GEO 580

• Edges -- The easiest analogy for an edge is an arc.  An edge is a feature that has some associated length, and through which a commodity flows (water, oil, electricity, people, and so forth).  Like an arc, an edge has two junctions (nodes), however, the intersection of two edges does not imply a junction -- for instance, in a coverage, the edge/arc representing a freeway overpass will cross over the road going underneath, however they do not intersect with one another. 

• Junctions -- Where an edge only has two junctions, a junction can have any number of edges.  A junction allows for the transfer of a commodity from one edge to another. 

• Edge-junction rule -- An edge-junction rule governs which edges can connect to which junctions.  For instance, a 240 volt edge can not connect to a 120 volt junction.
• Edge-edge rule -- An edge-edge rule governs how, and through what type of junction two edges can join.  For instance, the 240 volt edge can not meet directly with a 120 volt edge unless the edges meet through the proper volt adjusting junction.

Examples of weights (in transportation networks):

• Edge -- edge length / speed limit = travel time.  The travel time would represent the overall "cost" of traveling along that edge segment.
• Junction -- stop light wait time.  The average weight time could be used as the "cost" of traveling through that intersection.

Sources and Sinks:
     Sources and sinks are used in network analysis to establish flow direction in a network -- i.e., if the Edge C can only be traveled from junction A to junction B, or if junction A indicates the source of all flow, and so forth.

• Source -- The junction at which flow originates
• Sink -- The junction at which flow stops

• Network -- for selecting the network feature with which you wish to work
• Flow -- for turning on or off arrows indicating the flow of each edge in the network
• Analysis -- for enabling/disabling layers for analysis, clearing flags/barriers/results, or for modifying various options for the analysis (weights, etc.)
• -- for setting junction flags, junction barriers, edge flags, and edge barriers in the network
• Trace task -- for selecting the type of analysis that you wish to do 
  • Find common ancestors  -- find common features upstream from selected junction
    • Similar: Find disconnected 
  • Find connected -- find all of the edges and junctions that are connected to a selected feature
  • Find loops -- find all of the loops that make up paths between junctions
  • Find path -- find a path between two points
    • Similar: Find path upstream
  • Trace downstream -- find all of the network junctions and edges that lie downstream of a feature
    • Similar: Trace upstream
    • Find upstream accumulation 
• -- for "solving" network analysis

Note: Any modifications to a coverage must be done before the creation of the network.

     Examine the attribute table for the ushwy coverage.  Note that there are fields for speed limit (SPEED) and segment length (LENGTH_MIL) included in the table***.   These fields will be combined to create a weight field for our network. 

     First we need to create a new field in the table, so open the ushwy arc properties, and click on the Items tab.  Add a new field named "TIME_USED".  The field should be of type Float, have an input width of 4, display width of 12, and use 3 decimal places.  You do not need to include an alternate name.

     Now that the new field has been added, start ArcMap and add the ushwy coverage to the view. 

     Open the attribute table for ushwy, "turn on" the ArcMap editor toolbar, and select "Start Editing."
Right click on the TIME_USED field and select the field calculator.  Use the SPEED and LENGTH_MIL fields to calculate the value for TIME_USED.

     Once the calculation is complete, save your edits and stop editing.  You should now close ArcMap, you do not need to save the map file.

***SPEED and LENGTH_MIL do not reflect actual speed limits or length of edge features.  They have been invented for the purposes of lab analysis. 

Answer question 3: What formula did you enter into the field calculator to calculate the new TIME_USED value (write it as you entered it)?  Why do we want to use this value, rather than either SPEED or LENGTH_MIL alone?

• It may be difficult to create the network -- there seems to be a write lock problem in the 8.1 software (even if the file has not been used, it seems to be "in use").  If you have this problem, close all ArcInfo modules, then re-open them and try again.  If this doesn't work, restart and try again.

To create the actual highway network, we will use the Build Geometric Network Wizard in ArcToolbox.       Open ArcToolbox and navigate to this tool.        After reading the introductory screen, click on Next.  Use the highway feature dataset for your input feature dataset, and select the ushwy arcs as your feature class from which you want to build your network.  Enter an appropriate name for your new network.

     You do not want complex edges in your network,  the network features do not need to be snapped, we are not using sources and sinks, and we want to use weights (SPEED: type single, LENGTH: type single, and TIME: type single.  Do not fill in the bitgate size field.  To add a new weight, click on the  icon).

     Select next, and assign weights to the fields in the feature class as follows:
SPEED::Single -- SPEED
LENGTH::Single -- LENGTH_MIL
TIME::Single -- TIME_USED

     Finish the network building.

     If you refresh your geodatabase, you will see new icons representative of the newly created network.

• Display the highway edges (arcs) according to the segment speed limits, set the line width to 2.
• Add highway markers as labels .
• Add the usageog coverage as a backdrop for your network.
• Bring up the Utility Network Analysis toolbar.

Find path (part 1)

     We are now going to run some simulations with the network analyst.  We want to go see the Seattle Seahawks play the Atlanta Falcons -- but the game is in Atlanta, so we need to plot the path from Seattle to Atlanta.
 
 

We will consider the junction of I-5 and I-90 as representative of the city of Seattle, and the junction of I-85, I-75, and I-20 to represent Atlanta.

     To plot the path, we need to place flags at the "Start" junction (Seattle), and at the "End" junction (Atlanta).  The flags must be placed in the order in which stops will be made (i.e., Start -> End).

     Go ahead and place the Seattle and Atlanta junction flags.

     Select "Find Path" and the trace task, and click on the "solve" button. 
 

     Under "Edge weights" set both to-from and from-to weights to SPEED.  Since we do not have any junction weights, leave the junction weight as <None>.  Click on OK.  Now re-run the find path operation.

     Try calculating the path using each of the three weights that we set up (SPEED, TIME, LENGTH).

• I-94 through Minnesota will be closed between I-29 and I-35 (freeway lost under snow)
• The junction of I-70 and I-135 in Kansas will be closed

Side trips
     Unfortunately the official Seahawks Fan Roadtrip crew do not live in Seattle (the origin city), so the route must include the sidetrips to pick up our fellow fans (after all, it is for the 'hawks). 

     The cities in which we need to stop to pick up folks:

• Princeton, N.J. 
• Los Angeles, CA

• To clear your previously placed flags, click Analysis-->Clear Flags on the Network toolbar.

Answer question 7:  You have been hired to do network analysis to coordinate delivery for a local pizza parlor.  What data sets would you need/want, what would the datasets need to contain (i.e., what fields would you require for your analysis), and how would you set up the network for use? (This question requires a fairly detailed answer, spend a bit of time thinking about this one)