Spawn Points

Spawn points

At the very beginning of a battle, alien units are not scattered randomly throughout the map. Most aliens are positioned at one of a well defined spawn points. The easiest example is base defence missions, because you can locate the spawn points fairly easily, and use that knowledge yo your advantage:

As you can see all the aliens appear either on the red-marked line, or in one of the small enclosures. A blind shot of blaster bomb that goes along the the red line goes through a lot of spawn points and is likely to hit an alien.