I have a Windows Form App that shows a laser scan in a picture box.
The Picture box is 442 wide by 451 tall. The scale of the box is -138 to 138 on the "X" axis and 100 to 500 on the "Z" axis.
I can draw a rectangle on the screen but I can't figure out how to get the coordinates of the rectangle within the picture box scale.
Kind of depends on how you're doing your drawing.
If you used Transform.Translate and Transform.Scale to essentially setup your coordinate system to match your drawing area, then you would be doing your drawing in that coordinate system and you can save the matrix and use it convert client area coordinates into your scale's coordinates.
If you are doing all the scaling and offset calculations yourself, to draw the graph, then you would need to use the same math in reverse to scale the rectangle coordinates back.
I've posted one or two examples that might be useful. I'll do a quick search and post a link once I find one, or both.