Final answer:
In GIS applications, layers without an elevation value can still be displayed in 3D by extrapolating a third dimension from other data or by applying a default elevation.
Step-by-step explanation:
Layers can indeed be displayed in 3D in various GIS (Geographic Information Systems) applications, even if they do not contain an explicit elevation value. This is possible because the software can extrapolate or infer a third dimension from other available data or by applying a default base elevation. For instance, a 2D map of a city can be extruded to display 3D buildings by assigning a uniform height to all structures, or by using statistical data to approximate varying building heights. Furthermore, digital elevation models can be used to overlay 2D data onto a 3D terrain.