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## The Horizon Schema

The metadata that make up the Horizon Schema are described in the next three subsections. In Appendix A.2.1, dataset-level metadata are presented; these are names that might be returned by a `Viewable`'s `getMetadata()` method. Appendix A.2.2 lists metadata related to a dataset's world coordinate system. These metadata would normally be sub-metadata of the CoordinateSystem metadatum, making up part of the dataset level metadata; they would also be returned as top-level metadata (i.e. not sub-metadata) by a `CoordinateSystem`'s `getMetadata()` method. Finally, Appendix A.2.3 gives the metadata related to a single coordinate axis. These metadata would normally appear as sub-metadata of each element of the Axes metadatum.

The Horizon schema uses the convention that names begin with a lower-case letter unless they are of either type `Metadata` or type `Metavector`. The capitalization of the first letter therefore indicates that the metadatum contains sub-metadata. For example, the CoordinateSystem metadatum is of type `Metadata` which can contain the sub-metadatum CoordinateSystem.name. The Axes metadatum is of type `Metavector`; it may therefore contain the sub-metedatum Axes[1].type.

- Viewable Metadata
- Coordinate System Metadata
- Coordinate Axis Metadata