# JOSM - Creating an arc (or circle) from two tangeants

Hello all,

First of all excuse me but I’m not used to discuss geometry in English language, French only, so I lack the apropriate vocabulary to discuss this topic. I’ll try to translate everything literaly from French, and if something’s wrong please correct me.

If I remember well my geometry courses, a circle can be described by the following elements :

• 1 point (center) and radius

• 3 points

• 2 tangeants and 2 tangeant points

JOSM currently allows to draw circles with the first two options. If we want just an arc, then we can easily draw the full circle and remove the section we do not want.

However, I’m currently correcting the trace of railway lines, and usually arround curves you lack the center and the radius of the curve, so option 1 can’t be used. Option 2 can be used if the curve is entierely visible on the satelite image. But that’s alas not always the case. If part of the curve is hidden by trees or is in a tunnel, option 2 can’t be used.

So in most cases we have two tangeant directions and two tangeants points (start and exit of the curve), but alas JOSM doesn’t allow to draw the curve from this information. As such, correcting railway traces arround curved tunnels is extremely difficult and always result in goofy solutions.

Does anybody have a suggetion ?

The only idea I have so far is to attemps to trace two lines perpendicular to the railway on the curve entrance and exit, and where they cross this is the center of the curve. We can then draw a full circle from this information, and truncate it to the part we want. Altough this is possible, it is complicated (requires drawing 3 dummy objects before getting what we want) and takes time. It is also fairly inacurate, since drawing lines perpendicular to the railway line by hand is always ending to be somewhat aproximate.

Also there’s a command to transform some curve into an arc in JOSM, but alas this command never does what I’d like it to ! It usually makes an arc with the end of the arc going to a completely wrong directions, not tangeant at all, so this is not as useful as it should be.

If anyone has a suggestion to help correcting railway traces and make detailed curves that align correctly, that’d be helpful.