Is the Hydrographic office having a laugh?

[Sans Bateau]

Or do they just not know what they are doing?

So I buy a new folio of charts, the Solent 5600. Deciding that with this set I will keep them up to date, so I go to the UKHO web site and look for notice to mariners, the update section, easy so far!!

I start then to do the updates, but somethins is wrong. The charts are as one would expect WGS84 Datum. The updates are ETRS89 Datum!!!

I hadnt even heard of that datum until I googled it.

OK its probably me, what have I missed, anyone know? Or are the two departments at the UKHO office not talking to each other?

 

[stephenh]

Looked at one way they are equal -


"ETRS89 is our national coordinate system for 3D GPS positioning. It is a much more exacting definition of the GPS coordinate system than the better known WGS84 standard. Consequently, ETRS89 coordinates are also WGS84 coordinates, but be aware that general WGS84 coordinates do not necessarily meet the ETRS89 standard. ETRS89 is the GPS coordinate system standard used for high-quality GPS surveys throughout Europe."

but they might have said so........!!!

 

[Sans Bateau]

Where did you find that?

And yes they could have said so!!!!!

 

[stephenh]

Galadriel - sorry cannot quite remember ( senior moments and all that !!)

It was from an OS site and I certainly went here to start with :

http://www.ordnancesurvey.co.uk/osw.../coordinatesystemsinfo/coordinatesystems.html

Good luck.....

 

[wotayottie]

I start then to do the updates, but somethins is wrong. The charts are as one would expect WGS84 Datum. The updates are ETRS89 Datum!!!

At the moment the two datums are all but identical. However, the WGS datum has its base line in the USA whilst the ETRS one has a base line in Europe. So as continental drift occurrs, the two datums will drift slowly apart. Doubt you will have the charts that long.

However this discrepancy will be trivial compared to other inaccuracies in the charts arrising from , for example, triangulation from points on land which themselves arent quite where they were thought to be. Satellite mapping is allowing the correction of these errors which were as big as several hundred yards in a few places round the UK. So the accuracy of your modern WGS chart depends on whether it was simply translated from the one that went before ( as some were in the hurried conversion to WGS) or redrawn based on modern satellite data.

And then there is the fact that your pencil line on the chart can be as much as 100m thick. So the man who looks at his chart and plots a gps position thinking that he knows where he is to within a few metres can be badly mistaken.

This info is from the UKHO so it ought to be correct.

 

[Sans Bateau]

Thank you both. I searched the UKHO site in vain, it would be so easy for them to have made this information more prominent, I quote from their site:

"The service has been specifically designed to make updating your Admiralty Leisure charts an effortless task. Here you will find all relevant Notices to Mariners (NMs) released since the publication date of the chart's New Edition."

How difficult would it have been to add just a note to say that WGS84 is the same for update purposes as ETRS89?

When I did a Google search last night, the most information I gleaned was from the OS site, that indicated the ETRS89 datum was for the benefit of road maps, useful off St Cats.