How to get a segment of a polyline at a certain location?

Hi Everybody:

Given a polyline and a point on it, how can I find the segment of that polyline on that point? I checked the ISegmentCollection interface but couldn't find a way to do it. How can I do it?

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The reason I want to find the polyline segment for a given point
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I am trying to write a function to decide whether a given 3D point feature is on a given 3D polyline feature. After making sure the point is exactly contained by the polyline, I want to allow some Z-coordinate tolerance, which is decided according to the segment length of the polyline at that location:
-- The segment is long, which means the distance between polyline vertices is large, then allow a larger tolerance
-- The segment is short, indicating closer distance between polyline vertices, then allow smaller tolerance

Is there alternative way different from finding the segment? I want to find the local vertex density instead of an overall one for a long polyline (which can be calculated by length divided by number of vertices).

Thank you!