A good article, and timely reminder, by Chuck Husick about GPS and Chart variations on the Boat US website:
http://www.boatus.com/husick/n_gpschart.asp
http://www.boatus.com/husick/n_gpschart.asp
GPS and Charts
- Thread starter Warren Milberg
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Warren—
I take it you're a BoatUS member and get their weekly e-mails???
Good article... I'd add that if you're using chartplotter, have the paper charts on hand and keep a running plot on them just in case.
I take it you're a BoatUS member and get their weekly e-mails???
Good article... I'd add that if you're using chartplotter, have the paper charts on hand and keep a running plot on them just in case.
In addition to 'datum errors', GPS navigation (based on charts that were drawn by hand using hand methods for their fixes, etc.) is also subject to 'resolution errors'.
Resolution can be a big problem when using extreme magnifications that will exceed the base resolution of the original data. So if you happen to go aground when your GPS is at extreme high magnification .... it was probably a 'resolution error' caused by the HIGH MAGNIFICATION used on the GPS generated chart.
Resolution can be a big problem when using extreme magnifications that will exceed the base resolution of the original data. So if you happen to go aground when your GPS is at extreme high magnification .... it was probably a 'resolution error' caused by the HIGH MAGNIFICATION used on the GPS generated chart.
A very well written and timely piece. Thanks ,Warren. I copied the summary and will paste it here.
[FONT=Verdana, Arial, Helvetica, sans-serif]In summary, knowing precisely where you are on the globe in no way ensures that you are where the chart indicates you are. Always remember the universal navigation admonition, never rely upon a single source of information. Use your eyes, your depth sounder, radar, hand bearing compass, nose or whatever other means you have to validate your assumed position, especially when navigating in areas where a position error of a few meters can make a difference in the safety of your vessel. When navigating in unfamiliar waters or in restricted visibility keep a running plot, on paper, of your vessel's position. A simple interruption in electrical power to the Loran, GPS or chart plotter can suddenly leave you wondering where you are.[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]In summary, knowing precisely where you are on the globe in no way ensures that you are where the chart indicates you are. Always remember the universal navigation admonition, never rely upon a single source of information. Use your eyes, your depth sounder, radar, hand bearing compass, nose or whatever other means you have to validate your assumed position, especially when navigating in areas where a position error of a few meters can make a difference in the safety of your vessel. When navigating in unfamiliar waters or in restricted visibility keep a running plot, on paper, of your vessel's position. A simple interruption in electrical power to the Loran, GPS or chart plotter can suddenly leave you wondering where you are.[/FONT]
What!?! You mean the HUGE magnifying glass I've been using for years no longer is trustworthy!
And just when I finally learned to block the sun to avoid burning up my paper charts! And that bright moonlit night one time when the chart started smoldering...
I hit my second rock in 30 years exactly that way last summer. I was surveying mooring positions (I'm a harbormaster) in one of the coves so I had the GPS zoomed way in. On my way out, I saw the familiar clump of asterisks go by on the screen and turned left. Crunch! There was still one more astersisk off the screen. GPS zoom compounded by being in a very familiar area and letting my guard down.
That said, I have found the Blue Charts to be incredibly accurate on the coast of Maine. I probably could have threaded my way through that rock pile with the Garmin. I have to keep reminding myself they may not be that accurate everywhere.
I have a Garmin 72 that I play with. I carry it in my car and watch to see how closely my return track matches my outbound track. With an 800 foot "zoom" it is pretty good but if I zoom all the way in after 3 or 4 trips I can't see all of my tracks. The screen is too small. If I set the pointer on an intersection and get a lat-lon fix on it and then park the car and walk to that intersection my pedestrian position reasonably matches the pointer. We have very good local maps and someday I will compare the GPS the the map.
Most of the charts will tell you when you've zoomed in past their native resolution by saying "overzoom" on the screen on the Garmins at least. If you're interested in how accurate the GPS is, you can always set it to show the circle of accuracy, which graphically shows an estimate of the circle of error for the GPS at any given moment. On a good day, it can be as little as 30' across, on a bad day, it can be 300' across.
Yup! the greater the zoom the bigger the circle. And when I stop that circle pulses like a heart beat and finally settles down. The Icon drifts around like a boat without an anchor while I am parked.Most of the charts will tell you when you've zoomed in past their native resolution by saying "overzoom" on the screen on the Garmins at least. If you're interested in how accurate the GPS is, you can always set it to show the circle of accuracy, which graphically shows an estimate of the circle of error for the GPS at any given moment. On a good day, it can be as little as 30' across, on a bad day, it can be 300' across.
My laptop GPS will often have me a significant distance off of the road!!!! It is usually pretty good but makes me take notice some times. On the boat my garmin has put me 1/2 mile away a few times. I mean when you are between the jetties you know that you are not 1/2 mile offshore.
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