Foci method

Illustrator Draftsman 3 & 2 - Volume 2 Standard Practices and Theory
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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 2-47

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