Tried searching this extensively here and elsewhere so pre-apologies if this is already covered.

I am working on a poor man’s version of a plot plan for an architectural drawing. It’s your basic lines set to a scale drawing. However, it’s based off off typical city engineering plans which sometimes have curves (like curves in a road). Those curves have two components: a line length and an “R” variable which stands for the diameter of a circle which would represent the arc of the curve in question. So for example you can have a line that is 75 feet long and “R=50” meaning to represent the arc in that line is the same thing you’d get off a 50’ diameter circle.

I cant find anywhere to input the value of an angle or a line relative to the scale of the document. I’ve seen talk about pixel values in two adjoining lines but that won’t get the value you’re looking for here. I saw a more recent post talk about utilising an Adjustable Arc object but that didn’t work for me in my attempt- I basically made an arc of the prescribed length, then set its points at 0 and 90, and then in the Geometry section specified a 50’ height and it didn’t seem to check out.

I feel like there has to be something basic I’m missing and hoping someone else has tried to do a line like this. I attached a picture of the exact plot I’m trying to draw where you can see the dimensions in feet (decimals) as well as the two lines and their R values in question.

Yeah, as I suspected, in the above drawing R means radius, not diameter. I successfully recreated the drawing in Omnigraffle using arcs and the formula below to calculate the angles for the arcs (n being the angle).

Thanks for looking into this, I really appreciate it. However in my novice talent level I can’t figure out where to input any technical information for an arc object.

What you need to input in Omnigraffle is the width/height of the adjustable arc shape (which are equal to the diameter of the whole circle) and its angle.

You know the radius R, so the width and the height of the arc shape will both be 2*R. You can set these using the fields on the Geometry panel.

You know the length of the arc, so the angle can be calculated using the formula I posted above. Unfortunately in Omnigraffle the start/end angles can only be set by dragging the handles of the arc, which is cumbersome.

OK, I think one of the major things I was doing wrong was setting the height and width incorrectly. I was treating width like a line length which obviously it is not since OG isn’t treating an arc like a line from what I can tell.

So if I drag an arc and set the handles to 0° and 90° and then set the height and width to 2r, in this case “100” I seem to get exactly what I need. If I build out the drawing from the original post everything seems to check out: