Re: [Geotools-gt2-users] World shapefile wrapping

[Rueben S. <r_j...@ya...>]

Hi,

I am not exactly clear what you are doing, but I assume that you have a
shapefile with a geographic coordinate reference system (CRS) (longitude
(x) values from -180 to +180). Geographic coordinates (longitude,
latitude) are difficult to work with, since they wrap around on the
globe, but not on a flat map that you are displaying.

You could project it to a mercator CRS for display (or some other
projection). The central meridian of the projection would be the desired
centre of your map. This will work correctly for points, but
unfortunately the re-projected polygon geometries will be very messed
up. Some points in the same geometry will likely end up at opposite
sides of the map (also note that the mercator projection is undefined at
the poles).

Unfortunately the solution to this problem is not easy. I think that
besides just re-projecting the points in a polygon, polygons that now
get split over the outer edge of the map (ie England if your map centre
is +180 degrees longitude) need to have their geometries modified. They
need to be intersected with a line along the prime meridian (in the
England case) to split the polygon into two parts (one part on either
end of the map). Also polygons that were split onto two different sides
of the shapefile (eastern russia or alaska) will need to be merged
together. This last step is not required, but would make things look
nicer.

I think that code to do all of this does not belong in the coordinate
transformation codebase of geotools, but instead belongs in the
CRSServices class where geometries are being re-projected. Unfortunately
getting the above ideas working correctly for all cases will be a lot of
work. For example, the orthographic projection is undefined outside of
the hemisphere of the projection centre, so in this case, the undefined
boundary needs to be calculated (it will be a circle or ellipse) and
intersected with all the re-projected polygons that straddle this
boundary. The correct thing to do for each map projection is probably
going to be a bit different.

Also, for your problem, you could start to simply re-calculate the
points for your shapefile (add 180 to all of them and subtract 360 from
points that are now > 180 (to keep everything within +-180)). This
should result in the dateline as your centre (it is now longitude 0),
but you will run into the same problem as above, with some geometries
split over both ends of your map. 

Good luck, and remember that any general solution to this problem (or
feedback on my above thoughts) would be great. The ability to re-project
world maps correctly would be a nice addition to geotools. Unfortunately
95% of map re-projection does not need this ability, so it is a low
priority for us right now. Also note that we do not have any map
projections implemented that are suitable for displaying the entire
world well (such as the gall stereographic).

Rueben


On Wed, 2005-12-01 at 14:32 -0500, Philip Oliver-Paull wrote:
> Hi.  I'm working on a project that uses a map of the entire world.  We 
> would like to be able to center the map on any point on the earth and 
> still have the rest of the world visible on either side of the point.  
> This would be most helpful for center points in the pacific, but the 
> shapefile doesn't wrap and just drops off at the dateline.  It appears 
> that the coordinate system keeps going on past 180 degrees longitude, as 
> well.  Is there any way to get the coordinate system and shapefile to wrap?
> 
> Thanks in advance,
> 
> Phil
> 
> 
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