Many thanks Andy,
You have pretty much confirmed what I was beginning to believe - that I
needed to do some processing with the processed_p. Your idea of inverting it
is simple (in concept) - so simple I hadn't thought of it! I was mentally
doodling all sorts of things involving using processed_p shapes as a mask
and doing some mildly complicated image processing with them on the
relief/hillshade tiffs.
There is no panic, so I'd be happy to wait for your new improved sea polygon
data set.
Thanks for confirming that I wasn't quite as crazy as I was beginning to
fear!
Regards,
Philip
2010/1/12 Andy Allan <[email protected]>
> On Wed, Dec 30, 2009 at 1:20 PM, Philip Howarth <[email protected]>
> wrote:
> > Is anybody able to offer some ideas on how to render
> > hillshading/colour-relief in areas with coastline?
>
> Hi Philip, hopefully I can help.
>
> > I have used the methods described in the OSM wiki and elsewhere to get
> nice
> > colour-relief tiffs. I can render images using these so long as I put the
> > layer(s) containing the tiffs first.
> >
> > The problem comes around the coastline. I think that I really need to
> paint
> > the coastline using the processsed_p shape file (as the images produced
> from
> > the SRTM data aren't good enough to define the coastline - especially
> with
> > low lying islands).
>
> Beyond that in fact - it's impossible given that parts of the UK
> (around the Wash) and especially the Netherlands are below sea level
> in SRTM. I've seen naive attempts to make a zero-elevation blue before
> :-)
>
> > Clearly, if I just paint a filled coastline on top of
> > the hillshade, I lose the hillshade and vice versa.
> > I've tried lots of things but without the success that Opencyclemap have
> > clearly achieved.
> >
> http://www.opencyclemap.org/?zoom=12&lat=55.65104&lon=-1.85604&layers=B000
> > The best I've achieved is to use the linesymbolizer to draw round the
> > coastline shapes rather than fill them - but this is both a bodge and it
> > inevitably draws spurious lines. See:
> > http://www.philip.howarth.name/NorthumberlandRelief2.png If anyone is
> able
> > to give me some pointers, I'd be very grateful.
>
> There are two options I can recommend.
>
> 1) If you only use hillshading, then you can use mapnik's image
> compositing (as of 0.6). Use the normal coastline file as your first
> layer, and then a rastersymbolizer layer with a hardlight compositing
> option to blend in your hillshading tiffs. It works, but you get
> slight artefacts near the coasts since the hillshading isn't being
> clipped.
> 2) If you need colour relief, or accurate clipping, then you need an
> inverted coastlines file. I have a patch to an old version of the OSM
> coastline generator to make sea polygons instead of land polygons, and
> I'm in the process of updating it to work with newer versions of the
> coastline generator. Give me a week or so and it should be finished.
> Unfortunately the latest inverted coastline shapefile I have is from
> Feb 2009, and it's showing its age. I should have a new one soon too.
>
> Hope this helps,
> Andy
>
> > Just to make it harder... I'm using WIndows.
> >
> > Philip
> > --
> > Philip Howarth
> > Cambridge UK
> > email: [email protected]
> >
> > _______________________________________________
> > Mapnik-users mailing list
> > [email protected]
> > https://lists.berlios.de/mailman/listinfo/mapnik-users
> >
> >
>
--
Philip Howarth
Cambridge UK
email: [email protected]
web: www.philip.howarth.name
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