Elevation fields for planar lines

This document was published with and applies to ArcGIS 9.3.
A 9.2 version also exists.
Summary In a network dataset, elevation fields model the logical stacking of roadways. Streets at ground level have an elevation value of 0, a bridge has an elevation of 1, and a tunnel has an elevation of –1. Elevation fields in a network dataset have no relation to terrain elevations.

What elevation fields for planar lines provide

Most commercial street data contains planar line features and uses elevation fields to separate traffic flows at bridges, overpasses, tunnels, and interchanges. For workflow efficiencies, street data is often kept planar. The network dataset uses elevations to refine the connectivity model so that traffic cannot turn from one roadway to another at different elevation levels.
Elevation is relative to ground level. The vast majority of edges and junctions have an elevation of 0, representing ground level. The raised ends of a simple bridge or overpass have an elevation of 1. A highway interchange typically has levels 0–3. A simple tunnel has an elevation of –1 at the lowered ends of the tunnel line.
When you create a network dataset, ArcGIS searches for any existing field that has a name indicating it is an elevation field. Some field names recognized from earlier network software are FNODE_ELEV and TNODE_ELEV, F_ELEV and T_ELEV, and F_ZLEV and T_ZLEV. You can also specify another field for elevation values. Elevation values must be correctly populated before you build a network dataset.
Elevation fields refine the connectivity where edge features join. They do not override the connectivity policies for edge sources but rather enhance network datasets to allow modeling of overpasses and tunnels. Each source in a network dataset does not need an elevation field for the network dataset to support elevation fields. Elevation fields are not added to sources that do not contain them.
Elevation fields respect connectivity groups; coincident endpoints of lines with the same elevation field value will only connect if they are in the same connectivity group. Elevation fields are used if all features have a valid elevation value at a given location. If any features at a location have a null elevation field value, all elevation field values are ignored. Midspan vertices on edge features do not have elevation values and are treated as having null elevation values.
In the following illustration, overpass lines l1 and l3 have elevations of 1 at the ends adjacent to the overpass crossing and elevations of 0 at the other ends, where the roadway meets ground level. Underpass lines l2 and l4 have elevations of 0 at both ends. At the overpass junction, two system junctions are created, one with an elevation of 0 and the other with an elevation of 1.
The following illustration shows a map with one-way flow direction and elevations inside a highway interchange. Despite the complexity of all the crossing roadways, four elevation values are sufficient to fully describe the connectivity of this intersection.

See Also:

How to create a network dataset
About network connectivity