay, and
to
change it back again at will. Asssuming you change it to a datum which the
chart was not calibrated to, and there is a position difference between
the
two datums, all that happens is that the positions of buoys and other
marks
changes relative to the basic graphic. I usually place these to 2 decimals
only, since Reeds and other almanacs only gives the positions to 2 decimal
places (in UK).

Since the Lat/Lon shows up immediately I move the mouse cursor over a
buoy,
if the third decimal place is anything other than a zero, I know that
something has changed, and usually this is the datum of the map/chart.

The many facilities for self-calibration etc. in Ozi enable the
experienced
user to work out if something is not just as it should be, but with other
software it may not be possible to dig deep into the program and suss
things
out. Datums, calibration, and projections are the usual culprits, plus of
course an assurance that the basic graphic is dead accurate (not always
the
case, particularly with NASA satellite photographic downloads).

My guess is that the problem lies with the basic graphic, but with so many
potential variables, it's one hell of a job finding out. You could of
course
take a precise reading on a buoy on the chart, and compare this to the
position shown in your almanac, just as a check.

Do you have another chart of the same area? If not, tell me precisely
where
you are and I'll send you one, even if you are at the South Pole! How's
that
for an offer?

BTW, for the record, I too use a GPS48, but have recently acquired a
GPSMAP
60CS. Let me assure you that whatever else is wrong, it ain't your GPS! My
2
instruments are usually within .002 of longitude of each other, which I
make to be about 7 feet! (at 56 deg N.!)

Dennis.