Using groupings of centroids to split zones
Technical Note #28
By Dimitris Triantafyllos and Paolo Rinelli
Did you know that you can use Groupings of Centroids to split zones? The following workflow shows how to split a centroid into multiple centroids and distribute the trips generated and attracted with user-defined proportions.
A. NETWORK PREPARATION
First, use the Table View to ensure that all Centroids have been given different External ID values so that you can paste the trip values from the original OD Matrix into the one with split zones.
Figure 1. Verifying that all centroids have External IDs assigned
Duplicate the existing Centroid Configuration (“Eixample Barcelona – original”, see Figure 2) by dragging and dropping it into the Centroid Configurations folder. Once the new centroid configuration is duplicated, rename it (“Eixample Barcelona – split zones”, see Figure 3) and activate it by right-clicking on it and selecting Activate.
Figure 2. Duplicating and activating the split zone centroid configuration
B. SPLIT ONE ZONE INTO MULTIPLE ZONES
To split one centroid into multiple centroids, follow these steps:
- Create one new Centroid for each child zone
- Connect them from/to nodes and/or sections
- Set the same External ID as the parent Centroid for all of the child Centroids
- Delete the parent Centroid
Figure 3. Creating new child centroids and setting the same External ID as the parent centroid
C. CREATING GROUPING WITH NEW CENTROIDS
Create a new Grouping Category for Centroids. For each zone that you have split, create a Grouping and add to it all its child Centroids. Set the External ID of the grouping equal to the External ID of the parent Centroid so that you can paste trips later.
Figure 4. Grouping centroids and setting the same External ID
D. SPLIT PROPORTIONS
To set the split proportions between child Centroids of the trips generated and attracted by the parent Centroid, follow these steps:
- Open each OD Matrix in the split Centroid Configuration (AM Peak – split zones in Figure 5) and delete all trips (hint: use the Multiply operation to multiply all cells by 0).
- Order the cells by External ID (by setting Headers: External ID) so that the centroids that have been split are adjacent, and fill each set of rows and columns with numbers representing a split proportion (e.g. values from 1 to 100, adding up to 100 for each set of rows and columns). For example, to set that in 30% of the trips generated by the parent centroid should be generated by the first child, 20% by the second and 50% by the third (see Figure 5), put 30 in all cells of the line corresponding to the first child, 20 in those in the line corresponding to the second child and 50 in those in the line corresponding to the third child (hint: select the line and use the Add operation applied to a selection of cells, including empty cells); repeat for the columns to set the proportions of attracted trips.
- Leave the cells representing trips between the split centroids empty, as they correspond to intrazonal trips in the parent matrix (a cell on the diagonal); since they are generally null in the original matrix, you will have to fill in these trips at the end.
Figure 5. New OD matrix with departure and arrival percentages to split parent centroid
E. COPY TRIPS FROM OLD MATRIX
Open the original matrix (AM Peak – original matrix in Figure 6) and set Headers: External ID and Grouping: None. Select all the matrix cells (Ctrl+A) and copy (right-click) the values.
Figure 6. Selecting and copying all cells of the OD matrix of the original centroid configuration
F. PASTE TRIPS TO NEW MATRIX
Open the split matrix (AM Peak – split zones in the figure below) and set Headers: External ID and Groupings: Centroids. Click on the first cell, right-click and select Paste.
Figure 7. Pasting the values to the OD matrix of the split zone centroid configuration
When pasting, Aimsun looks in the target OD Matrix for a Centroid (for the zones that haven’t been split) or Grouping of Centroids (for the zones that have been split) with the same External ID of the original Centroid, hence the importance of setting unique External IDs at the beginning, and of setting the External ID of the parent Centroid in the Grouping.
Moreover, when pasting a value in a grouped cell, Aimsun splits it among the cells that belong to the group by calculating a proportion based on their current values, so the values you introduced in step D have the effect of split proportions.
To be sure that process was succesful, check that the total trips of the two OD Matrices match, and also that the total generated and attracted by the split Centroid is equal to the total generated and attracted by its corresponding Grouping of Centroids.
Figure 8. Comparing the total trips of the original and split OD matrix.