Why/when would I need to define a Z coordinate system in arccatalog for my dataset?

[Mr.T]

While defining the projection for a new feature dataset, it occurred to me that I have not defined a z coordinate in a while. Well, I have some field data now with elevation (z values). Out of curiosity, why or when would I need to define a Z coordinate system?

Here's whats bothering me. If the projection is the same when the data was collected (x,y,z), do I still have to create a z coordinate in arccatalog? I mean the xy coordinate defined should cover this as well...correct? considering mean sea level does not change.

I would appreciate some simple scenarios why a z coordinate would be required in addition to an existing x,y coordinate already defined.

Thanks for taming my GIS curiosity.

[vangelo]

I would recommend the Understanding Coordinate Management in the Geodatabase
whitepaper for the details, but simply put, *every* layer in ArcSDE has a coordinate
reference, which encompasses the coordinate system, the precision, the false offsets
and scales for the X/Y, Z, and M dimensions, and the tolerances for those dimensions.
The default offset and scale for the Z dimension are 0 and 1 (respectively), meaning
you could not capture data below sea level and only at 1 unit intervals. Therefore,
you need to define a Z component on the coordref any time you intend to store
coordinates with a Z dimension (that is, a Z value with *each* coordinate, not a
single "ELEV" attribute property for the feature).

- V