An attribute held against each contourable string which governs whether the string is to be used as a boundary of model formation.
Boundary strings must be closed. i.e. form a polygon.
A data base may contain numerous different boundaries. This enables any part or parts of the data base to be excluded or included in model formation as required.
For example:
- a single boundary in the data base would exclude from the model all data outside that boundary.
- multiple but separated boundaries in a data base result in island of modelled data separated by areas that have not been modelled.
- a boundary enclosed within another boundary defines a "lake" area not to be modelled.
- a boundary enclosed within two other boundaries defines an "island" to be modelled, within a "lake" not to be modelled, within a surrounding area to be modelled.