The Plane Calculation menu item is used to assign elevations to any geometry object based on a specific 3D plane. This is an inplace operation; existing geometry will be updated and no new geometry will be added.
Select Points
Use this option to restrict the data points to update, based on a filter.
All points:  All points will update their elevations. 
Only nulls:  Only points that have an initial elevation of NaN will update their elevations. 
Restrict by Polygon:  Points that are found within the boundary polygon(s) or region(s) will update their elevations. This option can be inverted (to accept points that are found outside all boundary polygon(s) or region(s). 
Apply Plane Equation
This defines the plane equation, used to calculate an elevation based on a formula plus the X and Y coordinates.
Three points:  Plane equation is defined by three unique points. 
Planar Equation:  Plane is defined by the equation Ax + By + Cz + D = 0. 
Point, Dip and Direction:  Plane is defined by a point, dip and dip direction. 
