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

[David Groom]

> ----- Original Message ----- 
> From: "Andrew Harvey" <andrew.harvey4 at gmail.com>
> To: "OSM Australian Talk List" <talk-au at 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+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.

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.