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Old 27-10-2010, 20:10   #1
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GPS Corrections on e-Charts

Paper charts will tell you that the positions derived by satelite nav systems can be plotted directly onto the chart OR need to be corrected by 'x" minutes and seconds North and 'y" minutes and seconds West.

Do electronic charts take this into account - even more so with Quilting?

Safe sailing, Tony

(btw I'm becoming a Huge Fan of Open Cpn!)
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Old 27-10-2010, 20:34   #2
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I think what you are alluding to is chart Datums. The datum in use by the GPS needs to be set to the same datum of the chart before you can be assured on any reasonable degree of accuracy/agreement between the two.
- - Theoretically the charts used in electronic navigation have been corrected to WGS-84 datum which is also the default datum in most GPS units. If the e-chart has not been corrected to WGS-84 then you will see a difference between where the GPS thinks you are and where you really are located. You have to read the GPS manual to find out how to access the internal datums.
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Old 27-10-2010, 20:55   #3
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Originally Posted by osirissail View Post
I think what you are alluding to is chart Datums. The datum in use by the GPS needs to be set to the same datum of the chart before you can be assured on any reasonable degree of accuracy/agreement between the two.
- - Theoretically the charts used in electronic navigation have been corrected to WGS-84 datum which is also the default datum in most GPS units. If the e-chart has not been corrected to WGS-84 then you will see a difference between where the GPS thinks you are and where you really are located. You have to read the GPS manual to find out how to access the internal datums.
Thanks - I'm on top of chart datums and how to change the gps if required but wasn't sure if e-charts were all WGS84. It'd be nice if someone could come up and say "definitely" rather than "theoretically" though!! T
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Old 27-10-2010, 21:07   #4
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Can't say definitely because some vendors do re-register their e-charts to WGS-84 and some do not. You have to contact the e-chart vendor to find out. Raster charts are scanned images of actual paper charts so you can look in the margin/corner and read what the datum is. Some chart management programs can read the "headers" on the e-charts and tell you what is the datum in use for that chart. Most e-nav programs have somewhere in their system a menu items to call up the chart information which would include the datum in use.
- - But in every case you do not bet your boat on the e-chart or even a paper chart being accurate especially if it is linked to an active GPS and has a "little boat" symbol displaying where it thinks you are located. You must confirm visually or by radar whether you really are there.
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Old 27-10-2010, 22:03   #5
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Originally Posted by osirissail View Post
Can't say definitely because some vendors do re-register their e-charts to WGS-84 and some do not. You have to contact the e-chart vendor to find out. Raster charts are scanned images of actual paper charts so you can look in the margin/corner and read what the datum is. Some chart management programs can read the "headers" on the e-charts and tell you what is the datum in use for that chart. Most e-nav programs have somewhere in their system a menu items to call up the chart information which would include the datum in use.
- - But in every case you do not bet your boat on the e-chart or even a paper chart being accurate especially if it is linked to an active GPS and has a "little boat" symbol displaying where it thinks you are located. You must confirm visually or by radar whether you really are there.
Agreed - Nothing beats the Mark One Eyeball
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Old 28-10-2010, 01:47   #6
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e-charts were all WGS84. T
Hi Tony,

All e-charts are WGS84.

You could just imagine the lawyers glee if, say, Garmin brought out their 2011 series with a new datum putting everyone 1 nm east.

It's always going to be the automatic selection - WGS84 and if the ever change (or its not) then everyone will know.
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Old 06-12-2010, 05:30   #7
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Many raster charts are not WGS84. Some of these are on a known datum like ED50 but some of them are to an unknown datum.
S-57 ENCs are all WGS84 (by definition).
Most ENCs are produced from paper charts via raster.
So how do chart producers get charts from an unknown datum into S-57 format? They do the best they can to work out what the datum is - but sometimes this is from pretty sketchy information.
Keep looking out of the window...
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