Analysing Kettle Holes in Mecklenburg in the Last 225 Years Using an Interdisciplinary Virtual Research Laboratory

Analysing kettle holes in Mecklenburg in the last 225 years using an interdisciplinary virtual research laboratory

Annelie Mai 1, Ralf Bill 1

Abstract Kettle holes are hollow forms smaller than 1 ha, of which some are filled with water. Until today, they are character- istical elements of the landscape of northern Europe, being originated glacially, quasi-natural or anthropogenic. Still, knowledge on the genesis and the embedding of these landscape elements is rather poor. Within the research project described here, our long term objective is to qualify the form of origin and development of each individual kettle hole. The research is done based on a virtual research environment for interdisciplinary research focusing on the landscape of the historical Mecklenburg (Bill/Walter, 2011). Georeferenced old maps from 1786, 1788 and 1877-1889 covering complete Mecklenburg are combined with current geo-information such as digital landscape/terrain models (DLM/DTM) and digital orthophotos (DOP) to support spatio-temporal research aspects in different scales in space (regional 1:200,000 to local 1:25,000) and time (nearly 225 years in three time steps). The Virtual Laboratory for Cultural Landscape Research (VKLandLab) is designed and developed by the Chair of Geodesy and Geoinformatics, hosted at the Computing Centre (ITMZ) and linked to the Digital Library (UB) at Ros- tock University. VKLandLab includes new developments such as wikis, blogs, data tagging, etc. and proven components already integrated in various data-related infrastructures such as InternetGIS, data repositories and authentication structures. The focus is to build a data-related infrastructure and a work platform that supports students as well as professional researchers from different disciplines in their research in space and time. With respect to kettle holes the time series of maps are used to extract several parameters giving a hint of the genesis. Amongst others, this includes existence, the geometric shape, the location in the glacier landscape as well as in the terrain and the distance to the next brickyard. With the help of automated methods like automatic colour recognition more than 50,000 water filled kettles were identified in the topographic base maps from 1889, revealing that until today lots of elements have disappeared and others are added. In this project all kettle holes in the large investigation area (15,500 km²) are considered, while previous investigations mostly focused on a few single objects. To facilitate successive researches, all gained attributes are saved in a VKLandLab database.

1. Introduction Kettle holes are hollow forms smaller than 1 ha, of which some are filled with water, but their genesis is different. For the kettle holes in Mecklenburg Klafs (1973) categorized three different types. The oldest form is the glacially originated kettle. Röpke (1929) developed the intramoraine ice-melting theory. Ac- cordingly kettles were formed during the last glacial period by glaciers, nearly 10,000 years ago. The second type is the quasi-natural or pseudo kettle. It is filled by groundwater or surface water, often because of its position in the terrain, in depressions. The third type is the anthropogenic kettle. It has been created by humans for example in the process of mining sand, clay or marl. Kettle holes often are subjects of anthropogenic change and use. For the agriculture they mostly represent a barrier. Hence, farmers plough near kettles, so that they get smaller and smaller. In the past they often were used as kettle watering tanks, fowl ponds, fishing ponds or for washing and colouring. All these an-

1 Universität Rostock, Professur für Geodäsie und Geoinformatik, Justus-von-Liebig-Weg 6, D-18059 Rostock, email: {annelie.mai;ralf.bill}@uni-rostock.de, Internet: http://www.auf-gg.uni-rostock.de.

thropogenic forces affected the water body and resulted in modifications of the shores. Today kettle holes in North-East Germany are saved by law (§ 20 I Nr. 1 NatSchAG M-V). For their flora and fauna as well as the aesthetics of the landscape they are worth to be protected. To get to know how a kettle hole is originated, sediment core drillings are a suitable possibility. Klafs (1973) described the layer structure of a glacial kettle as follows: mud, fen-peat, peaty mud and clayey mud on glacial till. In quasi-natural kettles the peaty mud and clayey mud are missing. However, for large investigation areas sediment core drillings are too expensive. Other methods have to be developed to allow conclusions of the genesis. Röpke (1929), Woldstedt (1955), Frielinghaus (1998) and Klafs (2000) found that glacial kettles typically exist in the ground moraine. Anthropogenic marl kettles are mainly located on the top in the terrain (Rixen 1805, Röpke 1929, Klafs 2000). These hypotheses have yet to be confirmed by a study using a large area under investigation. In the scope of this work we examined these and other proposals to deduce the genesis of kettle holes. While previous investigations mostly focused on a few single objects (6 in Riehl (2010), 22 in Florencio (2009), 144 in Kalettka (2006)) in a short observation period and small area, in this work a large area with much more kettles is focused. This investigation includes an area of nearly 15,500 km² in Mecklenburg (figure 1), nearly 50,000 kettle holes and an observation period of 225 years. Most of the area was formed in the last glacial period. The objective of this work is to get to know more about the spatial and temporal development of the kettle holes.

