With the release of ArcGIS 10, new tools were made available, allowing you to use 3D objects in geoprocessing tools, and opening up new possibilities for proximity analysis.  One of the new tools is the Near 3D tool, which calculates the three-dimensional distance from each input feature to the nearest feature residing in or more nearby feature classes. However, when using the new 3D analysis tools, it's always important to consider the geometry of the features being used. Suppose you have a 3D polygon and would like to determine whether 3D points are above versus below the surface of the polygon. Near 3D is a logical geoprocessing tool to determine which points are above and which are below, right? 3D Points with 3D Polygon In this particular case, the answer is no.  The results from the Delta Z created in the Near 3D analysis cannot be used because the geometry of the polygon is only maintained at the edge of the polygon.  Therefore, the results will be based on the distance from the point to the edge of the polygon, not the distance from the point to the polygon surface directly above or below the point. Points considered negative when only considering the Delta near Z To  determine whether the point is above and below the polygon surface, follow this workflow instead. 1. Convert the 3d Polygon to a TIN via the Create TIN tool. 2. Use the Add Surface Information tool to add the TIN values to the point. 3. Use the Add XY Coordinate tool if you do not have z-values for your 3D point data. 4. Add a field to calculate the difference. 5. Subtract the field with the TIN Elevation from the z-value of the point. Correct output with the field showing the difference field With this workflow, if the result is positive, the point is above the surface. So, when using the new 3D Analysis toolsets, be sure to consider the geometry of the features being used. References Editing polygons in 3DJeff S. -- Raster Support Analyst ... View more