Development of a Methodology for Map Quality Assessment
ISBN 978-85-88783-11-9
Authors
1Blana, N.; 2Tsoulos, L.
1NATIONAL TECHNICAL UNIVERSITY OF ATHENS Email: natal.blana@gmail.com
2NATIONAL TECHNICAL UNIVERSITY OF ATHENS Email: lysandro@central.ntua.gr
Abstract
Along with the augmented production and dissemination of spatial data caused – amongst others - by the development of National Spatial Data Infrastructures, there has been a considerable increase in maps/charts production. Although significant parts of the spatial data available bear some indication of quality, maps/charts resulting from these data are missing quality information. In certain cases, maps carry an abstract indication referring to the quality characteristics of their source data (scale, lineage or positional accuracy). This kind of information is not adequate to confer map quality to the user, making doubtful their fitness for use. This paper elaborates on the issue of quality assessment in model and cartographic generalization considering map as the product of composition of spatial data of known quality, which undergo a number of transformations during map composition process. It constitutes a part of the ongoing research on the development of a quality assessment methodology for maps aiming at the typification of the necessary quality controls through the development of a quality model. The proposed methodology considers map composition as a series of processes (model and cartographic generalization, symbolization) executed in a production line in the framework of a Quality Management System (QMS) according to the ISO 9001/2008 standard. The QMS provides for a quality model in every phase of the map composition process. The approach adopted is based on the assumption that cartographic data have the same characteristics with their source spatial data (cartographic database is derived from the spatial database) therefore it utilizes the ISO 19157 standard as a tool for the assessment of quality and the reporting of results. Quality assessment results include a quality indicator for every transformation executed during the map composition process, prohibiting the cartographer to proceed unless the assessed phase gives acceptable results. Four basic structural elements comprise the proposed quality model: • Map specifications • Map quality requirements • Map composition • Quality control tools Quality assessment in generalization is based on the aggregation of the results of the quality control in the discrete phases of model and cartographic generalization. In model generalization, certain issues like format compatibility between the source database schema and the new one of the map to be produced are very likely to occur and should be resolved before data transfer takes place from the source to the cartographic database. Otherwise information loss, object misclassification and ‘null’ values will occur. The proposed methodological approach for the implementation of model generalization considers the above issues and adopts the quality elements and the corresponding measures of the ISO 19157 standard for the identification of inconsistencies between the source and the cartographic database schema before data migration. It also provides a guide of the necessary operations that should be performed on data in the initial schema to match new schema specifications and the quality controls that should be carried out. The resultant cartographic database is considered as “error free” and its content is readily available for input to the subsequent phase of cartographic generalization. Quality assessment in cartographic generalization is carried out whenever a generalization transformation is executed. In this phase, quality model includes a measure for the positional displacement of the generalized features and an extra quality element that of ‘shape similarity’ for lines and polygons. ‘Shape similarity’ assessment is implemented through the use of the ‘turning function’ for line shape description and its change due to generalization. The map quality assessment process described has been implemented for the generalization of linear and polygonal features of the EuroRegional Map at scale 1:250.000 for the production of the EuroGlobal Map at scale 1:1.000.000.
Keywords
Map Quality; Quality Standards; Map Generalization