Inverse Tangent

When both legs are known, the tangent links them and $\arctan$ gives the angle. This is how a slope becomes an angle. A road that climbs 1 m for every 10 m along the flat has $\tan\left(\theta\right) = \frac{1}{10}$, so $\theta = \arctan\left(\frac{1}{10}\right)$, about 5.71°.

Road signs give that slope as a percentage: 10% means 10 m up for 100 m along, the same 1/10. A 10% hill sounds steep, yet its angle is under 6°. A slope of 100% is a rise equal to the run, an angle of 45°.