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Coastal zone seen from above: 25 years of land cover evolution around Brest

 

Changes in land use are an aspect of global change and are most often studied with remote sensing, either aerial or satellite imagery. But in spite of recent technological advances, satellite imagery remains very costly and high skills are needed for its use; in addition it covers only the most recent periods. It is thus difficult to get information both homogeneous and comprehensive at the space and time scales required to meet the needs of land planning public policies.

This is particularly true in coastal areas, where human pressures are increasing and which are covered with insufficient detail by geospatial data, particularly regarding the needs of Integrated Coastal Zone Management (ICZM). The paper presents and evaluates a methodology combining heterogeneous data, and studies how land cover changed since the late 1970s in the "Pays de Brest" (Brittany, Finistère department).

The data come from two sources: the "Permanent Inventory of the Coastal Zone" (IPLI, photo-interpretation of aerial photos taken on a 10 km-wide strip at the 1/25000 scale) and a 2003 SPOT satellite image (10 m spatial resolution).

Making typologies consistent was first necessary to combine these two sources: each image had its own classification of land uses, with differences both in quality and nature. In order to analyze the changes, three main types of land use were considered from the 24 of IPLI and the 21 of the SPOT image classification: artificialized areas, cultivated lands, forests and natural or semi-natural environments (plus a class "others"). A matrix of changes was elaborated from how land use evolved in each point within among three classes (for instance, "cultivated land in 1977 and artificialized area in 2003"). The quality of data, assessed through field observations (for 2003) or IGN aerial photos (for 1977), is globally satisfying, with more than 90 % correct classifications, even though it is poorer on some types of areas. On the whole, only 15 % of the land cover changes are impacted by all sources of error and uncertainty.

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Land use in the territory of Plouzané in 1977 (top) and 2003 (bottom). The comparison of the two maps shows the overall evolution but also the differences of land use classification between the two sources.

In 25 years the area around Brest experienced changes in land cover which were significant but localized, one quarter of its surface changed use, towards artificialization in half of the cases. These changes mainly concern the seaside on the north coast of Finistère, a circle around the city of Brest and the coastal towns in the southern part of the studied area. Closeness to the sea is a significant factor, since land cover changes were highest (more than 30 %) between 100 and 300 m from the shoreline. In spite of the legal protection provided by the "Loi Littoral" law, changes are also important, although lesser, at a closer vicinity (100 m or less) to the shore. The ring-shape artificialization around Brest and Landerneau is obvious: it shows the processes of urban sprawl and sub-urbanization, at the expense of cultivated lands. Conversely, very few natural areas in 1977 were put into cultivation in 2003.

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Part of the area artificialized between 1977 and 2003 in the coastal zone around Brest according to the distance from the sea

 

Using heterogeneous data is unavoidable to study long term changes of land use, since high-resolution satellite images are recent and previous periods are covered only by aerial photos. The analysis conducted around Brest has shown its evolution with a fair level of quality, but has methodological limits related to typology differences between the sources and to geometric problems of image superposition. This analysis could be refined by targeting smaller territories where more detailed land cover categories could be defined.

On thematic grounds, it would be necessary to go back to the 1950s (when Brittany experienced deep socio-economical changes that led to the current land cover evolutions), using other aerial photos and inserting intermediate steps to study the impact of some events like political decentralization, coastal zone law ("Loi Littoral") or the account taken of sustainable development in country planning principles.

 

The paper

Sparfel L. Le Berre I., Gourmelon F., 2011. Évaluation des changements d’occupation des sols en zone côtière à partir de données hétérogènes. Revue Internationale de Géomatique 21/3 : 381-403.

 

The authors

The three authors of this paper are members of Géomer, one of the components of the laboratory LETG (UMR 6554 Littoral, Environnement, Télédétection, Géomatique)

 

The journal

Geomatics is a multidisciplinary research field devoted to representation, modelling, integration, analysis and visualization of geographical data. The journal Revue internationale de géomatique (International Journal of Geomatics and Spatial Analysis) aims to cover this domain through the diffusion of innovating papers of synthesis, research or applications, written in French or in English. It also promotes the French-speaking and international research on geomatics. The paper is based on the PhD dissertation of the first author and was published within a special issue entitled "Changements d'occupation et d'usage des sols. Suivi et modélisation".

 

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