Viewshed Analysis for bridge polyline observer feature

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I am using the Viewshed tool in ArcGIS 10.2 to determine the cells from a DEM that are visible from a highway corridor that has a bridge crossing a river (my observation feature). My question is in regards to the z value of the observation feature, with several contingent follow-up questions: 1) does the viewshed tool interpolate the z value based on the DEM, or do I need to manually add a field and define this along the corridor? 2) If it does interpolate the z value, would I need to add OFFSET fields for the part of the corridor that is on a bridge structure (and therefore, not the same elevation as the DEM in that location)? 3) If I do need to add these fields, do I need to cut my line into multiple features/records to do this?

Any help is appreciated!!



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