In practice, holding trip productions constant is used in balancing trips rather than holding attractions constant. This is because the number of productions in a TAZ is usually strongly related to zonal demographics. On the other hand, there are many factors that might affect the number of trips attracted to a TAZ. It is more difficult to find good explanatory variables to estimate attraction models. For this reason, productions are typically considered more valid and thus are held constant.
Modeling Concepts: Joining Tables
With trip balancing, the TAZ dataview is extended: a new table is appended to the end of the original TAZ table. This new table is the result of the trip balancing that was saved as trips.bin. This file itself also contains a field calLED ID, which stores IDs of the "balanced" TAZs. transcad joined the TAZ dataview with trips.bin by matching the "balanced" TAZ ID to the original TAZ ID. The joined tables are displayed in another dataview window labeLED as TAZ+TRIPS and the ID of the "balanced" TAZ is named ID1 to distinguish it from the original TAZ ID.
Dataview file: If the number of balanced trips is needed (future model tasks will require this), save the TAZ+TRIPS dataview as a dataview file (*.dvw). A *.dvw file is similar to a *.map file and contains the settings (e.g., what tables formed the joined view) of the dataview, not the actual data. For example, if trips.bin is deleted, the balanced.dvw still exists, but it can not be openend directly.
Drop join: When two tables are joined, neither can be updated. For example, if trip balancing
