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Kompleksowa ocena dokładności wielkoskalowych opracowań cyfrowych miasta Zielona Góra

Dąbrowski W., Doskocz A.

Roczniki Geomatyki

2006

T. 4, z. 3

81-93

The paper presents the results of a project carried out over the last seven years by the Department of Geodesy and Property Management, Municipal Office of the City of Zielona Góra, in cooperation with the Faculty of Geodesy and Land Management, University of Warmia and Mazury in Olsztyn. The main goal of the project was to modernize the system of national geodetic and cartographic resources of the city of Zielona Góra, and to ensure its adequate quality. A restorable control network of the 3rd class was established in Zielona Góra during the years 1998 to 1999. Points of the existing horizontal geodetic control network of class I and II were incorporated into the newly-established geodetic network by classical surveys and GPS satellite technique. A total of 480 points of the existing horizontal control network of the 3rd class were adapted for this purpose. The modernization of the horizontal geodetic control network provided an accurate and reliable basis for conducting geodetic surveys and for performing cartographic work. The maximum error of position of a point within the restorable control network of the 3rd class, established in the city of Zielona Góra, is mp = 0,027 m. The accuracy of a large-scale digital map of the city of Zielona Góra, compiled based on the results of planimetric surveys conducted during the years 1974 to 1999, was estimated in 1999. The study was performed on seven control objects, using a total of 1619 control points of the following type: corner points of building contours (denoted by the letter B), boundary points of parcels (denoted by the letter G) and points of technical utilities (denoted by the letter U). The model coordinates of control points were determined based on new field surveys (with the use of an electronic tachometer - total station survey), including redundant data, which permitted to adjust the results of control measurements and to estimate their accuracy. It was found that the adaptation of the results of previous direct measurements based on an accurate geodetic control network enabled to compile a large-scale digital map with an accuracy of 0.10 to 0.20 m (i.e. at the accuracy level of previous planimetric surveys). The accuracy of a digital orthophotomap of the city of Zielona Góra, on a scale of 1:2000, was estimated during the years 2005 to 2006. The analysis was made based on 33 orthophotomap sheets in raster form (.tif files) with reference files (.tfw). A set of plane coordinates of 501 control points, determined by direct field measurements with an electronic tachometer, was used in the study. Detail control points, most of them belonging to the 1st accuracy group (i.e. the so-called .well defined points.), were considered. The points were easy to identify on the raster image of the orthophotomap, and available for precise positioning by manual vectorization. The error of position of the control points obtained by vectorization (on the raster image of the orthophotomap) was. This indicated the possibility to perform point planimetric surveys, using the raster image of the orthophotomap, with a high accuracy. With respect to considered detail points, a scale of 1:1000 can be used for the purpose of graphic representation of vector map data produced by manual vectorization. The results show that a digital orthophotomap of the city of Zielona Góra became a valuable supplement of a large-scale digital map as for the content of a cadastral map and a basis map. Both as an autonomous source of information or combined with the content of other digital maps, the orthophotomap on a scale of 1:2000 can provide the basis for studies and projects carried out within the framework of municipal GIS.