In the leveling between two points A and B on the opposite sides of a pond, the level is first set up near the point A and staff reading on A and B are 2.5 m and 2.0 m respectively. Then the level is moved and set near the point B, staff reading on points A and B are 1.2 m and 1.7 m respectively. Calculate the difference of heights between the two points A and B (in metre).

A

0

B

0.5

C

1

D

1.85