Comparison CCM Data with DLM1000 and DTK25

Bundesamt für Kartographie und Geodäsie

Investigation report: Comparison CCM data with DLM1000 and DTK25

File name: Comparison CCM data with DLM1000 and DTK25.doc Version Author Date Comments 1.0 Sonja Werhahn, BKG 24.7.2003

1. Executive Summary In order to briefly evaluate the CCM River and Catchment Database Version 1.0, the dataset was compared to official national small and large scale datasets for Germany. The differences found concerned the completeness, the topological relationship between the rivers and the positional accuracy of the CCM database.

2. Comparison

The CCM River and Catchment Database Version 1.0 was compared to the most up-to-date water data (2002) of the DLM1000 for the evaluation of completeness and topological correctness. For the evaluation of the positional accuracy the CCM data was compared to the DTK25.

The German topographic database 1:1.000.000 (DLM1000) was updated and densified in 2002 specially for the needs of the Water Framework Directive (WFD). The hydrography layer was updated through larger scale data (1:50.000), so the positional accuracy for this layer is at least comparable to map scale 1:500.000 or better. The river data was crosschecked by the German water authorities and can therefore be taken as topologically correct and contains all WFD relevant rivers and streams.

The DTK25 is the topographic raster map 1:25.000.

Three types of differences were found:

1.) Completeness: some rivers are portrayed in CCM but not in DLM1000; other rivers are not portrayed in CCM but in DLM1000.

2.) Topological relationship between rivers: in CCM rivers connect that do not meet in the DLM1000; the order of the confluences along a river in CCM does not correspond to the order in DLM1000.

3.) Positional accuracy: the position of the CCM river does not correspond to the position of the river in DLM1000 and DTK25.

The data was evaluated manually, no automated evaluation of the whole dataset was conducted. Because of the big differences between the compared datasets found during manual checking no statistical comparisons were made.

Bundesamt für Kartographie und Geodäsie, Frankfurt am Main, Germany page 1 of 5 Investigation report: Comparison CCM data with DLM1000 and DTK25

3. Results Most differences between the CCM data and DLM1000 were found in the flatter regions of Germany (North, river plains of Rhein and Donau). But all three types of differences occur also in other parts of Germany. The positional conformance of the compared datasets improves in mountain regions. The examples shown on the following pages do not show all the differences found! They are just examples for the type of difference. There are more differences of the same type in the CCM dataset.

Our checks could not cover the whole of Germany because the CCM data we received does not cover the catchments of the Weser, Ems and all other smaller rivers that flow into the North or Baltic Sea. So large parts of the north of Germany could not be checked. Because this part of Germany is more or less flat more differences of the types described above can be expected.

4. Examples for the errors found

Legend for the maps: Figure 1

1.) Completeness: some rivers are portrayed in CCM but not in DLM1000  Figure 2 other rivers are not portrayed in CCM but in DLM1000  Figure 3 Figure 1: Legend

2.) topological relationship between rivers: in CCM rivers connect that do not meet in the DLM1000; the order of the confluences along a river in CCM does not correspond to the order in DLM1000.  Figure 4

3.) positional accuracy: the position of the CCM river does not correspond to the position of the river in DLM1000 and DTK25. The topographic raster map in the background of the detail map is the DTK25.  Figure 5

Detail map  Figure 6

5. Conclusions

After our random check of the CCM data for Germany we come to the following conclusions: 1.) For a complete dataset of the hydrographic network not only the "natural" rivers are important, but also the artificial waterways and the headwaters must be included. Without these features the data can not be used for analysis properly. The CCM data does not seem fit for the use of the Water Framework Directive because not all relevant rivers are included. Compared to the (WFD conform) DLM1000 some 1st order rivers and quite a lot 2nd order rivers are missing. 2.) For network and other analysis it is not only important to have a closed network, but it is also very important that the topological relationship between the network features is correct. In the CCM data in some cases rivers are connected that do not meet in reality and the sequence of the confluences in some cases is not correct. 3.) The positional accuracy of the CCM data does not seem to correspond to the given map scale of 1:250.000 to 1:500.000. It corresponds to much smaller map scales.

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