Navicular drift and navicular drop measurements are not something I necessarily advocate in clinical practice, primarily because:
1) They are not that reliable as measurements
2) Knowing the exact value is not that helpful
However, they are useful concepts to understand and may be useful to do when learning to understand and appreciate foot function.
Essentially the measurements are done by placing a mark on the medial side of the navicular while the subject is standing; then use a business card to mark the height of the navicular; the STJ is then placed in its neutral position and the card marked again with the height of the navicular (this is the navicular drop); while the STJ is in its neutral position, the business card is placed flat on the ground and a pen is held vertically against the medial side of the navicular and its position is marked on the card; the foot is then moved to its relaxed position and the navicular position is marked again on the card (this is the navicular drift). The differences between the drift and drop marks can be measured (but just look at the technique and the potential for measurement error to creep in).
However, what is more important is the relative relationship between the two. I believe that navicular drop should be about the same as navicular drift.
If drop > drift, then that implies that most of the compensation is in the sagittal place (ie arch collapse) and is a lot easier to treat with something under the foot.
If drift > drop, then that implies most of the compensation is in the transverse plane and is a lot harder to treat with something under the foot. It really needs things up the side of the foot to control its motion (assuming that is what you want to do)..
We see this clinically all the time, but usually do not view it as the relationship between navicular drift and drop. We see those feet that predominantly compensate in one body plane or the other. We know from clinical experience that some are easier to treat than others.
As an aside, Brody a number of years ago wrote that a navicular drop of greater than 1cm is a pronated foot. He has been quoted many times that this figure has crept into folklore. Think about it .... any large foot will have >1cm of navicular drop!
Final word: How do I treat those with a predominantly transverse plane compensation (ie navicular drift > drop)? .... I refer to them to my enemies.
For more, see this thread on Podiatry Arena: Need help interpreting navicular drop and drift