Sagitta Rule

by Daris A Nevil

From Wikipedia:


 * In geometry, the sagitta of a circular arc is the distance from the center of the arc to the center of its base. It is used extensively in architecture when calculating the arc necessary to span a certain height and distance.

In Live Steam a Sagitta Rule is used to directly measure the radius of a curved section of track.

Calculating
The following spreadsheet is provided to calculate the sagitta value for different radius curves for 10 foot and 20 foot rail lengths.


 * Sagitta calculator spreadsheet (XLSX)

Here are the values provided by the spreadsheet above.

Construction
It is easy to construct a Sagitta Rule. Obtain a large paint stir stick (for 5 gallon buckets of paint) from a big box store or where paint is sold. Print the following paper rule. Cut out and tape or glue to the end of the paint stir stick.


 * Sagitta Rule for 10 Foot Rail (PDF)



Use
Using a Sagitta Rule is as simple as following these steps.