OK, so I've sorted this some.  It needs to be more clear that this `sigma` is the $w$ from the paper, this should be in the docs.  It should also be noted that this is in pixel coordinates.  Perhaps if the input is a map with plate scale info a flag can force it to use physical widths?  (perhaps that flag should default to true if it can be done?)

I'd also note in the docs that this is for visualization only.