Need to Filter Data

I have a loop that generates pairs of coordinates (X, Z) that are waypoints.

Eventually, a path is linearly interpolated between the waypoints.

Because of the nature of the data, there will be long sequences where Z doesn't change.  Within those sequences, I want to keep only the endpoints, deleting the superfluous waypoints in between.

Any suggestions on how to approach this would be appreciated (so far, my attempts are ugly).