Rule Name: Environment Root Spatial Reference Frame

Definition

A transmittal is allowed to have one or more spatial reference frames, which are individually defined Environment Roots (if there are any Environment Roots).

1. No two Environment Root instances may have identical spatial reference frame parameters.
2. All Location components aggregated below an Environment Root must be defined within the spatial reference frame of that Environment Root, unless they fall within the scope of an object which defines its own spatial reference frame, such as a Property Grid or Image Anchor.
3. No Location objects under an Environment Root may be invalid within that spatial reference frame; they must be either valid, or "extended".

Rationale

An Environment Root is the starting point for all objects in the same spatial reference frame in a transmittal.

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, Image Anchor). A Location in another spatial reference frame (e.g. geomagnetic when the reference frame is geodetic) is therefore undefined.

Example

1. If a Environment Root's spatial reference frame is AUTM, no Locations under that Environment Root may be invalid in AUTM; they must be either valid, or "extended".
2. Consider a transmittal spanning two consecutive UTM zones. The transmittal will have two Environment Root objects, one each of the two UTM zones.

FAQs

Why does a Property Grid have its own spatial reference frame, independent of that of its Environment Root?

The "griddedness" of spatial positions is dependent on the properties of the spatial reference frame in which they are defined. Coordinate conversions and transformations are not, in general, linear, so that a set of points that form a regular array of positions in one spatial reference frame may not be regular in another spatial reference frame. Therefore, in order to preserve "griddedness", a Property Grid specifies a spatial reference frame in which the data positions form a grid.

Why does an Image Anchor have its own spatial reference frame, independent of that of its Environment Root?

As with the "griddedness" of Property Grids, an Image Anchor specifies a spatial reference frame in which the anchor points specify the desired texture mapping, so that it is preserved without distortion.

In addition, an Image Anchor may be attached directly to an Image, in which case the Image Anchor would be outside the scope of any Environment Root.