wms scale calculation

In WMS 1.1.1 there is just a scalehint defined for calculation of scale boundaries. There is no need to define a pixel size for this.
But in the supplemental standard SLD 1.0.0 there is instead a scale denominator and a standard pixel size, which is specified to be 0.28 x 0.28 mm. This is the same definition as in the later WMS version 1.3.0. This correponds to an assumed resolution of the client screen of 90.7142857 dpi.

For this reason we used this value for calculating the current scale denominator in our WMS implementation, when scale boundaries, which are defined with scale denominators, are enforced at the server.

As uDIG uses the standard assumption of Windows for the screen resolution (96 dpi, or any other value, if the user has changed it), the user, who expects, that a map layer is not shown any more at a scale of for example 1:3000 or lower, sees, that the layer is still displayed at 1:3174. The WFS feature types on the other hand, which are displayed as layers using client-side SLD and for which the boundary is also defined at 1:3000, behave as expected, as there is no need to assume any pixel size at all in this case.

Though definition in the WMS/SLD standard is without ambiguity (if SLD is considered too for WMS 1.1.1), even UMN MapServer seems to use 75 dpi and there is much confusion in the community about how to calculate scales.

To solve the problem at least temporarily for our users, we have changed the dpi value in our version of uDIG. But this change will be lost or have to be redone, when switching to a newer version of uDIG. Therefore we hope, that some way can be found to solve this permanently in the uDIG base. Our suggestion would be to make the dpi value configurable as double instead of int and use the value from the getDisplay() only as default.