Right triangles on the job
Choose the ratio from the sides you have: sine and cosine involve the hypotenuse, tangent uses the rise and the run.
A worked example
A ramp rises 2.0 m over a run of 14 m: tan θ = 2.0/14 = 0.1429, so θ = 8.1°.
The mistake people actually make
An angle of elevation taken from eye level gives the height above the eyes, so add the eye height for the full height. On a scale drawing, multiply the drawn length by the scale to get the real length.