Vertical datum not been defined for Feature Class

[ZarehDemirdjian]

If there is feature like road, and this road two part and the connection point have the same XY but different Z like the sample below. I use the PLTS tools to find this type of error.
But there is massage appear ???Vertical datum not been defined for Feature Class??? so WHAT is the Vertical datum and HOW I can defined for Feature Class to find and solve such error like this case.

Thank you in Advance,

Zareh

[DanHopkins]

Have you looked at the following PLTS Discussion Forum thread?

http://forums.esri.com/Thread.asp?c=60&f=437&t=245839&mc=3#msgid750930

In ArcCatalog, launch the Feature Class Properties dialog and click the Z Coordinate System tab. You can define a Z Coordinate System for you feature class through the Z Coordinate System tab.

Information about vertical datum can be found at the following link:

http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Vertical_datums