[OSM-talk] Coastlines

[Nick Black]

On 8/16/06, Steve Chilton <S.L.Chilton at mdx.ac.uk> wrote:
> Further investigation leads me to think there might be some mismatch
> problems with this coastline data.
> If you zoom in in OSM and compare the alignment to the satelite image it
> appears to be misaligned.
> Following screenshot suggests that it is off slightly in a southerly
> direction. If you look at the truncated yellow roads they seem to align
> well with underlying image. One would really need higher-res satelite
> imagery to reach a more scientific conclusion.
> http://wiki.openstreetmap.org/index.php/Image:Iomoversat.png

Could this be a discrepency due to the definition of the tideline?  If
the landsat image was taken at high tide, but the shoreline data is
based on an average tide....

> It is even more evident if you overlay the data over Google map, where
> it appears to be all shifted to south-east.
> http://wiki.openstreetmap.org/index.php/Image:Iomovergoogle.png

Yeah, I guess the google data for IoM is based on Vmap/DCW or on SRTM
data - all too coarse to show small islands like the one here. (known
as teh Calf of Man).

> What is missing is any inland detail to verify against. This does in
> fact point up the poor quality of this particular Google map data. See
> for example how the map shows the large offshore island as being part of
> the mainland, encompassing the smaller island on the way!
>
> What I suggest would be useful would be to convert the dataset for the
> Isle of White, incorporate within OSM, and allow cross checking there
> against GPS data, OSM satelite data and Google map data. Is that
> posible? What does anyone think?
> I still think this might be a useful dataset, but we need to know what
> we are getting if we accept it on a larger scale as a coastline dataset.
>
> Cheers
> STEVE
>
> Steve Chilton, Learning Support Fellow
> Learning and Technical Support Unit Manager
> School of Health and Social Sciences
> Middlesex University
> phone/fax: 020 8411 5355
> email: steve8 at mdx.ac.uk
>
> Chair of the Society of Cartographers:
> http://www.soc.org.uk/
> 2006 SoC Summer School at Keele Uni:
> http://www.esci.keele.ac.uk/soc/
>
>
> -----Original Message-----
> From: dankarran at gmail.com [mailto:dankarran at gmail.com] On Behalf Of Dan
> Karran
> Sent: Tuesday, August 15, 2006 6:41 PM
> To: Steve Chilton
> Cc: talk at openstreetmap.org
> Subject: Re: [OSM-talk] Coastlines
>
>
> On 8/15/06, Steve Chilton <S.L.Chilton at mdx.ac.uk> wrote:
> > Data quality looks quite reasonable on this sample. I have made SVG
> > (saved as PNG) of the mainland bit of the example below.
> > See:
> > http://wiki.openstreetmap.org/index.php/WikiProject_Isle_of_Man#Assist
> > an
> > ce
>
> Moved to
> http://wiki.openstreetmap.org/index.php/WikiProject_Isle_of_Man#Coastlin
> e
>
> > There are two roads that go in the sea (1 in Port Erin and 1 in Port
> > St
> > Mary) but local gps work and editing could cure that. On balance I
> would
> > say "better in than out" (ie if upload has worked for whole of island
> > then repeat in stages for other areas). Is projection an issue over
> > larger areas?
>
> The road in Port Erin is based not on GPS tracks but on the Landsat, and
> the one in Port St Mary is the harbour I think. There are a couple of
> other close calls from real GPS data, but they coastline around those
> areas is easily fixed.
>
> > NB: renderer (osmarender2) doesn't recognise all of linework as some
> > is still in class= rather than highway= format. Maybe worthwhile to
> > also check in a more (data) populated area. Has Douglas been mapped in
>
> > detail? Maybe I'll have a look at that when I have time.
>
> There are some bits around Douglas, but not as much as Port Erin/Port St
> Mary yet. I should go through and recategorise PSM as Rob put a lot of
> work into that and it'd be a shame for it not to show up when rendered.
>
>
> Cheers,
> Dan
>
> --
> Dan Karran
> dan at karran.net
> www.dankarran.com
>
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