Hi Bill,
one problem is that it is not reported in other messages.
What bis reported is RelAlt-EKF and not RelAlt above home - as it was before in old GPS message. The problem is that there can be offests of several meters:
https://github.com/ArduPilot/ardupilot/issues/4468
and some discussion at:
http://discuss.ardupilot.org/t/relalt-disparity/10132/12
Best regards,
Thorsten