The change in elevation = Final Elevation - Initial elevation
Final elevation = Change in elevation + initial elevation
if a bird starts at 20 m in changes 16 m
Final elevation = 20 + 16
Final elevation = 36 m
a butterfly starts at 20 m and changes -16 m
Final elevation = 20 + (-16)
Final elevation = 20 - 16
Final elevation = 4 m