Rule Name: Model Spatial Reference Frame

Definition

1. All Models within a transmittal must either be defined in the spatial reference frame within which they will be instanced ((defined by the scoping Environment Root) or in a Local Space Rectangular (3D or 2D) spatial reference frame.

2. A Model defined in a spatial reference frame other than Local Space Rectangular may only be instanced by a Geometry Model Instance or Feature Model Instance in a matching spatial reference frame.

3. If a Model defined in a Local Space Rectangular reference frame is to be instanced into a non-LSR reference frame by a Geometry Model Instance or Feature Model Instance, then the model instance object must specify a World Transformation.

4. A Model defined in a Local Space Rectangular reference frame cannot be instanced into another Local Space Rectangular reference frame with different parameters (e.g. different direction_of_up specified).

Rationale

A Location object is not fully defined unless it falls within the scope of an object specifying its spatial reference frame parameters (e.g., an Environment Root). A Location in another spatial reference frame (e.g. geomagnetic when the reference frame is geodetic) is therefore undefined.

Transformation objects are required when a model instancing operation requires actual coordinate transformation or conversion.

Example

1. A Model defined in a geodetic spatial reference frame may only be instanced within an Environment Root with matching geodetic spatial reference frame parameters.
2. A Geometry Model Instance in a geodetic reference frame must have a World Transformation in order to instance a Model specified in a Local Space Rectangular reference frame.

FAQs

Are model Locations restricted to LSR and the spatial reference frames found in a transmittal's Environment Root objects?

No. A Model Library might be designed to be part of a multi-transmittal environment with Geometry Model Instance or Feature Model Instance objects occurring in other transmittals, so that the Model Library transmittal might not have any Environment Root instance at all.

Why aren't model instance objects required to have Transformation components in all cases?

If no coordinate conversion or transformation is required between spatial reference frames, and if (for LSR) the model instance is not being rotated or translated within the target spatial reference frame, then a Transformation is unnecessary.