Figure 1 Investigation area Mecklenburg with the area of the last glacial period according to Kalettka 2000

2. Methods and materials 2.1 Data sources Georeferenced maps and current geo-information are the basis for this work. The old maps of Wiebek- ing (1786, 48 sheets, 1:24,000), Schmettau (1788, 16 sheets, 1:50,000) and the topographic base maps (1877-1889, 168 sheets, 1:25,000) were scanned and georeferenced. The maps from 1786 and 1788 were treated as one map. Thus only one time layer of kettle holes will be produced from them. For the current layer we used the already digitized water polygons of the DLM 2011 (digital landscape model) and the orthophoto from 2007 (figure 2). Consequently, we got three time steps (1788, 1889, today).

Figure 2 Section of maps near to Papendorf (Rostock) Schmettau (1788), topographic base map (1889) and Orthophoto (2007)

The location accuracy of the examined objects is one of the main problems. For the map of Wiebeking the most points are in an area of inaccuracy of 40 m, for some it reaches up to values of 300 m. The old maps were evaluated in terms of their quality and their geometric origin by Kreßner (2009). He described possible quality aspects in his work, like measurement errors during the fieldwork, transmission errors and misprints in the cartographic implementation, warping of paper because of the storage or mistakes in digit- izing, georeferencing and vectorization. In this work these inaccuracies has to be taken into consideration. Additional base maps were the geological map of the federal state Mecklenburg-Vorpommern (Geo500, 1:500,000) from its geological service, department of the State Office for Environment and Nature Con- versation and Geology (LUNG) and the digital terrain model (DTM) from the Authoritative Topographic- Cartographic Information System (ATKIS).

2.2 Cultural Landscape Research - VKLandLab The processing of the kettle holes project is part of a bigger research project funded by the Deutsche Forschungsgemeinschaft (DFG) for investigation of the cultural landscape in Mecklenburg. Great im- portance is attached to a virtual research laboratory. All project participants work with a web environment, called Virtual Laboratory for Cultural Landscape Research (VKLandLab). This portal was designed and developed by the Chair of Geodesy and Geoinformatics. It is hosted at the Computing Centre and linked to the Library at the University of Rostock. VKLandLab includes new developments such as wikis, blogs, data tagging, etc. and proven components already integrated in various data-related infrastructures such as InternetGIS, data repositories and authentication structures. The focus is to build a data-related infrastruc-

ture and a work platform that supports students as well as professional researchers from different disciplines in their research. The Internet-GIS kvwmap retrieves data for example from a local PostGIS spatial database system or via Web Feature Service (WFS) and Web Map Service (WMS). The virtual research platform is especially applicable for simultaneous editing. Different editors can work on the same layer at the same time. Kettle holes can be digitized simultaneously. Everybody imme- diately recognises changes, digitized and non-digitized areas. With the help of different colours the kettle holes can be marked, for example if they already have all possible attributes or not. So it is easy to see, which one has to be handled next. The attributes are saved in the tables of a database (figure 3).

Figure 3 Data model for the kettle holes

It is possible to export the different layers in shape-format. In this way the data can be used in different desktop-GIS tools for further analysis. Afterwards the results can be imported back into the virtual platform so that the database is up-to-date. The ESRI program ArcGIS was used for further analysis, because specialized analysis functions are not available in the Internet GIS kvwmap.

3. Results and discussion 3.1 Counting and comparing the time slices The virtual research platform kvwmap was used for the digitization of the kettle holes and for filling in attributes that cannot be generated automatically, for example the name. At the beginning the geometric form and existence were filled by hand, because some objects would be needed for controlling the automated shape recognition and to get to know necessary buffer sizes. The area of investigation Mecklenburg contains 47,500 kettle holes for today (figure 4). In the second time slice 1889 are about 51,200 objects. For the oldest time slice 1788 the digitization is not finished yet. Only 1,200 kettle holes were digitized in the area of Rostock until today. It can be predicted that there will be less kettle holes in the investigation area than in the two younger time slices.

Figure 4 Distribution of kettle holes today in Mecklenburg

The water filled kettle holes of the topographic base map (1889) were identified with the help of automatic colour recognition. The 168 map sheets had different values for the colour blue. To get the same colour value for all the sheets, they had to be matched to one image. Some isolated sheets showed strange colouration after that process and some blue values got very dark. The alternative of this procedure would have been the identification of the blue colour values by hand repeatedly 168 times. To get polygons the blue values were used. To eliminate holes in the polygons expanding and shrinking the polygons helped along. The last automatic step is to convert the raster cells to polygons. Then the result was checked by visual control. Polygons bigger than 1 ha and rivers were eliminated, also missing kettle holes were digitized by hand. For the maps of 1786/88 this automatic procedure was not possible. While the map of Schmettau is monochrome the map of Wiebeking is coloured, but only a few kettle holes have blue colour, some are green or black and they often appear very blurry.

