The Principle and Methodology to Process Digital Cadastral Areas in GIS

Error analysis and processing for spatial data is one of the key issues in GIS research.In the establishing of geographic information system databases,the map digitization including manual map digitization and scanned map digitization are the main capturing methods for spatial data.So,it is necessary to study the characteristic and processing of the error in map digitization.In the land and housing fundamental geographic information system,the cadastral parcel is one of the most important objects.According to the feature classifications in GIS,a cadastral parcel belongs to one kind of closed polygon objects composed of series of digitized vertexes.The area of a parcel is the key attribute with legal authorization.However,in cadastral parcel digitization for capturing data,it is unavoidable to have errors (including systematic error and random error).As a result,with the propagation of errors in the vertexes of a parcel,the digitized cadastral area is not equal to the authorized area usually calculated by higher accuracy surveying method.Therefore,it is one of the focus problems to minimize the effects of the digitized errors to upgrade the accuracy of digitized vertexes to ensure the precision of the area attribute in GIS database. In this paper,the error processing in area of the digitized parcel is discussed.The digitized data of a parcel can be treated as observations that are the coordinates in the ground system obtained from the digitized coordinates in a digitizer or scanner by orthogonal or affine transformation.In a parcel,the known authoried area,the rectangular angles and circular arcs constitute the constraints of the digitized vertexes.For more correlated parcels,the constraints also become more.As a result,the redundant observations and adjustment problems are put forward.The principles for adjusting the digitized parcel areas are first presented.The adjustment models are then derived,including the condition equations for areas,areas with arcs,rectangular angles and circular arcs.The methodologies to process multiareas are further presented.The first is the adjustment model and method to process a single and independent parcel area.The second is to adjust the parcel areas with "holes".The condition equations of parcel areas are combined with those of "holes" to resolve together.The third is to adjust integrally the multi-areas that are correlated with each other.The key problem is to ensure that the shared vertexes and boundaries among interrelated parcels are moved simultaneously,therefore,the topologies between parcel polygons remain undamaged.The fourth is used to process the multiareas with fixed vertexes and fixed parcels.These fixed vertexes and fixed parcels keep unchanged in adjustment processing.And the graded adjustment idea and method are lastly presented to solve multi-areas with the larger data volume.The outer boundaries of the adjusting areas are firstly processed to control the whole parcels,the areas are then divided into several parts to process. Based on the above theoretical discussion of the parcel area processing models and methods,an area adjustment system for digitial cadastral parcels is developed.The implementation of the models and the methodologies are illustrated through case studies.And the results are further discussed and analyzed,leading to conclusions that adjustment processing for digital cadastral areas is helpful to ensure the quality of the data in GIS data capturing and database establishment.