A tilted container with water inside - how to calculate the distance A

We are trying to control a container with water inside. The container has two phases of operation. In one phase, the container is tilted. In this tilted state, the distance from the observation point (red dot) to the surface of the water (distance B) is known from a measurement device. However, we actually would like to know this distance when the container is in a flat position (distance A).


In the control system, the dimension of the container is known (C and E). The tilt angle (alpha) and the distance B are known from measurement devices. The position of the observation point is known (D).


The container cannot positioned in a flat position. But we want to know the distance A (distance when the container were laid flat).


Could anyone suggest how to calculate A ?


Thank you very much, Doy