Stupid bridge height indicators on charts

[RichardS]

As you begin to gain some experience as a navigator you will come to learn the merits of there being a difference.

.... if your eyesight is as poor as your navigation experience then I suggest up around +5 dioptre spec's will do the job.

What happened to the season of goodwill to all men?

It is a pity that this was felt to be an appropriate response to the OP.

Richard

 

[[70521]]

Should 'ave gone to Specsavers ...

I've just done so and am about to get my first set of varifocals the lady was a tat shocked when I said "HOW MUCH" in my best Aberdonian!

I am from the west coast by the way

 

[Robin]

Use electronic (vector) charts and set the units to read 'feet' that way the decimals are pretty insignificant.

Ooops that doesn't work either so excuse the red herring..

Best then to avoid all bridges, especially if you have poor eyesight.

 

[alant]

I don't know the formal answer for this but I suspect that it is done for clarity because depths are against the chart datum and heights (not only bridges, etc but rocks also) are against the chart's height datum.

So not as "Stoopid..." as you thought.

Bridges are measured from HAT & rocks measured from CD, with Lighthouses measured from MHWS.

 

[Plevier]

I've just done so and am about to get my first set of varifocals the lady was a tat shocked when I said "HOW MUCH" in my best Aberdonian!

I am from the west coast by the way

Severe Fred Drift here but try ASDA if you can much cheaper.
All part of the season of goodwill - so shall we be nicer to Boo2 too - whether agreeing or not? There you are that's back on topic now!

 

[jordanbasset]

I always have feeling of doom when we go under a Bridge, even when I know I have checked and double checked the height as I approach I question the fact we will fit under. I ended up crawling through at the lowest speed possible, even though I have 20 feet of clearance. Must be something to do with my perception of height as we approach the bridge, which does get worst the closer we get.

 

[nevis]

An interesting one carrys the main road to Skye over Loch Long at Dornie next to Eilean Donan Castle.
Historically up to ww2 there was a ferry ;then until the 1970s when the road was rebuilt a low level swing bridge.
Loch Long is a nice sheltered sea Loch and would make a lovely winter mooring.
I intend to measure the low water clearance from the bridge as I suspect my mast may well just clear the bridge at low water so long as the chanel can clear my 4 foot draft.
As my mast is currently down I can accurately measure mast.
Thought about this some years ago but mast height was a guestimate.

According to Martin Lawrence in the Skye and Northwest pilot there is only approx 2.5 headroom under the bridge to Dornie and a depth of 3m.I suppose you can still arrange to have the bridge opened and according to theCCC sailing directions for 1974 they require 24 hours notice but how accurate this is i have no idea.I intend to be in this area in June as i want to anchor at the head of the Loch to do some hillwalking

 

[webcraft]

.
Fabulous to see a trivial point humourously made escalate into a full scale flame war. Dyspepsia rules!

- W

 

[Trop Cher]

According to Martin Lawrence in the Skye and Northwest pilot there is only approx 2.5 headroom under the bridge to Dornie and a depth of 3m.I suppose you can still arrange to have the bridge opened and according to theCCC sailing directions for 1974 they require 24 hours notice but how accurate this is i have no idea.I intend to be in this area in June as i want to anchor at the head of the Loch to do some hillwalking

The bridge no longer opens, it was replaced some time ago with a fixed bridge!

 

[VicS]

Why don't they do them like they do depths with a subscript character for the 10ths instead of a decimal point between the characters ?

Stoopid...

Boo2

The reason is, I suspect, historical rather than logical.

The symbol for vertical clearance under bridges, power lines etc is a relatively new invention ( like wise the similar symbol for horizontal clearances)
On older charts the clearance under bridges is shown as illustrated by this extract from an old edition of 5011 (1973)

. . . . . . Now that is, IMO, clear enough.

When the symbol for vertical clearance was devised, sometime between 1973 and 1991, the numerical style was carried forward unchanged rather than being altered to the style used for depths and drying heights using a subscript for the decimal.

The result is as illustrated in these two extracts from the chart for Southampton for the Itchen Bridge and Northam Bridge

I do agree the heights are not as clear as they might be. Not as clear as they would have been in the old style illustrated above perhaps.
Whether they would be better in the same style as depths and drying heights, namely with the decimal as a subscript, I am not so sure. I think they might be more easily confused with the depths.

.

 

[geoid96]

I think Vic's Southampton example highlights the OP's issue well. In the Northam Bridge height, it is very easy to miss the decimal point. To avoid risk of confusing with depths, maybe the solution is for decimal the point to made larger and bolder.

 

[Elessar]

Let me help you out with one very simple example which I would have expected any navigator would realise or have taken what from I said as a lead to follow up for themselves.

