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For Checking Collision on your maps, a handy mode is available in the quick flags menu. Zero Extent and Rigid body collision modes will help you visualize the map collision better than the default view.
Terrain vs Landscape
As a General Note. Use Landscape, not Terrain. Terrain is outdated.
https://udn.epicgames.com/Three/Landscape.html
For more information:
Getting started with map editing.
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