GPS and Chart datums

[ssr56220]

OK this has probably been covered before but here goes!

If i have a load of waypoints in my GPS which were taken of a chart with one datum, and then buy another new chart of the same area with a different datum i.e ED50 to WGS80, assuming i change the GPS datum to WGS80 while working with the new chart, will the waypoints stil appear in the right place on the newchart?

 

[lw395]

Might depend on the GPS, will it change every waypoint's numbers when you change the datum? I doubt it!, but it might!
On my current GPS (Magellan) I've only ever used wgs84 and OSGB, the wp for my house does seem to work in os grid ref as well as lat/long.
Safest option might be to load the WP's into a PC, archive them, modify the co-ords (in a spreadsheet?) and load them back into GPS. You can also fill in the comment field on the PC, it's easier to type than add text on the GPS.

 

[lw395]

My magellan does indeed change the lat and long of the waypoint when you change the datum, it still puts ballast pile the same place relative to my office.

My garmin needs new batteries.

 

[Danny Jo]

If i have a load of waypoints in my GPS which were taken of a chart with one datum, and then buy another new chart of the same area with a different datum i.e ED50 to WGS80, assuming i change the GPS datum to WGS80 while working with the new chart, will the waypoints stil appear in the right place on the newchart?

I don't know what sort of GPS you have, but it would be a rubbish one if it were otherwise. I mean, what is the point of letting you change the datum if it doesn't adjust reference points, including waypoints, to suit?

If you want to make absolutely sure, invent a waypoint in the area covered by your non-WGS84 chart, with your GPS set to WGS84, using nice round numbers for the minutes. Now reset the GPS to the chart datum, go to your waypoint bank and look up the lat and long of your waypoint. The minutes won't be nice round numbers any more, but something like 00.07, corresponding to the offset which is mentioned on the chart.

 

[bedouin]

You will not get a definitive answer on this as it may vary from model to model. To me it would seem sensible that the waypoint was at an absolute point on the earth's surface, irrespective of Datum, but I can see a case for it being absolute coordinates instead.

As suggested it is easy to check for your GPS. Enter a waypoint, change datum and see if the quoted Lat/Long are the same.

As a rider I would say "Who cares". IMHO there should not be any circumstances in which the minor differences between say OSGB and WGS84 datums (or should that be data ) makes any difference to the safety of your boat.

 

[AntarcticPilot]

You will not get a definitive answer on this as it may vary from model to model. To me it would seem sensible that the waypoint was at an absolute point on the earth's surface, irrespective of Datum, but I can see a case for it being absolute coordinates instead.

As suggested it is easy to check for your GPS. Enter a waypoint, change datum and see if the quoted Lat/Long are the same.

As a rider I would say "Who cares". IMHO there should not be any circumstances in which the minor differences between say OSGB and WGS84 datums (or should that be data ) makes any difference to the safety of your boat.

In some cases the differences can be 100s of metres; that's certainly true of the difference between old OS maps and WGS84. And even in the Clyde there are places where an error of 100 m would put you on the rocks. Agreed you should not rely on accuracy at the limits of GPS, but there have been cases reported on here where having the ability to use the accuracy of GPS to make a harbour entrance has saved lives.

 

[lw395]

Absolutely,
having found my way into cherbourg by GPS without seeing the breakwater, I would say it's worth understanding the datum and getting it right.
I think the chart I was using at the time had a datum shift greater than the width of the entrance.
Also seeing your GPS achieve good accuracy is a reassurance that it still works as well as it ever did.

 

[bedouin]

Although a great aid to navigation, GPS and GPS sets are 100% reliable. As several threads on this board will testify, many people have experienced unexplained loss of signal or positional errors with GPS. And that is before you start worrying about the human factor - is that Waypoint entered correctly, using the correct Datum and so on.

While it is only my opinion, I would say that it is distinctly unseamanlike to put yourself in a position where the safety of your boat and crew is dependent entirely on the accuracy of your position as displayed on a GPS set. I would not do so and I would think very carefully about sailing with a skipper who did.

That is a very different matter to using GPS to get close enough to a known location to rely on pilotage, or where the safety of the boat can be confirmed from the depth, radar or other independent means.

 

[alan_d]

Although a great aid to navigation, GPS and GPS sets are 100% reliable.

From the context, I think you must have intended to say "...not 100% reliable".

 

[bedouin]

From the context, I think you must have intended to say "...not 100% reliable".

Oops - well it's only a little word - do you think it alters the sense much

 

[oceanfroggie]

Waypoints are stored internally in only one chart datum (ie usually WGS84 but that's not important in this context). The chart datum you set your gps to causes it to adjust the display of lat/long by the correct offset and the same when entering (ie the numbers you enter are modified when stored internally in the units base datum). The important thing is that the datum on you gps matches the datum on the chart you are reading co-ordinates directly off the chart, unless you have already manually corrected them to WGS84.

Changing the chart datum does not move waypoints it meerly adjusts the display of their lat/long to match the selected datum on the unit.

Many new BA and Imray charts are being issued with WGS84 datum which makes life a little easier. Anyway fewer folks now use paper charts as their primary planning and nav tool, preferring to use a plotter and paper as a fall back.

 

[KellysEye]

>unseamanlike to put yourself in a position where the safety of your boat and crew is dependent entirely on the accuracy of your position as displayed on a GPS set.

I agree, another issue is chart accuracy. For example the Imray charts for the southern Caribbean are out by half a mile. Witness to this is the number of boats up on the reefs of the Venezuelan out islands and Klein Curacao - all low lying coral. The last time we went pass Klein there were three yacht wrecks and one coaster. All went on the reef at night and all believed their GPS. We've also had charts out by three quarters of a mile in Europe. The only time you should trust a chart is if you've been there before.

Then there was the yacht using a chart plotter who sank when he hit a reef off Australia. The reef was marked on the paper chart (which foolishly he didn't have) but not his electronic charts. So electronic versions may be even worse than inaccurate paper charts.