[OSM-talk] Rocky beaches

[Steve Hill]

On Mon, 14 Apr 2008, Chris Hill wrote:

> High and low water marks vary every day, the height of the tides vary a 
> lot..

Correct - anyone who records high and low watermarks on maps/charts will 
be using the highest and lowest astronomical tides, not the high/low tide 
of an arbitrary day.

> Most measurements are made using Mean Sea Level, which 
> doesn't change (rising sea levels aside).

The options are generally to record lowest, highest or mean watermarks. 
ISTR that Ordnance Survey mark all three on their Explorer maps don't 
they?  And maritime charts rarely (never?) bother with mean - they are 
marked up in heights above/below lowest astronomical tide.

>  When you look at the Yahoo 
> images, how do you know what state the tide is?

You don't (other than being able to guestimate based on local knowledge). 
However, in some locations the coast line data is quite inaccurate and can 
be approximated by hand based on local knowledge and the Yahoo images. 
Some of the beaches around here (Gower, South Wales) are very flat and the 
very large tidal range of the Bristol Channel means the distance between 
high and low water marks can be over a kilometer.

At the moment, nothing states what natural=coastline is actually supposed 
to be documenting, so when estimating the coastline by hand you have no 
idea whether to put it at the top of the beach, at the bottom of the beach 
or somewhere in the middle.

I'm not expecting to be able to record the coastline with a huge amount of 
accuracy, but even an estimate is more accurate than the (big) distance 
between high/low water marks in some locations so defining what we are 
recording is important.

  - Steve
    xmpp:steve at nexusuk.org   sip:steve at nexusuk.org   http://www.nexusuk.org/

      Servatis a periculum, servatis a maleficum - Whisper, Evanescence