How do I properly read a clinometer?

Question

If the weight hangs down at roughly 42 degrees, would the angle be 90 degrees - 42 degrees = 48 degrees?

Answer 1

$\text{WANT}+\text{HAVE}=90^\circ$

Answer 2

This depends on which angle you are talking about. If you want to know the angle between the imaginary horizontal line and the black line in the picture, then you are correct. Call this angle $\alpha$.

To understand why this is true, imagine that the tree was your height. Then $\alpha$ would be $0°$ and your clinometer would read $90°$, as it would be horizontal. Now start increasing the height of the tree step by step. Your clinometer would show $89°$, $88°$, $\ldots$, while $\alpha$ would get bigger, namely $1°$, $2°$.

In general

$$\alpha = 90° - \text{ "the angle shown on the clinometer"}$$

(I'm aware that this is highly informal, but I think this might actually help OP.)