How Do You Construct an Altitude With a Compass?

To construct an altitude with a compass, draw two circles through the specified point, then draw a line through the intersecting arcs. Most often, this is done as a practice in geometry to find the altitude of a triangle.

It is a fairly easy process to find the altitude of a compass with a ruler. One simply has to use the ruler to draw a straight line from the specified point on the triangle. Using a compass, however, involves a few more steps.

  1. Draw the first circle
  2. A triangle has three vertices, usually designated by letters such as A, B and C. If the challenge is to find the altitude of C, use point A as the center point for the compass and the line segment from A to C as the radius to draw the first circle.

  3. Draw the second circle
  4. Using the example from step one, draw the second circle with point B as the center point. The radius from point B to point C is the radius of the circle.

  5. Draw the altitude line
  6. The circles will intersect at two points. The first will be at point C on the triangle. The second will be at a point somewhere below the triangle, directly below point C. Use a ruler to draw a line from point C to the intersection below. The measurement of that line from point C to the base of the triangle is the altitude.