Mission with absolute altitude


I’m planning a mission with absolute altitude and below is my mission file:

During takeoff, it is climbing to 1680m from the ground. But it is suppose to climb only 100m.
Also, when I arm the vehicle, I’m getting a message as

Terrain: clamping offset -1580 to -15

If I use relative altitude, it works fine. But I can’t use relative alt because of uneven terrain.
I’m not able to find a good tutorial/guide on how to use absolute and terrain altitude in mission planner.
Are there any params I need to set while planning in absolute altitude?