WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology - the Local Government perspective

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology – the survey 1.Approximately how many tables do you have in your GIS database? 2.Approximately how many were spatially constructed based on the cadastre? 3.Approximately how many are spatially referenced from aerial photography? 4.Approximately how many are spatially absolute (surveyed or by GPS)? 5.Please comment on the importance of maintaining spatial relativity. How much of a problem is it if your “home-built” tables don’t exactly coincide with the cadastre? 6. Do you currently check for slivers or overlaps with every cadastral update? Do you redefine points, lines and polygons to maintain relativity?

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology – the results Digitised from Absolute LGA No Cadastre Photography (eg GPS) A few Most Many Most

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP IMPORTANCE OF SPATIAL CONSISTENCY … little importance at this stage as all information is for reference only. If we require more accurate information then a survey is requested

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP 2250 A few Most ? IMPORTANCE OF SPATIAL CONSISTENCY … important for users of TPS data as we derive data based on spatial constraints.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP IMPORTANCE OF SPATIAL CONSISTENCY This is a big problem mainly with Planning schemes and the like but it occurs with any table that uses the cadastre at some point in time as its base.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP IMPORTANCE OF SPATIAL CONSISTENCY Due to the spatial queries within our GIS, if the layers don’t coincide closely with the cadastre, then incorrect/ambiguous details can potentially be displayed, so it is very important that accurate spatial relativity is maintained. Our corporate GIS viewer allows for an approximate tolerance for non-coincident boundaries, however it is difficult to find a “perfect” tolerance due to the differing alignments across the entire municipal area.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP IMPORTANCE OF SPATIAL CONSISTENCY Minor at this stage of development. Potentially major effect on future automated business procedures and results of spatial queries to public via Internet.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPS IMPORTANCEIP Many Most4280? IMPORTANCE OF SPATIAL CONSISTENCY Issues sometimes occur when doing a spatial query from our GIS. The query may sometimes return the wrong or multiple zonings, rcoding, ODP, DAP, contaminated site or other documents. While misalignments can have a tolerance set, it is difficult to set something that will work across the entire municipality.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Digitised from Absolute LGANo Cadastre A/P GPSIMPORTANCEIP IMPORTANCE OF SPATIAL CONSISTENCY Some of our in-house mapping is significantly displaced from the current Landgate cadastre and will be corrected over the next 12 to 18 months – more important mapping such as the Local Planning Scheme will have high priority.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology TYPICAL PROBLEMS AND COMMENTS Inaccurate reporting of constraints based on spatial queries; Inefficient or incomplete workflows generated; Misleading information provided to Public; Creates need to realign all tables spatially constructed from the cadastre; Lack of resources to fully QA input from “trusted” source; Probable impact on SLIP layers spatially based on cadastre – what version of cadastre was used? Is the problem with cadastral updates? Landgate’s stated accuracy usually greater than any spatial adjustment.

WA LOCAL GOVERNMENT GIS FOCUS GROUP Referential Topology Mapinfo30 LGAs ESRI10 LGAs Other 2 LGAs Mapinfo does not store attributed primitives (points, arcs and vectors) – need to deconstruct polygons (regions) into a set of topology tables before any automated adjustment can occur. Some proprietary applications have been developed – need further investigation. GRASS is a topological GIS that can read and write Mapinfo tables, is free to use (OpenGIS), but requires considerable technical expertise. A-Spec demonstrates ability of LGAs to work together to implement a new funded project with a committed project manager.