[josm-dev] to the devolopers of WMS-plugins

Tobias Wendorff tobias.wendorff at uni-dortmund.de
Thu Nov 6 19:03:07 GMT 2008


Hey Frederik,

Frederik Ramm schrieb:
> What you're doing is you request an EPSG4326 projected bitmap from the 
> server but with a height that doesn't fit. Example: Assume your JOSM map 
> view is 500x500 pixels and you're viewing an area around 60°N in 
> spherical Mercator. The area displayed is 5° wide. How high is it? The 
> answer is that it will be about 10° high (let us assume it's 55° to 
> 65°). You now send a WMS request for an area of 5° by 10° in EPSG4326 
> with a width parameter of 500 pixels; normally the image produced would 
> be 1000 pixels high, but you request a 500 pixel high image, causing the 
> WMS server to scale (scale! not reproject) the image.

Ok.

> Having said that, the effect is less pronounced if you're nearer the 
> equator, and if you are zoomed in far then the error is way smaller than 
> typical GPS accuracy errors.

Which means ± 15 Meter without WAAS/EGNOS or dGPS.

> The proper thing for JOSM would be to add the SRS parameter based on the 
> current projection, and evaluate a GetCapabilities response before it 
> even allows you to make a WMS request with the current projection. 
> (Landsat, for example, will always return EPSG4326 no matter what you 
> request.)

Yes, the server needs to support other projections. Perhaps it would be
a better solution to reproject files in JOSM? That would cover all
"EPSG:4336"-only servers.

Best regards,
Tobias




More information about the josm-dev mailing list