In ArcGIS Pro, there is the ability to mask layers using another polygon layer. I have tested that this can work using the steps outlined here:
I add two polygon layers, select the lower one in the draw order, go to the 'Appearance' tab, click on Masking, and tick the polygon layer above it. I click off this panel to activate, and the top layer masks out portions of the lower layer as expected.
However, if I then try it with an imagery or basemap layer, it doesn't work. I place the imagery layer at the bottom of the draw order, select it, go to the 'Appearance' tab, the 'Masking' option is there and it allows me to do exactly the same as above, but after ticking a polygon layer and clicking off it, nothing happens. The imagery layer is not masked at all.
Which leads on to a second follow-up question: my use case here was to try and clip/mask an imagery layer using a polygon layer. I actually want to do the reverse of the default here - I want to 'clip' the imagery, i.e. 'mask' everything that is *not* inside a polygon. I could achieve this by creating a 'doughnut' polygon where the masked portion is bigger than my layout, but that seems a little crude. Can anyone suggest another way?
Note that actually clipping (editing) the source image file is not an option.