HELP! - I need info and I am not near my boat..

Dave_Snelson

Dave_Snelson

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Porthmadog / Port Leucate
www.makeyourowngarments.com
What chart datum is used the UKHO small leisure craft charts?? I'm away at the mo and I have a mental block - especially as I cannot get to my boat. Cheers.

<hr width=100% size=1>Madoc Yacht Club
<A target="_blank" HREF=http://www.madocyachtclub.co.uk>http://www.madocyachtclub.co.uk</A>
 
Chart datum;
" means the Lowest Astronomical tide: which is the lowest sea level predicted under average meteorological conditions"
Or
The lowest depth of water you would find in that position normally on the chart. So all tidal depths are on top of that figure.
CD says 3.5 m and tide is 1.5 at Low Water. At LW you should have a depth of 5 m min there that day.
hope this helps.

<hr width=100% size=1>Julie ,
IMOSHO of course,/forums/images/icons/smile.gif
 
Julie - you are correct in your definitions, however, there is another use for the term Chart Datum. It refers to the shape of the world used in the making of the chart. The world is not round but an oblate spheroid (i.e. round but flattened in at top and bottom) There have been many attempts to describe this to a standard for a country and then for a world standard. The latest of these is World Geodetic Survey 1984 (WGS84) - this replaced WGS72 and is the standard observed by GPS. Thus the majority of new charts are being redrawn in WGS84 format. However, not all charts are in this format, and it is important to check exactly what the format is for each chart. There is normally a correction to WGS 84 positions shown on a chart that is not at that standard.

Ordnance Survey maps of UK are to a different standard again IIRC OS36, and needs yet another correction if using your GPS on land. Most hand helds come with a multiplicity of different chart datums already in memory and you just have to select the correct one - but it is important to know that there are differences and to be on the correct one! /forums/images/icons/smile.gif

<hr width=100% size=1>
 
Chart datum either OSGB36 or if new enough then WGS84
Depth datum is Lowest Astronomic Tide.
All tidal info is, I think, based on the variation between where you are and the Ordnance Datum Point at Newlyn. E.g. Salcombe's Datum is about 3m. below that of Newlyn. Exactly what this means in practice I don't know. /forums/images/icons/blush.gif - perhaps one of you Yachtmasters will be able to tell us /forums/images/icons/smile.gif

<hr width=100% size=1>
 
Here's something to think about. Check your charts to see when the surveys for that area were done and if they have been updated. The main ports are surveyed regularly but some of the smaller ports can be interesting reading./forums/images/icons/smile.gif

<hr width=100% size=1>Julie ,
IMOSHO of course,/forums/images/icons/smile.gif
 
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