Community Profiles Method for modelling catchments (2008 revision) Published by the Victorian Government Department of Human Services Melbourne, Victoria © Copyright State of Victoria 2008 This publication is copyright, no part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968. Authorised by the State Government of Victoria, 50 Lonsdale Street, Melbourne. Table of Contents Introduction ......................................................................... 4 Communities ........................................................................ 4 Modelling the catchments of rural towns (and suburbs) ...... 5 Small towns near large centres ............................................ 5 Models for defining catchments............................................ 6 Trade area modelling ............................................................ 6 Population-weighted travel-time catchment ............................. 7 Simple travel-time catchment ................................................ 8 Voronoi polygons ................................................................. 8 Rules for defining catchments .............................................. 8 Appendix A – Communities with rural catchments ............. 10 Towns .............................................................................. 10 Suburbs............................................................................ 11 Appendix B – Communities without rural catchments ........ 12 Suburbs of Ballarat............................................................. 12 Suburbs of Bendigo ............................................................ 12 Suburbs of Geelong ............................................................ 12 Suburbs of Melbourne ......................................................... 12 Towns .............................................................................. 14 Introduction The Town and Community Profiles bring together information about Victorian communities from a variety of sources, both internal and external to the Department of Human Services. In the current release of the Town and Community Profiles, all data items have been reviewed, and have been updated where new data is available. In particular, revision of the profiles has provided the opportunity to incorporate 2006 Census data. The Profiles include information on population, geography, services, facilities and social and health characteristics of each suburb, town and catchment in Victoria. The key aim of the Profiles is to provide Human Service planners and service providers with an understanding of the characteristics of the communities they work with, so that they are better able to meet the needs of those communities. The Town and Community Profiles are a companion volume to the Local Government Area (LGA) Profiles, available to staff from the Department of Human Services via SMap on the Departmental Intranet, and to funded agency staff via the Funded Agency Channel. The range of data available at community level is more limited than that which is available at LGA level. The Town and Community Profile data was selected for its availability, quality and capacity to inform users of an interesting dimension of community life. Communities Communities are formed by aggregating Census Collection Districts. While communities in the 2008 issue have not been re-modelled, they have been updated according to the Census Collection Districts used for the 2006 Census. Communities in Victoria have been divided into four categories: Community Definition Suburb (S) A suburb of Melbourne, Geelong, Bendigo, or Ballarat, based on Australian Bureau of Statistics (ABS) State Suburbs. Town (T) A town with a population of more than 200, based on ABS Urban Centres/Localities. Small Town An individual Census Collection District which is not an ABS (X) Urban Centre/Locality, but which appears to represent a small town with a population of fewer than 200, and which could be considered a community in its own right. Rural A rural, semi-rural, or wilderness area of the State which is not catchment (R) part of a town (ABS Urban Centre/Locality). These areas have been modelled into generic service catchments for ABS Urban Centre/Localities; each rural area therefore relates to a town (or suburb of Melbourne) and takes its name. For example, rural areas that have been assessed as relating to Swan Hill would be named ‘Rural catchment of Swan Hill’. The remainder of this document focuses on rural catchments and how they have been modelled. 4 Modelling the catchments of rural towns (and suburbs) The definition of catchments for Victorian rural communities came about due to lack of knowledge about which towns people living in non-urban areas are likely to travel to, to access services. How many and what kind of people live within towns is easily accessible knowledge but less is known about where people outside towns go to. This information is necessary if services are to be planned effectively. Mapping catchments is one way of defining communities. The natural catchments defined here attempt to strike a balance between how long people are prepared to travel to a town, and the number of services available when they get there. Smaller towns have smaller catchments and larger towns, with a greater range of services, have larger catchments. Some towns, especially smaller ones of less than 200 people, do not have a catchment. The catchment is considered to consist of the town itself, with no non-urban areas forming a catchment. Some suburbs of Melbourne have catchments, as it is more likely that people living in these catchments will travel to the suburb, and access its services, than travel to the nearest town. Small towns near large centres Due to technical limitations of the software, it was necessary to remove some Urban Centres/Localities from the modelling to reduce the required calculations to a manageable size. Those chosen were those that were likely to be too close to a Regional or District Centre to have their own catchment, and which would probably be swamped by the Regional Centre or District Centre. The criteria used were as follows: a small community was removed if it was X less than ten minutes from a District Centre, X less than twenty minutes from a Regional Centre, or X less than half an hour from the CBD. Towns were removed from the modelling if they were: X less than five minutes from a District Centre, X less than ten minutes from a Regional Centre, or X less than half an hour from the CBD. There was some flexibility in that communities which were marginal were included rather than excluded. 5 Models for defining catchments Four different models have been developed to define catchments for Victorian rural communities. The table below outlines their respective roles. Model Focus Role Trade area model Major centres The trade area model defines a large outer boundary for very large communities; it does not attempt to define a catchment for every community in the State. Weighted travel time Mid-sized Weighted travel time catchments are catchment towns similar to trade area catchments, but with less of a focus on very large communities. This model bridges the gap between the trade area model and simple travel time catchments. Simple travel time Smaller towns This model looks at the distance from catchment each Census Collection District to each town. There is no weighting for population, so every town has an equal chance of being assigned a CD, regardless of how small it is. This prevents very small communities from being swamped by larger, distant communities. The simple travel time catchment model is the closest to the original concept of mapping catchments, and therefore is the default model. Voronoi polygons Smaller towns Voronoi polygons are a straight-line geographic measure, and have been included to smooth out irregularities in the road network. Trade area modelling Trade area modelling (also known as gravity modelling or Huff modelling) is well- known in retail planning. It assigns a probability to a Census Collection District based on the likelihood that its residents will travel to a particular town, based on: (i) the distance between the collection district and the town and (ii) the relative attractiveness of the town. (Population has been used as a proxy measure of attractiveness; an alternative would be to use something like total number of services present in the town. Use of counts of businesses was considered but the data was unavailable at a sufficiently small geographic area to be useful). The model attempts to find a balance between distance and attractiveness. For example, a collection district could be quite close to a small community but a large District Centre could be just a little bit further away, and the model might calculate that there is a 35% probability that residents of the collection district will travel to the small community, but a 40% probability that they would go the extra distance into the District Centre, and therefore access a greater range of services. The collection district would therefore be assigned to the District Centre, 6 because there is the greatest probability of the collection district’s residents going to the District Centre. The model can be expressed as an equation, known as the Huff equation: ac where p= Huff probability d t a= attractiveness of town p= ac c= attractiveness coefficient town 1-n t= travel time between points Σ td d= distance decay coefficient The attractiveness