[josm-dev] pixel per degree

[Paul Hartmann]

On 09/19/2011 05:33 PM, Dirk Stöcker wrote:
> On Mon, 19 Sep 2011, Paul Hartmann wrote:
> 
>> Why is it called "pixelPerDegree" in ImageryInfo? Apparently, the value
>> is in pixels per unit in projected space ("pixelPerEastNorth").
> 
> Which is nonsense as well. It should not be relative to projected
> coordinates. 

What is the alternative? WMS cache as well as the global scale is
relative to projected coordinates.

One option would a scale in pixel per meter. But it changes with
location (also can be different in NS and EW direction). In order to use
the cache, you need the exact scale value in pixelPerEastNorth. When
converting back from pixelPerMeter to pixelPerEastNorth there will be a
rounding error.

> I think the sole idea behind this is the bookmark feature
> and this should not depend on projection.
> This is very old code (before we had proper projection support) and
> probably neeeds to be reworked. The idea of bookmarks is to have a
> cross-session caching possibility.

I was referring to everything that is named *pixelPerDegree or *PPD, not
just the bookmark feature.

Paul