For rocks that cover and uncover the depth can be given in brackets and with a subscript for the decimal. For rocks that do not cover the height is given in brackets and as a decimal for the fraction; the difference avoiding confusion.

One is against the chart datum and the other is against the height datum (I trust that you know the difference). Have a look at the international chart symbols and you will see what I mean. No one is going to make an exception for bridges.

So it would seem to me that you are being the "Stoopid" one and are insisting on continuing to displaying that is so rather than listening to those who have far more experience than yourself.

Err - you are not right and should be carfeul about calling him names.....

Depths and drying height are (usually - check chart info box) referenced to LAT or chart datum - which are for all practical purposes the same thing.

Drying heights have the integer underlined.

Numbers in brackets are displaced readings and can be depths or heights. It means its referrring to a place not where the number is. Often used for drying rocks it's true but thats NOT what the brackets mean. Depths and drying heights are not given with a decimal always a subscript.

 

[Elessar]

That is, just as I set out as the example in my earlier post #11, because rocks that do not cover are against the height datum of the chart, just as bridges are. Rocks that do cover are against the chart datum.

So it is not an exception it is just following what I at the start suggested was the rule - things which are against the chart datum use subscripts and those that are against the height datum use decimal points.

This is misleading - drying heights and depths are referenced to chart datum (LAT)

Land featured are referenced to MHWS.

You are wrong about "just as bridges are" because clearances (bridges, power lines) are referenced to HAT.

 

[Elessar]

Your experience does not sufficient to present a reasoned argument about it, and some might consider that to define stupid.

Boo2

I won't seek to define stupid but will categorically state he is factually incorrect in some of his statements about charts.

 

[prv]

Whether they would be better in the same style as depths and drying heights, namely with the decimal as a subscript, I am not so sure. I think they might be more easily confused with the depths.

I tend to agree, especially the Northam example. The lower line of the enclosing symbol could easily be seen as an underline since it's on a drying bank, and the upper line missed as part of the contour or some other thing in the mud.

The decimal point could do with being a bit bigger though.

On the gripping hand, it's fairly clear that a smallish urban road bridge like that isn't going to be 42 metres high! I'm sure some sites are more ambiguous.

Pete

 

[alant]

I tend to agree, especially the Northam example. The lower line of the enclosing symbol could easily be seen as an underline since it's on a drying bank, and the upper line missed as part of the contour or some other thing in the mud.

The decimal point could do with being a bit bigger though.

On the gripping hand, it's fairly clear that a smallish urban road bridge like that isn't going to be 42 metres high! I'm sure some sites are more ambiguous.

Pete

Shouldn't worry about any decimal points, big or small, with regard to the Northam bridge, you'd need a hinged mast to get under it.

 

[prv]

Shouldn't worry about any decimal points, big or small, with regard to the Northam bridge, you'd need a hinged mast to get under it.

Well you know that and I know that (I'm moored within sight of it) but I'm assuming anyone reading bridge heights on the chart is looking at an area with which they are unfamiliar.

As it happens, I do have a hinged mast. No desire to take the boat under Northam Bridge though.

Pete

 

[VicS]

Shouldn't worry about any decimal points, big or small, with regard to the Northam bridge, you'd need a hinged mast to get under it.

I only used Northam Bridge ( and the Itchen Bridge) to illustrate the way in which heights are displayed on current charts.
I dont think the Op mentioned any specific bridges, just that he was looking at the Solent area.
I suppose I could have used Langstone Bridge 1.3 m, Burseldon Bridge 4m, the nearby railway bridge 6m or the M27 bridge over the Hamble 4.3m.

BTW I have sailed under the Haven Bridge, River Yare, Great Yarmouth while it was closed. Charted clearance 1.8m

 

[Elessar]

I only used Northam Bridge ( and the Itchen Bridge) to illustrate the way in which heights are displayed on current charts.
I dont think the Op mentioned any specific bridges, just that he was looking at the Solent area.
I suppose I could have used Langstone Bridge 1.3 m, Burseldon Bridge 4m, the nearby railway bridge 6m or the M27 bridge over the Hamble 4.3m.

BTW I have sailed under the Haven Bridge, River Yare, Great Yarmouth while it was closed. Charted clearance 1.8m

It's an excellent example. I hadn't considered that it isn't clear but it isn't.

Also interesting that the dredged area off princes wharf and the wharf upstream of it (that is now southampton dry stack) is a decimal. I presume that that is because the number indicates dredged depth, not the depth (which may be different of course). That's a guess though....

1.8m and not a huge tidal range there - what were you in?

 

[VicS]

1.8m and not a huge tidal range there - what were you in?

17ft Lysander. Rigged so that the mast could be dropped and raised again from the cockpit for sailing on the broads.