[Tagging] Tagging Digest, Vol 143, Issue 12, water heigth values

[Graeme Fitzpatrick]

On Mon, 9 Aug 2021 at 15:33, Mateusz Konieczny via Tagging <
tagging at openstreetmap.org> wrote:

> See say
>
> https://forum.mazury.info.pl/download.php?id=8366&sid=370c6708968b028624b55a39760a0a4a
> for an actual example
>
> In my limited experience they appear to be expecting a normal situation,
> someone on a boat in
> other situation is likely expected to be an expert able to apply a
> correction to a signed value.
>

Clearance heights to bridges, cables, & similar boating obstacles are all
calculated using the HAT (Highest Astronomical Tide, which is the highest
tide that can be expected for a location during the year, & also known as
Spring, or King, Tide), which will result in the smallest possible amount
of clearance.

So if, as per your example, the bridge clearance sign shows 3, then the
minimum clearance under that bridge will be 3m. Looking at that photo, the
current clearance would appear to be 3.5-4m - 3m would probably be the
level at the high tide mark shown on the pilings.

If your boat height is less than 3m from waterline to top of mast /
superstructure, you will always fit under the bridge (barring extreme
conditions such as flooding, which raise the river height abnormally). If
you're 3.5m high, you'll have to wait till low tide (assuming a .5m tidal
range), but if you're 4m high (& only .5m range) then you're never going to
fit under that bridge!

Thanks

Graeme
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