How is scale computed?

[Keith Sibson]

Hi,

I can't find anything in the archives about this so here goes...

I'm using map server to display a very large amount of data points, so
what I'm doing is to automatically generate shapefiles across several
layers. The top level layers are heavily clustered and the lowest layer
not clustered at all. Since these layers show the same data, their
display is mutually exclusive. In this way, I intend only 50-100 points to
be visible at any one time. This can be done by setting an appropriate
MINSCALE and MAXSCALE for each of them in the map file.

The problem is that I need to automatically determine MINSCALE and
MAXSCALE. 

If I know the initial map extent and the zoom factor per level of detail,
how do I go about computing the scales at which the layer divisions occur.
Assuming that the layer division will be at the mid point between zoom
levels.

I'm using DD as my shapefile metric, and I understand that scale is
affected by this. However, I'm willing to tolerate even a rough
approximation here.

Thanks for your help,

Keith.