[geotk] Mapping coordinate system axis order to i, j indices on a map images

Jonathan Blower j.d.blower at reading.ac.uk
Mon Apr 19 09:01:55 EDT 2010


I'm not sure whether this is a Geotk question, or one for the WMS/ISO
19111 community.  Given a general 2D coordinate system, how do I know
which axis should map to the horizontal dimension of a map image, and
which axis should map to the vertical?  For example, EPSG:4326 uses
lat/lon order, whereas CRS:84 uses lon/lat, but in both cases one would
reasonably expect longitude to map to the horizontal direction.
However, I don't know how a computer would make this inference.

The WMS spec states "Coordinates shall be listed in the order defined by
the CRS and shall be mapped appropriately to the Map CS i and j axes,
*swapping axis order as needed during the projection operation*.

Is there anything in the Geotk API (or even in GeoAPI) that defines when
the axis order shall be swapped?  Or is this really up to applications?


Dr Jon Blower
Technical Director, Reading e-Science Centre
Environmental Systems Science Centre
University of Reading
Harry Pitt Building, 3 Earley Gate
Reading RG6 6AL. UK
Tel: +44 (0)118 378 5213
Fax: +44 (0)118 378 6413
j.d.blower at reading.ac.uk

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