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 wait 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 asmnt_st coverage.  Note that there is field for average speed limit (Speed-Limi) ***.   You will create a Length_Miles field based on the LENGTH field. These two fields will be combined to create a weight field describing the length of time it takes to traverse each segment of the network. 

     First we need to create two new fields in the table, so open asmnt_st in ArcMap.

Open its table, Click Options and add a new field named "Time_Used", and a field "Length_Miles".  Both fields should be of type Float, and use default settings. (You might need to close out of ArcCatalog first)

First use the field calculator (Calculate values) to convert the meters values found in LENGTH into units of miles in the Length_Miles field. (Remember to right click on the field name to bring it up, the conversion is ~1609.344 meters per mile)

     Now right click on the Time_Used field and select the field calculator.  Use the SPEED_LIMI and Length_Miles 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-LIMI does not reflect actual speed limit.  It has 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_Limi or Length_Miles alone?

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

All other students, PRESS ON!
 

To create the actual highway network, we will use the Build Geometric Network Wizard..   Right click on your feature dataset and select new-> Geometric Network       After reading the introductory screen, click on Next.  Click Next again to build a network from existing features. Select the asmnt_st arcs (or whatever you named it) 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, and we want to use weights (Speed_limi: type integer, 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_Limi::Integer -- Speed_Limi
LENGTH::Single -- Length_Miles (or as you named this field above)
TIME::Single -- Time_Used (or as you named this field above)

Select next until you finish

Note - If you did not define your fields with the correct type (i.e. integer, float, etc..) you may have trouble with this step.

     Finish the network building.

     If you bring up your geodatabase again in ArcCatalog, you will see new icons representative of the newly created network.

• Display the highway edges (arcs) according to the segment speed limits (unique values symbology).
  
• Add highway markers as labels . This is a good time to explore the label placement options available in the labels tab of the layers properties. - Try limiting the number of markers to interstates with SQL and set the Label Placement Options to display the symbols on top of the roads, and horizontal to the page, not the lines they are based on.

• Add the pnwstates shapefile 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.  You are now the proud owner of the Pacific Northwest Hazardous Waste Delivery Service. Your fleet of trucks needs careful planning to ship deadly toxins from municipal HazMat sites to regional centers. In our first case, a research hospital in Seattle has a large supply of slightly radioactive heavy metals from their new x-ray laboratory. You need to determine the most efficient route to transport the material to the regional center located in Ogden, Utah..
 
 

We will consider the junction of I-5 and I-90 as representative of our location in the city of Seattle, and the junction of I-15, I-84 just south of Ogden as the nearest point to our hazardous waste facility.

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

     Go ahead and place the Seattle and Ogden junction flags.

     Select "Find Path" as the trace task, and click on the "solve" button.  . The results might be difficult to see unless your roads are not too coarsely displayed.
 

     Under "Edge weights" set both to-from and from-to weights to Speed_Limi.  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_LIMI, TIME, LENGTH).

• To represent this, use the suitable flag from below to create barriers along all the road segments entering Oregon from the North.
• To add these barriers, use the  Edge barrier flag, and the  Junction barrier flag.  These flags are placed in the same way as the junction and edge flags.

Side trips
     Unfortunately our trip is more complicated than this. We need to retrieve waste from not only Seattle, but Eugene, Spokane, and Billings before stopping in Ogden. Fortunately, ODOT (Oregon Department of Transportation) has agreed to allow our vehicles to utilize the Oregon roads as necessary. Clear the barriers using the analysis menu in the newtwork toolbar.

       Create a multi-stop path (or a path that must go through certain junctions) - simply place a junction flag on the junctions to be visited -- remember that the flags must be placed in the order that the stops will be made. 

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