Chart/GPS kweschun

[Magic_Sailor]

I've just received my Reed's Channel Almanac. It comes with a "waypoint" booklet.

It says

"The waypoints listed have been lifted from Admiralty charts prior to WGS 84 conversion"

I feel as if I'm asking a bit of a stupid question but.....what does that mean?

(I do of course understand the need to correlate the GPS with the chart datum).

Thanks in advance

Magic

 

[LadyInBed]

meens there knot rearly wurf a lite.
Not that I would use a published set of W.P's anyway.
Imagine, several boats all homing in on the same W.P.

 

[Twister_Ken]

It means if you're running on WGS charts and have diddled your GPS onto WGS datum, the waypoints will be a little off real life. Typically, though you'd still be within a 100 metres of the real thing, which would have been considered pretty damn impressive a few years back. So, if the WP has clear water all around it, I wouldn't worry too much.

 

[Magic_Sailor]

I didn't formulate my question properely Ken.

I fully understand the need to set your GPS to the same datum as the chart you are using but.... if these waypoints were lifted from admiralty charts PRIOR to WGS84...then what datum would they be using?

Magic

 

[dk]

OSG 36.

 

[Magic_Sailor]

Thanks DK

So why don't Reeds just say that!

Magic

 

[tome]

Careful here - channel islands more likely to be ED50!

 

[whisper]

Because that doesn't sound nearly as impressive and is far too simple for such a publication.

 

[rhinorhino]

What ever the datum, I would be very careful about using published lists for three reasons.
First, are they accurate? The only way to find out is to plot them. Once you have done that then you might as well pick your own anyway. A certian publishers way piont directory was consigned to the gash last year due to very high level of errors detected.
Second, if evey one used same list, likelyhood is evey one would end up on same track, crash!
Third, the best waypiont may well depend on wind, tide boat etc. For example in fog I might want to get to within 100m of a cardinal bouy to vefrify my position and see how tide was running while in wild weather I might want to stay a mile or so from a cardinal to be sure of clearing hazard.
Much better imo to plot your own.
Also check datum on GPS and chart are the same!

 

[Magic_Sailor]

Thanks Rhino

I must fess up to not actually using the list (I get them off the chart taking into account the things you describe).

I just could't understand the statement in the almanac.

Magic