Definition

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

1. For TR, no two <Environment Root> components may have identical srf_context_info field values.

2. All <Location> instances appearing in the component tree of a given <Environment Root> instance shall be specified within the SRF of that <Environment Root> instance, unless such <Location> instances fall within the scope of a DRM object that specifies its own SRF, such as a <Property Grid> instance or <Image Anchor> instance.

3. No <Location> instances in the component tree of a given <Environment Root> instance may be invalid within that SRF; they shall be either valid or extended.

Rationale

A <Location> instance is not fully specified unless it falls within the scope of a DRM object specifying an SRF.

Example

1. If the SRF of an <Environment Root> instance is AUTM, no <Location> instances in the component tree of 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> instance specify its own SRF, independent of that of its <Environment Root> instance?

The griddedness of spatial positions is dependent on the properties of the SRF 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 SRF may not be regular in another SRF. Therefore, in order to preserve griddedness, a <Property Grid> instance specifies a SRF in which the data positions form a grid.

In addition, a <Property Grid> instance can exist without appearing in the component tree of any <Environment Root> instance (e.g., in the component tree of a <Data Table Library> instance).

Why does an <Image Anchor> instance specify its own SRF, independent of that of its <Environment Root> instance?

As with the griddedness of <Property Grid> instances, an <Image Anchor> instance specifies an SRF in which the anchor points specify the desired texture mapping, so that it is preserved without distortion.

In addition, an <Image Anchor> instance may be a component of an <Image> instance, in which case the <Image Anchor> instance does not appear in the component tree of any <Environment Root> instance.