[talk-au] National borders (was: import of state borders?)

[Ian Sergeant]

Hi,

If it is really ABS2006, then it can always be better aligned.  The
fact that it is still in the database when all of the ABS2006 import
was removed, means that it has probably changed relative to the ABS
import, and I wouldn't rely on the source tag.

The convention in OSM is that coastline should be mean high water
mark.  In reality, it usually corresponds to the most inland area that
appears to be inundated by water at some time from imagery.

Some areas the beach is aligned the the coastline, and in others it
extends out to sea.  I can see the reasoning for making a beach an
area that is exposed at lower tides, but I personally don't do it that
way - I align the beach to the coastline.

If you realign the coastline, remember to change the source to what
you realign it to.

Ian.

On 18 September 2012 10:23, Brett Russell <brussell237 at live.com.au> wrote:
> Hi
>
> I have noticed that in Devonport Tasmania the coastline has appeared with
> reference ABS-2006 along with an area called beach.  It can be a little out
> so just curious what level of detail/accuracy is this and worthwhile better
> aligning it.  Also am I correct to assume that it is the high water mark?
>
> Cheers Brett
>
>> From: inas66+osm at gmail.com
>> Date: Tue, 18 Sep 2012 09:34:46 +1000
>> To: ohrosm at gmail.com
>> CC: talk-au at openstreetmap.org; cadmanager at live.com.au
>> Subject: Re: [talk-au] National borders (was: import of state borders?)
>>
>> Hi,
>>
>> If I understand correctly, the differences should only correspond to
>> the amount that our coastline differs from the low water mark? Are we
>> missing parts of the true coastline, is our coastline just inaccurate
>> do you think?
>>
>> I'd be surprised if there was any definitive low water mark data for
>> the entire coastline.
>>
>> Should we start with a simpler section, like NSW?
>>
>> Ian.
>>
>> On 12 September 2012 23:04, Michael Krämer <ohrosm at gmail.com> wrote:
>> > Hi,
>> >
>> > 2012/8/31 Michael Krämer <ohrosm at gmail.com>
>> >>
>> >> 2012/8/31 Ian Sergeant <inas66+osm at gmail.com>
>> >>>
>> >>>
>> >>> Well, I think the baseline is defined here..
>> >>>
>> >>> http://www.comlaw.gov.au/Details/F2006L00525
>> >>>
>> >>> I don't think we have any issues using those facts as a source.
>> >>
>> >>
>> >> That looks great, combining this with the coastline should work. The
>> >> coastline can be either drawn via osmosis from a planet extract or
>> >> perhaps
>> >> also from OverpassAPI. But I guess we'll have to generate all those
>> >> line
>> >> segments in QGIS to get the coordinate systems right.
>> >
>> >
>> > A quick update from my side on this: I'm afraid it's not as
>> > straightforward
>> > as I had assumed...
>> >
>> > I managed to generate an osm file from the points given in the
>> > proclamation
>> > [1]. This gives the straight pieces of the baseline. But the problem is
>> > that
>> > the coastline doesn't really give the right baseline for the rest (high
>> > vs.
>> > low water mark). When I checked briefly I came across some pronounced
>> > differences for example in the gulf of carpentaria. I guess it will be
>> > the
>> > same along the Great Barrier Reef.
>> >
>> > So we would have to do some guesswork to combine coastline and straight
>> > segements.
>> >
>> > BTW, it's very likely not a projection issue. I made sure by doing a
>> > check
>> > of my calculations against the data Geoscience Australia provides.
>> >
>> > Michael
>> >
>> > -------
>> > [1] https://dl.dropbox.com/u/1222615/baseline.osm
>>
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