ET Network

The Network module of ET GeoWizards allows to build networks and to perform network analysis.

What is Network

GIS networks are networks composed of linear features (generally called edges or links) that can be used to represent interconnected features (roads, rivers, etc.), and to perform special spatial analysis on them.The edges are connected in junction points (nodes).

• Based on the way an agent can move along the networks they can be classified as:
• Directed Networks - the agent (for example water) can move only in one direction along the edges
• Undirected Networks - the agent (for example cars on the roads) can travel in both directions of the edges (roads)
• Based on the graph they can be modeled with the networks can be classified as
• Planar - can be drawn in a plane so that no edge cross (for example rivers).
• Non-Planar. These graphs cannot be drawn in a plane so that no edges cross (for example roads).

Type of Networks in ET GeoWizards

• Single Directional Networks (SDN). This network is modeled as a Directed acyclic graph . There is no way to start from node A go trough a sequence of links and nodes and reach back node A.
• The SDN is generally planar, but it can have non-planar elements that can be modeled with attributes as for the BDN (see Network Attributes below)
• The directions of the edges can be maintained as the physical directions of the source polylines or using attributes (see Network Attributes below).
• The loops should be avoided.The Check for Loops function can be used to find the location of loops.
• A typical river with an island. The flow direction in both channels is the same (from Node 1 to Node 2), so no loop is present

• The transported agent travels from node 1 to Node 2 then returns from Node 2 to Node 1 - this is a loop. The source data for the network should be corrected.

• Bidirectional Networks (BDN). These networks are Undirected - the agent (for example cars on the roads) can travel in both directions of the edges (roads)
• The BDN is generally non-planar. In a road network for example, most intersections happen on the same level( we need to have nodes at all these intersections). Some of the edges however cross at different levels (over & under passes on the highways for example). There are 2 ways to handle such cases:
• do not intersect the crossing roads (do not create a node at the intersection)

• Create a node at the intersection and handle the intersection with elevation attributes (see Network Attributes below).


Each of the above approaches has advantages and disadvantages, but the second method is more appealing because it needs proper topological relations between the features of the source data which can be accomplished for the entire dataset at once.

• Most of the edges in a BDN are bidirectional (travel in both directions is allowed). There however are some edges (highways represented by 2 lines - one in each direction, on & off ramps, etc.) where the restrictions should be applied to some of the edges. This can be handled by using "Direction" attribute (see Network Attributes below).

Modeling Networks in ET GeoWizards

The sources for GIS networks are normally polyline feature layers. In order to prepare the source polyline layer for network analysis, we must ensure topological correctness of the input data

• Use Clean Polyline function to create nodes at all intersections, remove duplicate polylines, fix small inconsistencies in the input polyline dataset.
• Use Clean Dangling Nodes function to clean all dangling polylines.
• Use Export Nodes function to verify that all the polylines are correctly connected.

Network attributes

We need to set values in the attribute table of the input polyline layer.

Cost Field

The values in the Cost Field will be used for cost (length, time, etc.) for each link. If not specified the geometric length of the polylines will be used. ET GeoWizards allows 2 different fields to be used for the different directions of Bidirectional Networks (BDN).

Direction Field

Single Directional Networks use the physical directions of the input polylines, so they do not need this attribute. For Bidirectional Network (BDN) we need to set direction attribute, that will indicate how the edges can be traversed. Since all polylines have a direction, the values should be set according to the direction of the polylines. The values that can be used are:

• "FT" (meaning From => To ) - traveling only along the direction of the polyline is allowed.
• "FT" (meaning To => From ) - traveling only against the direction of the polyline is allowed.
• "N" - No traveling is allowed.
• "B" or all other strings mean that traveling in both directions is allowed.

Below is a small example of the "Direction" attribute". Note that the example is for Left hand side of the road driving. For Right Side of the road driving the links with FT should get TF and the ones with TF should get FT

Elevation Fields

A couple of numeric (recommended integer) fields which values will define the elevation of the Start and End point for each edge. These values will be used to model under and over passes.At the beginning (since most of the polylines will intersect at the same level) all values in these fields can be calculated to 1 and then change the values only in cases of over or under passes

Below is a small example of the "From_Level" (the level of the FROM node) and "To_Level" (the level of the TO node) attribute".

Illustration how the above works for the network connectivity.

Building a Network

After you have modeled your network dataset, you just need to run one of the functions below (depending on what type of network you want to create). The network is stored in a single file with extension .etn (for example MyFirstNetwork.etn) in the location you specified. You can use this file as input in all Network Analysis functions of ET GeoWizards.

• Create Bidirectional Network
• Create Single Directional Network