Apollyon
New member
Can anyone give me a quick laymans explanation as to why GPS derived positions differ to those on Admiralty Charts.
GPS v Admiralty
- Thread starter Apollyon
- Start date
rogerroger
New member
some Admiralty charts are based on surveys first done years ago so simply aren't very accurate.
Roger Holden
www.first-magnitude.co.uk
Roger Holden
www.first-magnitude.co.uk
LadyInBed
Well-known member
1 Older charts have different chart datum (OSGB not WSG84)
2 Some surveys are Very old
3 Errors when converting survey data to digitised data
4 combination of all above
2 Some surveys are Very old
3 Errors when converting survey data to digitised data
4 combination of all above
G
Guest
Guest
the hydrographic office have issued a note on this point, and i believe that it has been picked up by the magazines
the above factors are the cause. the accuracy of charts varies (from memory) from a few meters to 100 or so, with the worst u k areas being n scotland and lands end. in the pacific, it is even worse.
dont forget also that the 5 or 10 meter quoted accuracy of gps post selective availibility is the best achievable figure, with satellites at the optimum position etc. combine this with chart accuracy, and it seems to me that you navigate within 50m by gps alone at your peril.
whats the bets on some plonker in his tupperware tart trap attempting to negotiate a 15 m gap in fog on gps at 20 kn?
the above factors are the cause. the accuracy of charts varies (from memory) from a few meters to 100 or so, with the worst u k areas being n scotland and lands end. in the pacific, it is even worse.
dont forget also that the 5 or 10 meter quoted accuracy of gps post selective availibility is the best achievable figure, with satellites at the optimum position etc. combine this with chart accuracy, and it seems to me that you navigate within 50m by gps alone at your peril.
whats the bets on some plonker in his tupperware tart trap attempting to negotiate a 15 m gap in fog on gps at 20 kn?
ccscott49
Active member
I love that expression, "tupperware tart trap" I'll remember that one! tee hee.
Apollyon
New member
Thanks for the help - I have subsequently found the following links that help explain the difference as well...
Related link: http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
Related link: http://www.chersoft.co.uk/safe_gps_navigation.htm
Related link: http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
Related link: http://www.chersoft.co.uk/safe_gps_navigation.htm
claymore
Well-known member
Yes
I think you should share that one with the brylcreme team over on the motorboaty flora
The thread is an interesting one - My chartplotter sometimes puts me on Dock Street or in Freeport when the boat is in Fleetwood but there is practically no error for my mooring in Loch Melfort
regards
JS
What care we - tho' white the Minch is,
What care we boys, for wind or weather?
I think you should share that one with the brylcreme team over on the motorboaty flora
The thread is an interesting one - My chartplotter sometimes puts me on Dock Street or in Freeport when the boat is in Fleetwood but there is practically no error for my mooring in Loch Melfort
regards
JS
What care we - tho' white the Minch is,
What care we boys, for wind or weather?
Chris_Stannard
New member
Be careful, there are a number of datums in use, WGS84 being the main one, but the older Admiralty charts use a previous datum. The French use a continental datum. On each chart you will find the datum, for GPS, stated and you have to change your GPS set up to allow for this. Some charts even require to add a correction in addition to this.
Why do I say be careful, in a race off Guernsey, I was close off shore to stay out of the tide and came up just in time to stop a dozy helmsman hitting a rock at speed. The reason was that I had the wrong datum for the chart entered and I was about 150 yards to the South of where I thought I was. I was still shaking two gin and tonics later!
Chris Stannard
Why do I say be careful, in a race off Guernsey, I was close off shore to stay out of the tide and came up just in time to stop a dozy helmsman hitting a rock at speed. The reason was that I had the wrong datum for the chart entered and I was about 150 yards to the South of where I thought I was. I was still shaking two gin and tonics later!
Chris Stannard
ean_p
Well-known member
The UKHO advice is not to change the datum within the gps but to leave it set at WGS84 and then to apply any correction quoted on a particular chart for any position plotting on to that chart using data from the gps and that even if the chart datums of two consecutive charts are the same (eg OSGB 1936) to use the correction specific to each particular chart as the corrections do differ......
G
Guest
Guest
At this point it is wise to remember that all electronics are merely AIDS to assist navigation and should be used as such when navigating "properly" (whatever that means to you).
ponapay
New member
It is not only that they may be using different data but also there are errors in each GPS display, errors in chart plotting and, not least of all errors in users plotting the data they are given from GPS.
Using the same data on GPS and a charting system produced a track (in the Baltic) 2 cables inshore for over 1 1/2 miles. That was due to chart (land ploting) errors from an old survey.
Be cautious GPS is only a guide and is not fool proof.
ponapay
Using the same data on GPS and a charting system produced a track (in the Baltic) 2 cables inshore for over 1 1/2 miles. That was due to chart (land ploting) errors from an old survey.
Be cautious GPS is only a guide and is not fool proof.
ponapay
