[Viking-devel] EPSG:4326 support

[Guilhem Bonnefille]

Hi all,

Today, a friend of mine come to me and ask: Is it possible to hack
viking in order to access this Tile Map Service?
As I have a poor geomatic knowledge, I laugh and replied: Of course
guy, viking is tile oriented and a simple declarative section is
needed for such stuff.

After a whole day of procrastination, I have to admit it is not so simple.

I discovered there is many way to understand tile cache system. The
most funny is that TMS implemented by OSM (and Google) use an origin
at the top of the map, while OSGeo TMS specification considers it at
the bottom. But I also discovered an other difference: OSM (and
Google) consider latitude between -85,xxx and 85,xxx. IMHO, such
difference is really significative for viking. I understand this mean
that OSM (and Google) uses a projection system different than the one
used in the EPSG:4326 (the tile service I'm currently interested in
use this projection).
Am I right?
I think so because, reading some pages on OpenLayers, I found that
OpenLayers have to convert coordinates when using a Google layer and a
EPSG:4326 TMS layer.

As viking handle correctly OSM tiles, if I want to support such
service in viking, I understand I have to add the EPSG:4326 projection
in viking?
And I fear it will not be really easy.


Any information is welcome about this topic. As I said, I do not have
geomatic knowledge.


References:
http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification

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Re: [Viking-devel] EPSG:4326 support

[Greg Troxel]

I'm not really clear on this myself, but I would suggest that a good way
forward is to try to grasp things and spiff up the comments/docs.

There are several things out there:

  WMS: arbitrary areas and scales
  WMS-T  : above, *usage* constrained to a subset
  TMS: really is tiles with specific 2^-k type alignment/size

I think the OSM tiles are in 'google spherical mercator', which is
certainly supported by proj (which viking doesn't seem to use), and is
basically a mercator projection that assumes a spherical earth.

http://docs.openlayers.org/library/spherical_mercator.html


So I would ask: where is the code that turns lat/lon into spherical
mercator coordinates to then bin them into tile indexes?


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