Channel Islands chart datum and GPS

[zvidoron]

Hello,

I am defining GPS waypoints on Admiralty Channel Islands Charts. The 1:150,000 chart (2669) says clearly that the datum is European Datum (1950) but the three 1:50,000 charts (3654,3655 and 3659) only say European Datum without a year.

My Garmin GPS has two options for European datums - 1950 and 1979. Are the two datums identical or very similar in terms of positions derived, and if not, which should be used for waypoints derived from the 1:50,000 charts?

Any knowledge shared would be much appeciated.

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[starboard]

Put in a waypoint lat and long. then goto noting Range and Brg with datum 1950 set. Then from exactly the same position change datum to 1979 and goto same WP. Check range and bearing and note the differance if any. This may give you some idea of diplacement from either datum but I guess if any it will be negligable.

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[graham]

Enter a waypoint from the chart then check the distance and bearing from your present position using both datums in turn.

This will show if there is any difference .

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[zvidoron]

Thinking about it now after a good night sleep the answer is actually very simple - enter a waypoint with the GPS set to one datum, say Eu 1950, using round lat and long, i.e. N 51 00.000 and W 005 00.000 then switch to Eu 1979 and read the lat and long. All waypoint are kept in the memory as WGS84 and only display according to the datum currently set, so with a precision of 3 decimal points after the minute of arc, any discepancy will show clearly.

Thanks for your comments.

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[LadyInBed]

Re: WP store

Is that right? I thought WP’s were just stored as numbers, so range and bearing would change as you change datum no matter what datum you were using to enter the WP.

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[zvidoron]

Re: WP store

GPS works internally on WGS84. If you take the coordiantes of a waypoint from a Euro 1950 chart and enter them to the unit after setting it to this datum, the correct geographical position will be recorded. If you are 10 miles west of that point, the range and bearing to it will remain constant at 090 degrees / 10 miles irrespective of which datum will be set at the time. The only difference will be your lat and long displayed position which will change based on the current datum. This is by the way why I never plot lat and long directly from the GPS on a chart (on top of this being easily plotted wrongly with no alert of any kind) - I may unintentionally be on the wrong datum for the chart in use. I much ratherprefer to take 3 bearings and distances from the Nearest Waypoint list and plot them as if they were say very accurate compass bearings and radar ranges. That gives a tiny cocked hat made of 6 simultaneous position lines and an eror in plotting any single one of them will show immdiately. I normally use lighthouses which I keep permanently as GPS waypoints, which enables a quick reassuring manual check with a hand bearing compass.

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[LadyInBed]

Re: WP store

Yes, we agree.

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