clarify Z values

[mattyschell]

When the vendor delivers planimetrics to CUNY and OTI it is in the form of an ESRI file geodatabase. All spatial data is in spatial reference id 2263 aka "New York State Plane Lambert Conformal Conic." Some of the feature classes in the geodatabase have Z values. These are noted in the repository with geometry type Polyline Z or Point Z (see #49).

Speaking personally here, as a mid-career civil servant with a similarly mid understanding of coordinate reference systems, this is nonsense. Lambert conformal conic projections are flat surfaces. At best the Z values are maybe related to the NAD 83 datum where elevations exist. But then why deliver Z values in an X/Y projection? And more importantly, is this even true?

My colleagues usually look for the "stop generating" button at this point in the conversation. Sometimes an example helps.

If one coordinate of a curb at the top of a hill in Central Park has a Z value of 10, what does this mean? Is this point 10 feet above the surface of the hill? Is it 10 feet above some general contour of the topography of Central Park? Is it 10 feet above mean sea level? Is it 10 feet above the geoid?

Much of the time Z values don't matter. But bread and butter GIS operations like nearest neighbor will return different results when the third dimension is included. Not to mention the fact that collecting, accounting for, and documenting the Z values in each format requires a fair amount of effort for data points that provide seemingly no use.