Which of the following best describes z-values in a digital terrain model?

Z-values in a digital terrain model refer to numerical values that represent elevation or height above a reference point, typically sea level. These values are crucial in geospatial analysis as they provide the third dimension in a three-dimensional model, allowing for the representation of terrain relief. This elevation data is essential for various applications, including modeling water flow, analyzing land suitability for construction, and assessing visibility from a given point in geographical studies.

The other options do not accurately describe z-values: temperature variations relate more to thermal modeling, urban planning requires a broader set of data beyond fixed coordinates, and index values can denote a variety of data types but don't specifically refer to elevation in a terrain context. Thus, the numerical value representing elevation is the defining characteristic of z-values in a digital terrain model.