As an adjunct to efl's excellent post, the designer producing an LNAV/VNAV procedure takes into account the lowest recorded temperature at a particular aerodrome, rounds it down to the nearest 5deg and calculates the temperature error at the glide path intercept altitude. This lower altitude is then used to calculate an obstacle identification surface (OIS) - any obstacle that penetrates is used to calculate the DA by adding an appropriate clearance.
If the intercept altitude is more than 5NM from the threshold, then the equivalent ILS "W" surface is used instead of the calculated OIS - which is an even lower surface (about 1.63deg)