Returns the parameter of the curve at the point
Supported Platforms: Windows only
(vlax-curve-getParamAtPoint curve-obj point)
Type: VLA-object
The object to be measured.
Type: List
A 3D point (in WCS) on curve-obj.
Type: Real or nil
A number representing a parameter, if successful; otherwise nil.
Assuming that ellipseObj points to the ellipse shown in the example for vlax-curve-getArea, set OSNAP to tangent and select the point where the line is tangent to the ellipse:
(setq selPt (getpoint)) (7.55765 5.55066 0.0)
Get the parameter value at the selected point:
(vlax-curve-getParamAtPoint ellipseObj selPt) 4.58296