----- Original Message -----
From: "Andrew Harvey" <andrew.harv...@gmail.com>
To: "OSM Australian Talk List" <talk-au@openstreetmap.org>
Sent: Friday, February 03, 2012 10:20 AM
Subject: Re: [talk-au] Mapping Coastlines (Was: Re: Boundary removal.)
On Thu, Feb 2, 2012 at 11:24 PM, Ian Sergeant <inas66+...@gmail.com>
wrote:
If there is a man made seawall or barrier, I use that. If not, I try to
estimate how high the water comes at high tide from the look of the
terrain. If all we have is one image, then that's all we have.
If there was a seawall that the water reached once a day that would
make it easy as that is your mean high tide mark, but few places have
such a wall.
On Thu, Feb 2, 2012 at 11:43 PM, David Groom <revi...@pacific-rim.net>
wrote:
With the high resolution imagery its usually quite easy to differentiate
between permanently dry areas, and areas which was been covered by water
in
the last 12 hours.
I'm not an expert but for areas like a coastal beach which have waves
coming in won't the peak point where the water comes to be higher that
mean high tide?
For a lake with no upstream influence that would work well, but I was
thinking about where you have beach with waves coming in.
If anyone has some expertise and knows if this peak water point caused
by the waves is generally close to mean high tide or not that would be
good to know as then we can just trace/measure that mark.
That will depend on the gradient of the land between the highest high water
spring tide, and the lowest high water spring tide, and what location you
are in, since the tidal variation between spring and neap tides varies
enormously depending whereabouts you are.
I think you are possibly seeking a higher degree of accuracy than is
required. Also then next question would then be to concern ourselves with
whether we took the peak water point of the largest expected wave, or the
average wave.
David
Having said that, tropical regions where there may be large areas of
mangrove etc, it is quite common for the coastline way to be drawn at the
mangrove / water interface rather than the mangrove / land interface .
This
boundary is usually quite visible on even the low resolution imagery.
I think I'm going against what I said in an earlier thread on this
list, but I think now that the tide mark should be mapped
independently of what plant life is growing in that area of
land/water.
If all else fails, then you have guess when to put the coastline, someone
with more knowledge can always come along later and correct it. If its a
choice between no coastline, and inaccurate coastline then I'd always go
for
inaccurate. You could always tag the ways with a fixme if you wanted to
flag them up.
There are some lakes which I've observed over the full cycle (but even
that isn't really a mean, but just a sample of one day) and mapped
those more accurately, but it isn't so easy when you have waves coming
in.
Lastly, if you are redrawing coastline ways then can I make a reminder
that
the direction of the way is important. They must be drawn with the water
on
the right hand side.
Yep, defiantly.
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