wrapping around the dateline using geographic coordinates

[Brent Wood]

--- percy <percyd at PDX.EDU> wrote:

> Thanks Frank!
> 
> I'd rather stick with something more standard so that we can integrate 
> with other services.

Hi Percy.

We have done this two ways (New Zealand has the same problem). 

By a custom mercator with a central meridian at 100 & lat of true scale at -41.
Geographic coordinates are noticeably lacking however :-(


I have used for some years a 0-360 longitude EPSG:4326 coordinate system.

Note that EPSG:4326 does not explictly define the longitude range, but most
implementations assume +-180, although many, including mapserver, are quite
happy with 0-360 data.

I have a request in to EPSG to consider tidying this up & providing a 0-360
WGS84 "projection" with a formally specified EPSG code. I'm not sure where this
is at, hoping to hear soon.

Proj.4 now allows users to specify the central meridian of a lat/long
coordinate system, so you can specify 0 (default) or 180 (ie, 0-360).  

GMT supports (effectively) a mod360 longitudinal model, which is great, but not
much use with mapserver.


With raster tiles, I have used +-180 based tiles, then symbolic linked the
-180-0 tiles & generated world files which are 180-360 for the same tiles. This
avoided having to replicate tiles & gives a -180 to 360 cover.

Using PostGIS for my vector data, something along the lines of

create table temp as
select id + (select max(id),
       ...,
        setsrid(translate(the_geom,360,0,0),4327);

insert into table
select * from temp;

replicates all the -180 to 0 features with 180-360 longitudes, (where my 4327
is a custom projection, 4326 with the proj central meridian at 180).

I'd guess a view on the 4326 tables unioning translated vectors would also
work.
        

Lines & points work fine, polygons require tidying up by selecting
intersections with a polygon of each longitudinal hemisphere to close them at
the new limits, but it is quite workable. Hope this makes sense...

If you have to work with other data which is stuck at +-180, I think stuck
might describe the situation well.

If EPSG come to the party it may provide a solution longer term.


Cheers,

  Brent Wood