Figure 5 5 Figure disappeared kettle holes in Mecklenburg since 1889 in Mecklenburg holes kettle disappeared Number of kettle holes in Mecklenburg 1889 and today & today 1889 and Mecklenburg in holes of kettle Number Appeared and Appeared

The distribution of kettle holes is shown in figure 5. Each cell shows the number of kettle holes in a 5x5 km area. The minimum value is 0 and the maximum 352. The average number of kettle holes is 77 per grid cell in the year 1889 and 68 today. Especially to the south of the federal state and near the Baltic Sea the number decreases. In the south of Rostock and near to Grevesmühlen the most kettle holes per grid cell occur. Comparing the time slices of 1889 and today the result is that nearly 57 % of the kettle holes got lost. On the other hand there are about 54 % new ones. Only about 22,000 of 47,500 were existent in both time slices. For detection of identical objects in the time slices of 1889 and today the polygons were selected by location with a buffer of 40 m. That means polygons of the two time slices with a maximum distance of 40 m get marked as identic. Figure 5 shows two maps with the gains and losses of kettle holes for each grid cell. Throughout the country a lot of objects disappeared, but on the other hand many new ones appear on the maps of today. Especially to the south of the state capital Schwerin a lot of kettles holes disappeared since 1888 in terms of percentage. On the other hand many new ones are there today. Reasons of this fluc- tuation are still unknown. The region is characterized by agriculture, typical for Mecklenburg. In Rostock a lot of kettle holes disappeared, too (483) and 413 new ones are there today. Reasons for disappearing are the extension of the town and the port area. In the peripheral of Rostock small villages get to big parts of the town. Some new kettle holes are anthropogenic ponds for relaxation today. Furthermore the wet lands of 1889 are drained and small water areas have remained.

3.2 Location in glacial landscape With the help of the Geological map (Geo500) and the information of Katzung (2004) a map with the different glacial landscapes was produced. The polygons of the kettle holes were intersected with the map of the glacial landscapes, so for every object it is known if it is part of the ground moraine, end moraine, outwash plain, glacial valley or another part (table 1).

kettle holes 1888 kettle holes to- kettle holes 1888 glacial landscape area in % in % day in % & today in %

ground moraine 62,6 84,5 85,0 88,2 end moraine 3,5 3,8 4,2 4,2 outwash plain 14,0 4,8 4,4 3,5 glacial valley 7,3 2,2 2,4 1,1 others 12,6 4,7 4,0 3,0

Table 1 Percentage distribution of kettle holes in the glacial landscape of Mecklenburg

The greatest change is to find in the areas of the glacial valley. Only 20 % of the kettle holes existed in both time slices. One reason could be peat mining. Today some of the remained holes are filled with water and look like kettle holes. Another reason can be changes in the courses of the rivers. Nowadays the old riverbeds are often wet land with a lot of small ponds. In the outwash plain the percentage of the kettle holes, which exist at least since 125 years is lower than the percentage of the two time slices on their own. 37 % existed continuous. The anthropogenic mining of sand for buildings could be one reason.

The landscape of Mecklenburg is characterised by ground moraines, which accounts for 63 % of the area. In the moraines the proportion of kettle holes is higher than the percentage of landscape. Additional the most kettles holes existed in this area in both time slices, nearly 48 %. The statement of several literatures that kettle holes typically exist in the moraines can be confirmed in this work for a large investigation area. But if these kettle holes are especially glacial kettles is not identified till now.

4. Conclusion In the federal state of Mecklenburg-Vorpommern a number of characterizing landscape elements exists. Amongst others kettle holes are such components of the landscape. The oldest kettle holes were formed 10,000 years ago in the last glacial period. In this project the kettle holes are to be registered for three different maps (1788, 1889, today) in the investigation area of Mecklenburg (nearly 15,500 km²). The main objective is to identify the glacially formed kettles under the use of different attributes and to show in which part of the landscape they primary occur. In the project nearly 50,000 kettle holes of the current time slice and roughly the same number for 1889 were already digitized. The research results show, that a lot of kettle holes have disappeared and new ones developed in the last 225 years. This applies to the whole federal state of Mecklenburg, as well as all the different forms of the glacial landscape. Especially human factors are regarded as the cause of disappearing and appearing. For- est clearances were already a reason in the 12th century for rise of groundwater and consequently the development of a lot of pseudo kettles in depressions of the terrain (Kalettka 1996). The fill up of kettle holes and mining activities of different materials also affected the number of objects. Only 22,000 of the todays 47,500 kettle holes already existed 125 years ago. Initial random investigations revealed that the number of kettle holes that have been existed for 225 years will be even lower. Considering how many objects in the last 225 years disappeared it is hard to imagine that there still are glacially originated kettles that exist since 10,000 years. Consequently finding out how the distribution of glacial kettles in the landscape looks like appears not to be easy. The investigations have not yet reached their conclusion. The methods which have been developed cur- rently for automated shape recognition and detection of position in the terrain must be enhanced. Further- more the kettle holes of the oldest time slice (1788) have to be digitized to contribute to the evaluation. In- tersections with the land use, distance to settlements or streets could not been evaluated yet, because re- quired information must be generated within further subprojects of the VKLandLab project.

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