[MetaCRS] proj4js and axis order

[Christopher Schmidt]

On Thu, Oct 01, 2009 at 08:03:00PM +0200, Bart van den Eijnden (OSGIS) wrote:
> Indeed, thanks for the detailed explanation Chris.
>
> So there are no plans to extend the proj4js projection object with this  
> info? That will make it really hard to make a generic WMS 1.3 javascript  
> client ..... maybe I can take a shot at it with a few pointers.
>
> Chris, can the axis order info be retrieved through  
> spatialreference.org, and if so, how? TIA.

not that I'm aware of. Some projections (not supported by GDAL) may
have "AXIS" things in the WKT, but since the primary source for a lot
of them is GDAL/OGR/proj (which don't store this information), any that come 
from that source will not have the information available.

-- Chris

> Best regards,
> Bart
>
> Christopher Schmidt wrote:
>> On Wed, Sep 30, 2009 at 09:11:01AM -1000, Harley Powers Parks wrote:
>>   
>>> Hi, just curious.
>>>
>>> What is meant by "axis order"?
>>>
>>> Axis orientation from the origin..0,0 on a 2D plane? X is horizontal,
>>> increase from left to right, decreases from right to left; y is vertical
>>> perpendicular to X axis, increase from low to high, decreases from high to
>>> low.
>>>     
>>
>> Axis order, as it relates to coordinate systems, is the definition of  
>> which axis comes first in a series of coordinates. So, for example, if 
>> you
>> have Boston, is the point:
>>    * lon, lat -- -71, 42
>>  * lat, lon -- 42, -71
>>  * x, y,   y, x,  x, y, z, etc.
>>
>> The EPSG database defines, for each projection, the order of the coordinates
>> within a set of coordinates. Generally, these coordinates are lat, lon
>> ordered in the case of geographic projections, and lon, lat ordered in the
>> case of projected coordinate systems.
>>
>> However, there are exceptions to all of these cases.
>>
>> More modern WMS specs require clients to know what the coordinate order 
>> for a set of coordinates is, and request a bounding box (4-tuple 
>> defining
>> a 2d box) in that projection. So for Boston, you have to request in
>>     srs=EPSG:4326&bbox=41,-72,43,-70
>>
>> The problem with this is, of course, the fact that in order to make 
>> this request correctly, rather than assuming you can just do 'min_x, 
>> min_y, max_x, max_y', you have to have a complete knowledge of the 
>> coordinate systems that are possible to use in your application, or 
>> have some way
>> to communicate or obtain that data.
>>
>> Best Regards,
>>   
>
>
> -- 
> Bart van den Eijnden
> OSGIS, Open Source GIS
> bartvde at osgis.nl
> http://www.osgis.nl
>

-- 
Christopher Schmidt
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