ORMs vs. object models
Reference Surface: An ORM is often selected to contain one or more RDs of category oriented surface to correspond to (physical or conceptual) surface(s) which is significant to the modelled spatial object.
Example: an oblate ellipsoid as a model of the shape of the Earth.
An RD is chosen and its position with respect to the object is bound so that the RD instance is a “best fit” to the object in some application-specific sense.
In particular, if the RD surface is “fitted” to a specific part of the object surface, the ORM is a local model.
If the RD is selected to best fit the entire surface, the ORM is a global model.
Other Surface models:
geoid: gravity equipotential surface that approximates the mean sea level of the Earth.
pressure equipotential surface: equipotential surface of a pressure field.
topographic surface: interface between the solid and liquid/gas portions of a celestial object.
Some of these surfaces are treated in the SRM as vertical offset surfaces (a later topic), while others are outside of the scope of the SRM.