## Wndsn Telemeter Instructions

Telemeters setup, operation, and maintenance.

Many uses of the Telemeter are apparent after simple instructions, but some techniques require some in-depth practice and special attention to detail.

## Initial Telemeter Setup

To install, knot the string to the card through the provided hole. Measure a length of 57.3 cm (22.44 inches) from eye to card. You can make a knot at the end or a loop; for measuring distances, hold onto the string with your teeth.

Note that you can measure any dimension; width, height, etc., as long as it's on a plane that is perpendicular to you, the trigonometry doesn't care where in space the triangle is located.

Why is the string length so important, and what's special about the 57.3?

It says "baked-in trigonometry"; also, on the tool itself, there is a hint: `For d = 57.3 cm, arctan(s/d) in deg = s => 1° = 1 cm`

The explanation is the definition of radian, or rad: The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3° (when the arc length is equal to the radius). The circumference subtends an angle of 2π radians.

Now, at a radius (hence the string length) of 57.3 cm, 1 cm in measured size subtends exactly 1° and makes for elegant calculations; `D = S * d / α` where:

• D = distance from eye to observed object of known size
• S = known size of the object observed
• d = distance from tool to observer's eye
• α = the angular size of the object observed at the distance d