Rocks

[Yealm]

Sorry probably schoolboy question- but what are the criteria for including rocks on charts?

Specifically, the cross icon, I think indicating a rock always under water, but how far below chart datum gets inclusion? Apart from a vague ‘danger to shipping’ criterion is there a more precise value? ie 20 cm below would presumably warrant a mention, but what about 2m, 5m etc ?
And why isn’t the rock height displayed ( I can’t see anything on Navionics)? It would be useful to know which rocks are badduns and which can be ignored by small sailing vessels.

Apologies if I’m missing the obvious

 

[Bodach na mara]

If you're missing the obvious then you're not the only one. As you state, the criterion for displaying a cross representing a rock is that it can be a hazard to navigation. Not to all boats of course but if you're draft is greater than the distance between the surface and the top of the rock its a hazard to you. The west coast of Scotland is littered with rocks that present no hazard to my boat most of the time, but there are several that are well known to contributers to this forum that we are sure can jump up when one of our boats is in the wrong place. I don't use Navionics myself, preferring Admiralty Charts both in paper and electronic forms where there are usually depths shown either of the actual rock or of the surrounding area.

 

[Sandy]

Sorry probably schoolboy question- but what are the criteria for including rocks on charts?

They eat ships and sailors.

Chart 1 is a much overlooked document. A delightful read on a cold winter's night with a glass of malt whisky with a drop of water in it.

Section K, page 51, may help answer your question.

 

[Poignard]

 

[Skysail]

Admiralty chart 5011 and US Chart 1 are much the same except for buoyage systems. The major difference is that the UK charges for this essential safety information.

 

[Yealm]

If you're missing the obvious then you're not the only one. As you state, the criterion for displaying a cross representing a rock is that it can be a hazard to navigation. Not to all boats of course but if you're draft is greater than the distance between the surface and the top of the rock its a hazard to you. The west coast of Scotland is littered with rocks that present no hazard to my boat most of the time, but there are several that are well known to contributers to this forum that we are sure can jump up when one of our boats is in the wrong place. I don't use Navionics myself, preferring Admiralty Charts both in paper and electronic forms where there are usually depths shown either of the actual rock or of the surrounding area.

Thanks yes, I think Navionics is the issue - they don’t seem to give rock heights/depths.

I wonder why- maybe the Admiratly charges more for a ‘rock’ license?!

I can’t remember if my Garmin plotter (Garmin charts) has rock depths/heights..

 

[CM74]

This is a really good book:
"How to read a nautical chart" (Nigel Calder)
Contains the information in 5011 (but cheaper) and a whole load of other information about how charts are compiled, reasons for particular symbols and all sorts of other useful gems of information as well as how it all applies in electronic charts

 

[penberth3]

They eat ships and sailors.

Chart 1 is a much overlooked document. A delightful read on a cold winter's night with a glass of malt whisky with a drop of water in it.

Section K, page 51, may help answer your question.

I thought "Read The Manual" would be the "text-book answer".

 

[Yealm]

I thought "Read The Manual" would be the "text-book answer".

I don’t think there’s a manual for Navionics. They just don’t give the rock data…

 

[Sandy]

I thought "Read The Manual" would be the "text-book answer".

I enjoy Highland Park 12 year old while reading, what is your tipple?

 

[emandvee44]

They eat ships and sailors.

Chart 1 is a much overlooked document. A delightful read on a cold winter's night with a glass of malt whisky with a drop of water in it.

Maybe, but I fell asleep after page 7 (of 134)

M.