2.11 Toolbox Documentation Return to Table of Contents

Non-Existent Links, FT & TF,
and Other Important Concepts

Important differences between ArcInfo and EMME/2

First, a bit of terminology. A segment of street in ArcInfo is represented by an arc, while in EMME/2, the same segment is called a link. For our purposes, "arc" and "link" are interchangeable. (Confusingly, ArcInfo also has a feature type called a link, but these are not used by the 2.11 Toolbox, and are not discussed in this documentation.)

There are several important differences between ArcInfo and EMME/2. In ArcInfo, a street is represented by a single arc, regardless of whether it is a one-way or two-way street. In EMME/2, a one-way street is represented by a single link, but a two-way street is represented by a two distinct links rather than one, each representing the traffic flow in opposite directions. EMME/2 really models a two-way street as a pair of one-way streets.

Links and arcs have one thing in common -- they both have directionality. That is, links and arcs both have a from-node and a to-node. (This can be seen in ArcInfo by drawing an arc coverage by setting the draw environment to include "arc arrows".) In the ArcInfo diagram below, A is the from-node, and B is the to-node. In the EMME/2 diagram, A and B are actually both from-nodes and to-nodes because there are two links running in opposite directions to one another.

FT and TF

In order to represent two sets of link attributes on a single ArcInfo arc, the Toolbox uses a scheme that stores these attributes in relation to the arc's directional arrow. Those attributes representing the direction "along" the direction of the arrow can be said to represent the direction from the from-node, to the to-node, or ft for short. Likewise, those attributes running "against" the arrow can be referred to as tf. If you examine the Base Network's Arc Attribute Table, you will see "ft" and "tf" appended to the various EMME/2 link characteristics.

ArcInfo Arc Model EMME/2 Link Model

This concept of along and against is used throughout the 2.11 Toolbox to assign and manipulate link attributes. On the Link Attributes menu, for example, the icons and are used to view and edit attributes "along" and "against" a project link.

Non-Existent Links

Another important difference between ArcInfo and EMME/2 is that in ArcInfo, since a single link is used to represent both directions of travel along a link, in order to represent a one-way street, one of the directions must be marked as not existing. EMME/2 has no similar concept because if a street is one-way, it is simply represented by a single link, with no link for the opposing flow, as there would be for a two-way street.

In order to represent non-existent links, the 2.11 Toolbox flags a link with a user-definable code. In one implementation, a non-existent link could be flagged by setting the Link Type to 99. Alternatively, if one of the user attributes is used to store link capacity, then that attribute could be set to 0 on non-existent links. The ideal attribute and value for this flag is dependent on your particular EMME/2 model.

Non-existent links are used in another situation as well: When you create a project that represents a new road or transportation link. Since projects are based on ArcInfo's dynamic segmentation, in order for a project to be defined, a corresponding link in the Base Network must exist. If the project represents a new road, then the associated links in the Base Network should be flagged as being non-existent.

When you build an EMME/2 network, links and link-directions flagged as non-existent are simply dropped from the resulting EMME/2 network.


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