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2 2005

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Wieloskalowe modelowanie rzeźby terenu. Cz. 2

Kochman M., Olszewski R.

Polski Przegląd Kartograficzny

2005

T. 37, nr 4

267-273

W drugiej części artykułu omówiono wybrane koncepcje generalizacji numerycznego modelu rzeźby terenu, ze szczególnym uwzględnieniem metody wagowanej filtracji lokalnej opartej na uogólnieniu linii strukturalnych. Zaproponowano także koncepcję budowy wieloskalowego NMT o strukturze hierarchicznej, umożliwiającą monoskalową reprezentację modelu rzeźby na określonym poziomie uogólnienia.

The concept of hierarchical model of relief is a logical conclusion of the idea of a multi representation topographic database. It permits a combined analysis of all the components of the environment and uncovers new possibilities in preserving the integrity of spatial constraints between the model of terrain and other thematic layers (primarily river network). It is possible due to, among other factors, defining of spatial relations between particular objects and object classes. It also provides a new way of looking at the process of relief generalization understood as an approximation of the relief model preserving topological relations, and not as a simplification of contour lines. The basic feature of the generalization of relief should be the preservation of its structure (morphological skeleton). What seems important for the construction of multi scale DTM is not only a hierarchical approach to relief modelling but also the method of approximation of source data. Simple algorithms of DTM generalization such as the conversion of TIN model into regular GRID structure of certain spatial resolution and the application of global filtration allow for a relatively low reduction of complexity of the source model. The proposed concept of hierarchical model preparation through iterative elimination of points from the data source combines elements of local filtration and heuristic approach. In the conducted research it was assumed, that while building the hierarchical structure of DTM model one should define relief structural lines and assign to them appropriate numeric weights. The analysis bases on the division of source data of the model into structural and mass points. In the process of generalization the points are removed according to the combined evaluation of several criteria: their vertical significance (established locally), their density (horizontal significance), and for structural points also by the weight of structural lines and local curvature of those line (established horizontally as well as vertically). The selection of relevance of particular criteria is fully parameterized, permitting a free assignment of weight factors. Hierarchical (multi scale) model of relief can be adapted to the scale of presentation chosen by the user, while retaining the relief structure relevant for that level of generalization. Suggested algorithms make it possible to preserve significant topological relations between structural elements of relief.

Wieloskalowe modelowanie rzeźby terenu

Kochman M., Olszewski R.

Polski Przegląd Kartograficzny

2005

T. 37, nr 3

171-184

W artykule omówiono wybrane koncepcje generalizacji numerycznego modelu rzeźby terenu (NMT), ze szczególnym uwzględnieniem metody heurystycznej opartej na uogólnieniu linii strukturalnych terenu. Przedstawiono także zróżnicowane sposoby wyznaczania linii strukturalnych i ich znaczenie dla wieloskalowego modelowania rzeźby terenu oraz generalizacji NMT.

In order to automatize the process of generalization of DTM (Digital Terrain Model) in TDB (Topographic Data Base), information about relief should be recorded in such a way, as to preserve real location of characteristic key forms and structure lines. Automatic generalization of DTM should involve generalization of database rather than graphic simplification of contour lines. Among the existing methods of DTM generalization one can distinguish various types of filtering (global, local) and heuristic approach based on a concept of generalization of structure lines. It uses digital operators in interactive cooperation between operator and the digital system. Here, generalization bases on the approximation of source SLM (structure line model) through the use of classic generalization methods such as, among others: magnification, deletion, combination, and simplification of single structure lines, followed by recomposition of secondary DTM basing on the resulting SLM. In the conducted research it was assumed, that for the composition of a hierarchical (multiscale) DTM basing on a generalization of significant morphological forms it is especially important to properly establish structure lines which constitute the topographic ..skeleton" of terrain. These lines were established basing on the existing topographic maps as well as on the analysis of TIN and GRID model. The aim was to see if it was possible to fully automatically determine structure lines basing on irregular (or regular) source data including no additional information on structural forms. Most algorithms which determine structure lines of relief basing on the regular model involve advanced analysis of potential surface flow accumulation. The research applied a modified D8 algorithm implemented in GM Grid 5,2 module, which is a functional extension of GeoMedia Professional package by Intergraph. To establish erosion network (drainage lines) it is vital to determine a threshold of accumulated flow, over which drainage network becomes a stream. Although the process is fully automatic, subjective setting of such threshold significantly varies the results. Authors of the concept of so-called extended Morse-Smale's decomposition (E. Danovaro L. DeFloriani, P. Magillo, M. Mesmoudi, E. Puppo 2003) claim that generalization of automatically determined structure lines allows for hierarchical DTM generalization, preserving significant topological relations. The research results do not support that hypothesis. Because the actual structure lines were not taken into consideration, in the process of standard Delaneuy's triangulation there appear triangles with edges crossing structure lines. It appears that although DTM generalization (or its hierarchical composition) can be successfully executed through the generalization of structure lines, the automatization of the process of determining these lines does not yield expected results. The need to determine structure lines through direct surveying, e.g. with photogrammetric techniques is therefore justified. Basing on thus determined ..skeleton" it is possible to properly generalize the model of relief, while preserving significant topological relations.