This question has been answered here within the past 5 days.
A search should turn up the solution.
What version of Inventor are you using? (solution depends answer to this question)
Actually I read this as arc length.
But if you really need arc height - simply create construction line and dimension.
If you attach your ipt file here it might be clearer what you need to solve the problem.
Please mark this response as "Accept as Solution" if it answers your question.
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Autodesk Inventor 2013 Certified Professional
Autodesk AutoCAD 2013 Certified Professional
Certified SolidWorks Professional
Inventor Professional 2013 SP 1.1 Edu 64-bit
GeForce GTX 560M i7-2670QM @ 2.2GHz 8GB RAM
http://home.pct.edu/~jmather/content/DSG322/inventor_surface_tutorials.htm
http://www.autodesk.com/edcommunity
Still waiting for -Draft option on any Rib feature.
