Detect round objects with JOSM

Maybe this ticket is related: https://josm.openstreetmap.de/ticket/16283

One neat fact about circles is that they have the smallest possible perimeter (circumference) for their area of any shape. The area of a circle = (perimeter ^ 2) / (4 * pi). So if you find the area and perimeter of a given shape, you can calculate the following ratio: (perimeter ^ 2) / (4 * area). For a circle, this ratio will be pi. For a square, it will be 4. For approximate circles like those in OSM, it will be somewhere between. The more circular the shape, the closer to pi.

If you do write a plugin, you could configure it for a range, say between pi and 3.25, and find shapes within that range.

...However, calculating the area of a shape defined by an arbitrary set of node coordinates can be a little complex, so if you don't have a built in routine for this calculation you might want to do something simpler. You could find the maximum distance between any two points in the shape and compare that to the perimeter. For a circle, this maximum distance will be the diameter, and the ratio of the perimeter to the diameter should be pi. For nearly-circular shapes, this ratio will be a little less than pi. (For a square it will be 2 * sqrt(2), about 2.83.)

Note that, depending on the range of ratio values you decide to consider circular, these techniques will not necessarily restrict your answers to "factory" circles with a lot of identical angles. IMO this is probably a good thing -- there are probably many circular ways that are drawn by hand or otherwise slightly deformed.

Not sure I have understood your question. Do you mean draw a true circular shape? Not used JOSM much but thought it must have a tool for it. I searched and found this https://learnosm.org/en/josm/josm-tools/