Definition

Consider a <Transmittal Root> instance TR having one or more <Environment Root> instances as components.

1. For TR, no two <Environment Root> instances may have identical spatial reference frame parameters.

2. All <Location> instances appearing in the composition tree rooted at a given <Environment Root> instance shall be defined within the spatial reference frame of that <Environment Root> instance, unless such <Location> instances fall within the scope of an object that defines its own spatial reference frame, such as a <Property Grid> or <Image Anchor>.

3. No <Location> instances under an <Environment Root> may be invalid within that spatial reference frame; they shall be either valid, or "extended".

Rationale

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

A <Location> instance is not fully defined unless it falls within the scope of an object specifying its spatial reference frame parameters (for example, an <Environment Root> or <Image Anchor> instance). A <Location> in another spatial reference frame (such as celestiomagnetic when the reference frame is celestiodetic) is therefore undefined.

Example

1. If an <Environment Root> instance's spatial reference frame is AUTM, no <Location> instances under that <Environment Root> instance may be invalid in AUTM; they shall be either valid, or "extended".

2. Consider a transmittal spanning two consecutive UTM zones. The transmittal will have two <Environment Root> instances, 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 Grid> instances, an <Image Anchor> instance 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>.