Sea polygons

[Nop-4]

Hi!


I am looking for a set of sea polygons for use with mapnik.

The shapefiles that are usually referred to as sea polygons are actually
land polygons drawn on a blue background. I cannot use this technique as
my land needs to be transparent.

Are there sea polygons that actually cover the sea area?
Is there a way to invert the land polygons to sea?
Is there a way to create sea polygons from the coastlines similar to
however the land polygons are created?

thanks
        Nop


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[Frederik Ramm]

Hi,

Nop wrote:
> Is there a way to invert the land polygons to sea?

I guess you could easily create a world-size water polygon and then
subtract the landmass polygons from the existing shape file using
PostGIS functions. That would, however, leave you with one very big
"oceans of the world" polygon, whereas the existing landmass polygons
are diced into smaller chunks for (I suppose) more performant
processing. There is probably a PostGIS way to achieve that too - if he
doesn't answer here, maybe e-mail kleptog directly, AFAIK he's the one
who did the PostGIS magic for the existing land mass polygons.

Bye
Frederik

--
Frederik Ramm  ##  eMail frederik@...  ##  N49°00'09" E008°23'33"

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