[talk-au] Mapping Coastlines (Was: Re: Boundary removal.)

[Andrew Harvey]

On Thu, Feb 2, 2012 at 11:24 PM, Ian Sergeant <inas66+osm at 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 <reviews at 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.

>
> 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.