[josm-dev] to the devolopers of WMS-plugins
tobias.wendorff at uni-dortmund.de
Thu Nov 6 19:03:07 GMT 2008
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.
> 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
Yes, the server needs to support other projections. Perhaps it would be
a better solution to reproject files in JOSM? That would cover all
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