sdr_calculate_prweighted_population: Calculates the total probability weighted population for a...

In station-demand-forecasting-tool/sdft: Model to forecast demand for new local railway stations

Description

Calculates the total probability weighted population for a station by weighting the population of each postcode by the choice probability for the station and a distance decay function and then summing across all postcodes. In addition, takes account of population data in the exogenous inputs table.

Usage

sdr_calculate_prweighted_population(con, schema, crs, tablesuffix)

Arguments

con	An RPostgres database connection object.
schema	Character, the database schema name.
crs	Character, the crscode of the station.
tablesuffix	Character, the suffix of the probability table - either crscode (isolation) or 'concurrent' (concurrent) is expected.

Details

The calculation is different depending on whether the concurrent or isolation method is to be used. For concurrent treatment there is only a single probability table and a spatial query is used so that only those postcodes within the 60 minute service area of an individual proposed station are included. This is because a single merged service area (within 60 minutes of all the stations) was used to generate the postcodes in the probability table. But only those within 60 minutes of each proposed station should be considered when generating the weighted population. It is also necessary to only include the exogenous population for postcodes that fall within a station's 60-minute service area.

There are three CTE queries involved. The first (nw_pop) is population weighted just by probability when the distance to a station is <= 750m. The second (w_pop) is population weighted by probability AND by distance decay function. The third (adj_pop) gets the probability weighted population based on the exogenous data table, applying the decay function for access distances > 750m.

For stations treated in isolation the FROM table for nw_pop and w_pop is the probability table for that station and census postcode population is joined by a left join, so only relevant postcodes will ever be included. In the case of adj_pop the from table is the exogenous table and the probability is left joined from the probability table. This will be null if the postcode is not present in the probability table and it will not in those circumstances contribute to the sum - as in SQL, NULL times anything is NULL.

station-demand-forecasting-tool/sdft documentation built on July 11, 2021, 4:23 a.m.