What is my vertical datum

[MathieuCain]

I have a feature class of Polygon ZM type, but without a vertical coordinate system specified, but a couple of elevation fields. Is there any way of telling whether the elevations in the fields are in NGVD29 or NAVD88 vertical datum?

I considered creating copies of the FC and defining the Z coordinate system for each as NGVD29 and NAVD88, then adding a new Z field, and calculating the "Max Z of geometry". Unfortunately both FCs yielded the same Z value (matching one of the pre-existing elevation fields).

Any thoughts?

[NeilAyres]

The Z will not differ, that's part of the geometry.

And as regards which V datum the data is actually based on that will only be known by the source.

Depending on the history of the data (perhaps it's capture date), you may be able to guess.

National Geodetic Survey - Frequently Asked Questions FAQ 

[MathieuCain]

Based on the National Geodetic Survey's VERTCON tool (https://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl), the datum shift turned out to be fairly negligible, particularly given that the elevation fields were only being reported to the nearest whole number. So use of either in this particular case was adequate.