Geoffers, as part of the pre-flight planning there are certain safeguards;
Firstly an Obstacle Plane Value (OPV) is established, above which all obstacles are supposed to be marked on the relevant maps or in the Digital Vertical Obstruction File (DVOF) for those using electronic mapping systems.
Next, a Minimum Operating Height (MOH) is established for the type of NVG you are doing.
Then, mark your map with obstacles out to a suitable distance from your desired track, especially highlighting those that are above the level you are flying at.
For each leg you will calculate a Minimum safe Height (MSH) which you will climb to if you are unsure of your position, have a goggle malfunction or can't see an obstacle that is above you by a certain minimum distance.
You will also calculate a Reversionary Altitude (RA) which you can climb to with a total navigation or NVG failure which will ensure you are above 500' for that route.
All mil pilots have to carry out reversionary (normal) night flying on a regular basis to ensure the basic skills are not lost - specifically making an approach to a lit site/ NATO T or similar.
For SAR work, the mil generally flew above the OPV and used lookout and normal map reading techniques to avoid obstacles, knowing that you were above the highest normal pylons ie above 250'. If forced down by bad weather then the navigation skills became much more important and speed was reduced commensurate with height - even down to the hover taxi and the use of white light with NVG was the norm.