The mass appraisal process is used to determine property values in many jurisdictions around the world as a basis for taxation and has been for a considerable time. The need to be able to transparently demonstrate the fairness and equity of this process is of prime importance to both government and the taxpayer. Currently this is primarily achieved in Australia and New Zealand through a series of assessment ratio studies based on standards established by the International Association of Assessing Officers (IAAO). Both United States and Australasian jurisdictions have started to spatially represent aspects of their mass appraisal process in order to improve the auditing function which leads to the certification for use as a taxation base. Against the broad background of international practice this paper investigates the spatial interpolation of the sales price to the assessed value ratio as a way of contributing to the quality control and assurance of the fiscal cadastre. This is considered through a case study of the metropolitan area of Adelaide, South Australia. Refereed/Peer-reviewed