Net Density (Regions)

Introduction

Nett Dwelling Density is the ratio of the number of dwellings over area of land utilised as residential land within a neighbourhood.

Inputs

Open the tool (Tools → Walkability → Net Density) and enter the parameters as shown in the image below (these are outlined underneath the image)

[Click to Enlarge]

[Click to Enlarge]

  • Name: Output name of the resulting polygon layer.
  • Neighbourhood: Polygon data set representing.
  • Land parcel type data set: Polygon data set representing cadastral parcels to constrain the extent of land to be classified as residential.
  • Residential points data set: Point data set representing residential features in the study area.

Outputs

The output of the Neighbourhood Generator tool is a polygon data set, which will contain the attributes of the input Neighbourhood data set and the following attributes:

Additional Information

Background

Christian et al (2011) from the Centre for the Built Environment and Health devised a method of utilising rateable features (data from Western Australian Valuer General’s Office) and to extract the residential dwelling features to perform a point in polygon analysis to identify cadastral parcels that could be considered residential land. This process returns the count of residential features over the area of cadastral parcels (with duplicate parcels and overlaps eliminated) to provide the net dwelling value. This process relies on a look up table to identify a subset of rateable features which qualify as residential (from 1000+ feature types down to ~30). A spatial selection process is used to select cadastral parcels which intersect (wholly or partly) a participant’s neighbourhood. These parcels are then used to select residential features, which are both used in the calculation of the metric.

Data Sources

The source of residential features is the Valuer General in the respective state. The Valuer General’s Office will usually hold a database of sold and/or rateable features which the Local Government Authorities are charging rates on. In Western Australia these features are stored in the Val Sys database. The features should contain attributes that identify their type, which can be used to filter the residential features from others. In the Val Sys database there is an integer field (“Classification_Code”) and a text code which distinguish the types very specifically (about 1000 types). These fields can be used in conjunction with a look up table to help subset residential features.

For a simpler approach, but with a loss of specificity, the Valuer General’s data set may also contain an attribute that broadly categorises features and can be used without the need for a look up table. In the Val Sys database there are two such attributes; “Land_Use_CFMIR” and “Property_Use_VCFMIR”, with the “R” in the suffix equalling residential.

Cadastral parcel data is available from state government land data agencies for their jurisdictions, with national coverage available from PSMA. However, when examining urban fringe or recently redeveloped urban areas it is important to obtain cadastre and residential features that are of the same era. otherwise erroneous residential areas can be reported.

It may also be possible to capture residential features through aerial photography interpretation or field based data capture.

Analysis Process

This process assumes that the base data sets (service areas, cadastre and residential features) have proper attribution. The user may need to use the Tabular Join tool to join a look up table to the Valuer General’s features in order to identify and filter the residential features from the other feature types.

This is a spatial over process, where for each neighbourhood polygon being analysed the following operations are performed:

  1. Select all cadastral parcels touching the neighbourhood
  2. Select all residential features touching the selected cadastral parcels;
  3. Remove from the selected parcels in step 1 any cadastral parcels that do not touch a residential feature;
  4. Dissolve the remaining cadastral parcels to remove any overlaps in parcels.
  5. Write the summed area of dissolved polygons to the output attribute table
  6. Add an attribute of the count of selected residential features to the output attribute table
  7. Calculate a ratio of the count of residential features over the area (in hectares) of dissolved cadastral parcels
Process of Identifying Residential Land for the Nett Dwelling Density.  Image to the Left shows residential features identified from the Western Australian Valuer General’s database, which is then filtered to the extent of a neighbourhood and subsequently used to spatially filter cadastral parcels.  The image at right shows the final extent of land considered residential.

Process of Identifying Residential Land for the Nett Dwelling Density. Image to the Left shows residential features identified from the Western Australian Valuer General’s database, which is then filtered to the extent of a neighbourhood and subsequently used to spatially filter cadastral parcels. The image at right shows the final extent of land considered residential.