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#1
posted to rec.boats.electronics
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WGS84 vs. NAD83
Huh?
"Dennis Pogson" wrote in message news 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!) |
#2
posted to rec.boats.electronics
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WGS84 vs. NAD83
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$xH.38@dukeread03... Huh? "Dennis Pogson" wrote in message news Len Krauss wrote: Having a little problem with my chart display showing my position approx 150 ft west of actual, I checked chart datum (NOAA BSB raster type) and found it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just for the heck of it I set the GPS to NAD83. That didn't correct the position error. From what I understand, WGS84 and NAD83 are practically identical, so is there any reason not to just leave GPS set to WGS84 (this is in SW Florida, USA)? Any advice on what else to check to correct the westerly error would be appreciated. (using Capn nav software and Garmin GPS 48). Thanks, Len It's your calibration data that is off by 150ft. Re-calibrate (if your program allows it!) |
#3
posted to rec.boats.electronics
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WGS84 vs. NAD83
Oh, thanks.
"Gordon Wedman" wrote in message news:XQ2Jf.1383$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$xH.38@dukeread03... Huh? "Dennis Pogson" wrote in message news Len Krauss wrote: Having a little problem with my chart display showing my position approx 150 ft west of actual, I checked chart datum (NOAA BSB raster type) and found it was NAD83, whereas my GPS (non-WAAS) had been set to WGS84. So just for the heck of it I set the GPS to NAD83. That didn't correct the position error. From what I understand, WGS84 and NAD83 are practically identical, so is there any reason not to just leave GPS set to WGS84 (this is in SW Florida, USA)? Any advice on what else to check to correct the westerly error would be appreciated. (using Capn nav software and Garmin GPS 48). Thanks, Len It's your calibration data that is off by 150ft. Re-calibrate (if your program allows it!) |
#4
posted to rec.boats.electronics
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WGS84 vs. NAD83
William Andersen wrote:
Oh, thanks. "Gordon Wedman" wrote in message news:XQ2Jf.1383$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? |
#5
posted to rec.boats.electronics
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WGS84 vs. NAD83
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? |
#6
posted to rec.boats.electronics
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WGS84 vs. NAD83
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. |
#7
posted to rec.boats.electronics
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WGS84 vs. NAD83
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. |
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