Marking out an ellipse

How to mark out an ellipse 4 m x 2m

[clelanj]

Excellent demo which will prove very useful to me. Thanks for sharing

[FourApramanas]

Many thanks for providing instructions for this rather ‘niche’ topic!
We adapted it to go around a tree, using a plastic hula-hoop… Fortunately I was able to buy a tubular plastic hoop of sufficient diameter to go around the trunk. It came with a join where the two ends of the tube are held together by an inserted plastic connector, the join being covered by a piece of tape. The hoop was not an entirely symmetric circle but I marked four ‘cardinal points’ at approximate quarters of the circumference (“that’ll do!”).
After removing (or cutting through) the tape over the join, the hoop was pulled open to encircle the trunk before reconnecting the two ends over the plastic connector-insert. The hoop was then fixed temporarily in situ by pegs to stop it from shifting about.
The central tree meant that we lined up the two axes mainly by eye and by cross-checking measurements. Then we ran a length of string from the *major* axis ‘cardinal points’ marked on the hula-hoop to the respective terminals of that axis (i.e on ‘good-enough’ opposite sides of the tree), to serve as a ‘line’. When calculating where the halved measurement from the *minor* axis terminal should cross this line, I factored in the diameter of the hoop; then we ran a second piece of string (string-B) from these new ‘line-crossing points’ around one minor axis terminal, before removing it from the latter in order to spray the curve using the slack string.
The tree meant a couple of further adaptations: 1) only half of the ellipse could be drawn at a time before unhooking string-B from one ‘line-crossing point’ to reattach it to use on the other side of the tree; 2) the trunk of the tree prevented string-B from being able to fully complete the curve at the major axis terminals, so these relatively small parts of the curve were finished freehand by eye.
Despite some necessarily approximate measurements, I am very pleased with the resulting ellipse (which will be a small meadow area in the lawn).