Accuracy assessment of old large-scale maps and reducing positional error in land use change analyses

Abstract

Abstract Context Old large-scale maps are one of the main data sources on historic landscapes and form the basis of many landscape studies. However, few studies have addressed the issue of assessing the accuracy of map sources and the impact of this accuracy on the results of spatiotemporal analyses of landscape evolution.Objectives The purpose of this study was to verify the positional accuracy of large-scale maps used in landscape analyses and to test the possibility of eliminating the influence of mutual positional inconsistency of map sources on the results of this analysis. Narrow residual polygons, referred to as sliver polygons, arising during overlay operations because of positional errors in old maps can affect the results of the analysis, so it is appropriate to determine to what extent this happens, whether and when it is necessary to eliminate their influence and by what methods.Methods The positional accuracy of the vector models derived from old maps was verified in three model areas around the Vltava River by quantifying the mean positional error of a set of control points. Different methods for removing sliver polygons were proposed and tested for the selected test area within the model area by comparing the selected results of the spatiotemporal analysis.Results The achieved values of the mean positional errors for the historical data models from the mid-19th and mid-20th centuries are in the range of three to four metres for the model areas, which is highly accurate considering the scale values of the old maps used, confirming the suitability of these maps for landscape studies. The reverse vectorization of the time series of the maps eliminated the residual polygons due to positional error and thus reduced the false change areas, which was most evident in the change maps. The change maps after using this procedure better reflected the true changes. A method of identifying them based on their position within a buffer of a given width and then eliminating them by joining them to a neighbouring polygon was proposed as the most appropriate method for removing sliver polygons in overlay analyses.Conclusions Old large-scale maps are a very valuable source of historical data and have a place in landscape studies, especially when researching smaller areas, such as municipalities or cadastres, where they allow work at the level of land parcels. It has been confirmed that the positional inconsistency of map sources can be eliminated to a certain extent by the chosen time series vectorization procedure. Considering the type of study, the type of spatial data used, and the type of results that characterise the change in the area, it is advisable to choose an adequate method for refining the results.