Monday, September 15, 2008

Creating a Feature Class of Pairs of Intersecting streets from a street centerline network

Step 1: We start with a set of features of Feature Class STREET_SEGMENTS (total 3793 features)

  • Each street segment consists of one continuous line feature and has the street segment name and a polarity (a way of showing that one end is the start and the other end the end of the line).
  • Different segments could have the same segment name
    * The segment name is actually the street name and has the following components
    a. Direction (N, S, W, E or Null)
    b. Street_Name
    c. Street_Type
  • Use Functional Attribute to generate a single string out of the three components. One such Functional Attribute would be to CONCATENATE the three fields together with a space and treated the Null direction as the empty string ‘’.

Step 2: Apply Analytic Merge to STREET_SEGMENTS to merge by attribute Segment Name to get STREETS. This will ensure that each Street Feature has a unique name (843 Features)

Step 3: Output Feature Class of the results of the STREETS query to a Geomedia Warehouse calling it MILPITAS_STREETS. While we could/should work with the query STREETS, at some point we will need to have an ID for referring to the streets, and writing out the Feature assigns an ID field.

Step 4: Apply Spatial Intersection of MILPITAS_STREETS that touch MILPITAS_STREETS to get intersections of all the Streets. (3565 features). The result will be a set of point and line features.

Step 5: Apply Attribute Query to the result from Step 4 with the Filter being ID – ID1 <> 0. This will remove features which consist of streets that are spatially equal to themselves. Details on why ID <>ID1 as a filter does not work is given in Identifying duplicate Spatial Features in GeoMedia (There are 2722 features which is 3565-843 as expected)

Step 5 is a query that produces a feature class where the every intersection appears at least twice, in different orders of the street names creating the intersection. In the case of junctions where 3 streets meet, there will be 6 features with the same intersection point, and in the case of 4 streets, there will be 12 features with the same intersection point. A illustrative example is the intersection of N and S Main Street and E and W Calaveras Blvd.


Step 5 produces a feature class that may then be modified for other purposes.

Suggested Modification 1: Eliminate duplicate combinations of cross streets so that each intersecting pair of streets appears once. One way of doing this may be to apply an Attribute Query to the Step 5 results with ID - ID1 > 0

Suggested Modification 2: By applying an Analytical Merge of the results of 5, and counting how many intersection point create each result, it may be possible to rename the results where the count is greater than 2. For instance, the case of N and S Main St intersecting E and W Calaveras Blvd may be considered Calaveras Blvd crossing Main St.

Application and Business Requirements:

The choice of modifications is in the realm of business requirements that determine how the data is to be used, and how cross streets should be defined. One application is to describe traffic accidents, and in that case if the order in which the street names are stated is not important, Modification 1 may be adequate for the purpose.

A second application is to create a street index of all streets cross each street. In this case, the data from Step 5 may be exported to an EXCEL spreadsheet and then manipulated.

1 comment:

Anonymous said...

Keep it up, looks great.