There is a solution to this (sort of). There does not seem to be a way to specify an environment setting mask that would automatically constrain the dispersed markers to a polygon; however, you can edit your dispersed markers after the fact. It's not an elegant solution, but it will allow you to make maps that don't have markers spilling outside of boundaries where they don't belong! The thing to remember is that dispersed markers are a type of cartographic representation. You can use the representation editing tools to modify how your markers are displayed. You can set up polygon masks by specifying a many to many relationship in your geodatabase between a new polygon layer and your feature representation (dispersed markers) layer. Using the editing window, you can then specify your masking polygons interactively. An alternative, quick and dirty approach is to use the Representations toolbar, turn on the Editor, and use the direct select or lasso tools to select the stray markers and either erase them or move them. You can read more about masking feature representations at http://resources.arcgis.com/en/help/main/10.1/index.html#//00s500000051000000
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