|
The SEDRIS Data Representation Model
APPENDIX A - Classes Edge Direction |
|---|
An instance of this DRM class specifies, for the associated relationship, whether the target edge should be traversed forwards (from start node to end node) or backwards (in the opposite direction).
A <Linear Feature> instance representing a stream is associated with several <Feature Edge> instances that define the path of the stream. The <Edge Direction> link object associated with each <Feature Edge> instance has forwards = SE_TRUE if the <Feature Edge> instance is oriented so that the stream flows in the direction from its starting node to its ending node, and SE_FALSE if the stream flows in the opposite direction.
Consider a <Linear Feature> instance representing a one-way street. Its path is defined by a <Feature Edge> instance, so that the <Edge Direction> link object between the <Linear Feature> instance and the <Feature Edge> instance indicates whether the orientation of the <Feature Edge> instance matches the permitted direction of travel along the one-way street.
The external <Feature Face Ring> instance of a regular <Feature Face> instance consists of three <Feature Edge> instances. In traversing the ring, the first <Feature Edge> instance might be traversed from starting node to ending node, so that its <Edge Direction> link object has forwards = SE_TRUE If the next <Feature Edge> instance was traversed from its ending node (which in this example is the same <Feature Node> instance as the ending node of the previous <Feature Edge> instance) to its starting node, its <Edge Direction> link object has forwards = SE_FALSE (that is, backward).
In such cases, the forwards field of the <Edge Direction> link object shall be set to SE_FALSE.
While this information can be derived by comparing the starting and ending nodes of consecutive edges within the face ring, it is provided as a convenience and as a validity check.
If the value of the forwards field is SE_TRUE, the edge is oriented in the same direction as the object with which it is associated, so that it should be traversed from its starting node to its ending node.
If the value of the forwards field is SE_FALSE, either the orientation of the edge is the opposite of that of the associated object, or the associated object has no clearly defined orientation. In this case, the edge should be traversed from ending node to starting node.
|