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Image Matching The DCDB

MODEL IMAGE MATCHING – In most jurisdictions considerable resources have been invested in the improving the Digitized Cadastral Data Base (DCDB) and Geodata has developed a process that migrates existing DCDB data into a Parcel Fabric survey database and adjusting it to match existing imagery.

In this process the original DCDB parcel dimensions become the original parcel dimensions stored as attributes and it is adjusted to desktop control from imagery. The business case is that the cost to migrate the DCDB minimal and collecting control from imagery becomes a desktop exercise.

Geodata manages the migration and overcomes topology and connectivity to produce a packet of up to 3000 - 4000 parcels that is adjusting. This provides a cost effective starting parcel fabric in preparation for adding control and data from survey plans (back capture or digital data) to improve spatial precision.

As the parcel dimensions are initially generated from the DCDB, there is no survey integrity in the data and adjusting one point to an image feature or a surveyed control point does not mean that surrounding points will necessarily adjust to the same level of precision. Over time the DCDB parcels can be replaced with surveyed dimensions derived from input of the bearings & distances from the survey plan.

The benefit of this option is that the model can be constantly improved in-house by staff or external resources by simply moving around the model/image comparison and identifying where the model can be improved to match the imagery. Miss-matching occupations on the image can provide coordinates to apply as a control point to the relevant point in the model. A small selection of parcels (10-200) can then be selected and adjusted, which moves the control point to match the image occupation. This process could be done in minutes and does not require senior technical staff.

This method provides a cost-effective method to get a DCDB to a “fit for purpose” database that will generally match imagery available to the public. This outcome allays the fears of the public that may be distressed by the vision of mismatching database boundaries and imagery where they do not understand this is not a true picture of the location of their legal boundary.

This is not a rigorous solution but moving forward, parcel attributes can be replaced by original dimensions and other back captured attributes. This does not change the spatial location of the model until a new adjustment around that parcel utilizes the new parcel dimensions. Once the model has been populated with all accessible original parcel attributes and sufficient survey control is available, a rigorous solution is achievable. There may be remnant parcels with DCDB attributes in the model that have indeterminate original dimensions but their weighting in the adjustment will be low and the surrounding original measurement-based parcels will adjust those DCDB parcels to the most accurate location.

To move to the next level of spatial accuracy the complete back capture of parcels with field survey control is required but in many areas the fit for purpose solution outlined may meet the needs of many users of the DCDB in remote areas or areas where survey plans have limited information or spatial integrity.

1 An Example of a DCDB Section Adjusted to Air Photography Images 1569 Parcels at circa <1m. It should be noted that the Google Earth Photography is Unlikely to be of the Same High Standard as the Photography Used


Ian Harper : harper@geodata.com.au - Tel: +61 (0) 412 453 170

Roger Lee : lee@geodata.com.au - Tel: +61(0) 407 333 078

Website : www.geodata.com.au - Geodata Australia P/L

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