Ellipse Construction, Continued
Foci method
The foci method of ellipse construction involves plotting a series of points
along the circumference of the ellipse by drawing a series of intersecting arcs
using the foci on the major axis as centers.
To construct an ellipse using the foci method, use this table:
Step
Action
1
Lay out horizontal (AB) and vertical axes (CD) that intersect at
right angles (0).
2
Locate the foci (F, F’) by setting the compass to one half distance
of the major axis AB and striking arcs along AB using C as the
center.
3
Mark a minimum of five equal distances between F and 0. The
more distances marked, the more accurate the ellipse construction.
4
Set the compass for the distance from A to 1. Strike arcs above and
below line AB using 1 and F’ as centers.
5
Set the compass for the distance A to 2. Strike arcs above and
below AB using 2 and F’ as centers. Continue plotting points this
way until all five points form an ellipse circumference between CB
and BD.
6
7
Mark a minimum of five equal distances between 0 and B.
Plot the five points forming an ellipse circumference between CA
and AD using the same previous procedure but in reverse using F
as the center.
8
Once all points are plotted, connect the points using french curves.
Continued on next page
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