Hi 2012/9/18 Ian Sergeant <[email protected]>
> 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? > Well, to me there are two major reasons for differences between the coastline in osm and the baseline used to compute the boundary: (a) osm's coastline definition as the high water mark vs. the baseline defintion as the low water mark. Obviously the difference is quite small at cliffs and increases a shallow beaches. But the biggest difference is caused by something off the coast inundated at high tide but showing at low tide - e.g. high parts of a reef. (b) Smaller bays are cut off straight. These are not included in the list of points [1], in there are only the additional larger sections. I've tried to illustrate this a bit generating an image in QGIS [2]. It combines OSM with the data from [3] to show the situation. > I'd be surprised if there was any definitive low water mark data for > the entire coastline. > I don't know how it has been generated but I'm afraid there is one. As this directly translates into the extension of the territorial waters etc. there's quite some incentive to have it defined. > Should we start with a simpler section, like NSW? > This probably wouldn't really help much as it doesn't really solve the problem. Last night I have spent some time on the topic and would currently suggest the approach to simply use what's there. Use coastline from OSM and combine with the straight sections from [1]. For both a 12 nautical mile buffer can be computed in QGIS and combined. This will not be accurate as it neglects the reefs etc. But it's probably the best we can get. BTW I haven't checked yet, if there are reefs mapped in OSM. If so they could perhaps be added into the process. Michael ---- [1] http://www.comlaw.gov.au/Details/F2006L00525 [2] https://dl.dropbox.com/u/1222615/coastline.png [3] https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=63565
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