[OSM-talk] Coastlines

[Steve Chilton]

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
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
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