With version 10.2.49 of the GP Tools, a new GP tool named ‘Interpolate/Extrapolate Z-Values’ is provided. This tool will calculate the z-values that are 0 or NaN for polyline vertices by extrapolating/interpolating the known z-values from other vertices. The tool will store a z-value of zero for vertices that cannot be calculated.
This functionality is offered for the business purpose of applying elevations to previously 2d polylines that have been Z-enabled but are not yet populated.