Downpointer

Except at level zero, the downpointer always points down.

[…] this data structure differs from the tree […] illustrated in Fig. 1, only by having downpointers replaced by downtrees.

By using the so-called functional approach to data structures Chazelle [1988] developed a compression scheme to encode the downpointers used by Willard [1985] to reduce further the space requirement.

During one session , the subject used a canopy-mounted downpointer to estimate position in space relative to the ground, and in the other session the subject used a joystick-controlled, inside-out, aircraft attitude indicator.