Calculate Change in Accessibility

See also

• MMA Process Fundamentals - Compare Scenarios

This is the geoprocessing interface for calculating the change in accessibility between two tables containing accessibility summaries for a set of zones under two alternative conditions or scenarios.

To understand how accessibility is modified by a project altering the transportation system and/or land uses, for example, provide a no build table reflecting baseline accessibility scores and a build table reflecting new accessibility scores assuming the project is implemented. For each zone affected by the project, subtract the no build scores from the build scores to determine the changes wrought by the project.

No build table : ArcGIS Table or Table View

The table containing accessibility scores for a set of zones in the “no build” or “baseline” condition.

Build table : ArcGIS Table or Table View

The table containing accessibility scores for a set of zones (matching those in the no build table) in the “build” or “change” condition.

ID field : Field

The field - present in both the no build table and the build table - containing zone IDs allowing records in the two tables to be related to each other.

Accessibility fields : [Field,…]

The fields containing accessibility scores for which the differences between the build and no build conditions will be calculated and stored in the output table. Field names must be the same in both input tables and will carry over to the output table.

Output table : ArcGIS Table

The output table storing the differences between the build table and the no build table.

gp_calcChangeInAccessibility.tableDifference(table_1, table_2, id_field, diff_fields, output_table, skip_nulls=True, null_value=0)

Given two tables of identical structure and similar content, calculate the differences (table_2 minus table_1) between values in a selection of fields for records identified by a common ID value in each table.

This function assumes two tables of identical structure and similar content as shown in the example below. They each have an ID field with at least some common ID values in both tables. They also share common numerical fields, for which the analyst wants to know the differences between values in each table for each distinct ID value. If an ID value is present in one table and not the other, it will be treated based on the provided null_value (default is 0).

Example table_1:

ID	Field 1	Field 2
1	1000	900
2	2000	600
9	3000	300

Example table_2:

ID	Field 1	Field 2
1	1200	750
2	2100	1019
3	3500	25

Example result table (table_2 minus table_1):

ID	Field 1	Field 2	notes
1	200	-150	presnt in both tables
2	100	419	present in both tables
3	3500	25	present in table_2, not table_1
9	-3000	-300	present in table_1, not table_2

Parameters:

• table_1 (ArcGIS Table or TableView) – A table that organizes data by distinct values in an id_field and containing measures in one or more numerical fields.
• table_2 (ArcGIS Table or Table View) – A second table identical in structure to table_1 with similar distinct values in the id_field and different values for measures, representing an alternative condition or scenario, e.g.
• id_field (String) – The field in table_1 that is also in table_2 that uniquely and consistently identifies records such that each table’s rows can be directly joined and compared to each other.
• diff_fields ([String,..]) – The list of numerical fields in table_1 that are also in table_2, representing measures for which the differences between the tables are to be calculated (by distinct values in id_field).
• output_table (String) – The full path to the output table storing the differences (table_2 minus table_1).
• skip_nulls (Boolean, optional) – If True, null (missing) values in each table will not be considered in the calculated of differences between table_1 and table_2. If False, null (missing) values in each table will be included in the difference calculation and
• null_value (Float, optional) – The value to assume whenever a null (missing) value is found in a table. Default is 0. Onyl applies if skip_nulls is False.

Returns:

Writes an output table with the differences between table_1 and table_2 .

Return type:

None