Cadastral Coordinates: A Case Study

UNSW School of Surveying and Geospatial Engineering

GeoCadastre

Overview

GeoCadastre is windows based software from the company Geodata Australia specially designed to adjust survey parcel networks. It describes itself as a 'record-to-fabric' programme designed to build a seamless parcel network from survey plans, survey data and electronic subdivision data. The GeoCadastre software has been developed over the last 20 years by Michael Elfick and Michael Fletcher. Recently the worlds leading GIS Software Company ESRI (Environmental Research Systems Institute) has bought the exclusive rights to use the GeoCadastre software approach in its GIS software packages. ESRIs decision to use the GeoCadastre methodology is affirmation that this software is the most effective model of a cadastral parcel management system.

The GeoCadastre solution creates a coordinated cadastral database which is survey accurate. This has always been an intuitive process where a surveyor would consider the title plan dimensions as well as physical measurements on the ground. It is impossible to completely replicate this process but the software should consider all survey information and provide coordinates which are the best approximation of the legal cadastre.

Cadastral model in GeoCadastre showing parcel accuracies

The first adjustment iterations in GeoCadastre had a large average coordinate shift and many distances exceeding tolerance. On investigation it was found that many of these distances were part of the same few lots. One reason for this is the way that the parcels were joined manually distributed most of the misclose to these few parcels. GeoCadastre has a scale of 7 parcel accuracies; 1 being the parcel held fixed and 7 not included in an adjustment. When each parcel is entered into the system it is assigned an accuracy based on the date of survey or whether it is a compilation or not. These are able to be freely adjusted manually also.

The parcels all have a colour displayed which corresponds to its accuracy. The purple coloured parcels have an accuracy of 4 because they are the oldest plans which were surveyed before the steel band was used. The yellow parcels are accuracy 5; they are all the compilation plans in the network and they have a default value of 5 assigned to them. The dark blue parcels which form the majority have an accuracy of 3. These have been surveyed in the time when surveyors used steel bands for measuring distances and better quality theodolites for directions. The light blue parcels have an accuracy of 2. These plans are on MGA and are the most recent. They have been surveyed using modern high quality total stations. The orange and red coloured lots have an accuracy of 6 and 7 respectively. The four parcels have been assigned these accuracies because all of their lines exceeded tolerance. With these newly assigned accuracies the adjustment was run again; the average shift in easting and northing direction was 0.002m and 0.001m respectively. There were 10 lines which exceed tolerance however they come from the same deposited plan and could be dealt with by raising the parcel accuracy of this particular plan.

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