If I want my plane to fly 100m above the launch site and stay 100m above the ground whether the ground rises or falls (i.e. follow the terrain), do I check both the 'Hold Default Alt' and 'Veryify height' boxes or just the 'Verify Height' box and leave the other 2 unchecked?
Hold Default Alt is just for the RTL function. You would just need to have Verify Height check for terrain following.
Here is an excerpt from the wiki:
Default Alt is the default altitude when entering new waypoints. It's also the altitude RTL (return to launch) mode will fly at if you have "Hold Default ALT" checked; if you don't have that checked, your aircraft will try to maintain the altitude it was at when you switched on RTL.
Verify height means that the Mission Planner will use Google Earth topology data to adjust your desired altitude at each waypoint to reflect the height of the ground beneath. So if your waypoint is on a hill, if this option is selected the Mission Planner will increase your ALT setting by the height of the hill. This is a good way to make sure you don't crash into mountains!
Thanks Kurt. Can you help with 1 other mission planner question?
If I don't specify an airspeed between waypoints will the plane sit at the default throttle setting for the whole flight - my throttle (nitro plane) set to 50% for example.
I think that is correct. APM allows you to "nudge" the throttle while flying in Auto mode but on the other hand if you set the cruise to 0 it may chop the throttle on you and then you would have to bump it up. Hopefully the more informed will chime in.