GRID product detail

Semi-automated, multi-source cartography of land cover. 1. Geneva transboundary area


Duration: February 2011 - on-going

A land cover map is an essential layer of information for the management and planning of the territory. Within the context of a trans-boundary endeavour between Switzerland and France (Projet d’Agglomération Franco-Valdo-Genevoise, hereafter AFVG), a medium-resolution land cover map (CCSA04) was developed for the Canton of Geneva, part of the Canton of Vaud, and parts of the French departments of Ain and Haute-Savoie. Within the constraints of the available data, a 27-class legend was chosen to suit the needs of the AFVG project. This legend can be converted to the Swiss OTEMO and European CORINE standards. The applied mapping methodology can be described as multi-source, multi-scale and object-oriented. Multi-source refers to using the SPOT satellite imagery at two different resolutions (5m for the visible bands, 10m for the infrared), a Digital Elevation/Digital Height Model, and several vector layers. Multi-scale means that the analysis, within the Definiens Developer™ software, proceeded on different levels or scales. The object-oriented approach is based on the concept that the semantic information necessary to interpret an image is not contained in isolated pixels, but rather in objects and their mutual relationships. These objects, obtained through a segmentation process, were classified using multiple criteria, such as their spectral values, morphology and context. Some legend classes, such as buildings and roads, are defined by their spatial coincidence with a shapefile. Others, such as forests and water, are derived from their spectral values. The altitude criterion was used to differentiate mountain grasslands from low-altitude meadows. The automated classification process was completed by some manual editing in the following cases: objects adjacent to roads, under-estimation of coniferous forest, agricultural surfaces falsely classified as deciduous forest, confusion between hard surfaces and bare soil, and objects hidden by shadows. In order to control the quality of the land cover map, a statistical procedure was applied. Two control points were randomly selected within each square kilometre of the AFVG area, and their land cover class was visually estimated from colour aerial photographs, based on a simplified legend. A contingency matrix was computed, yielding a Kappa coefficient of 89%, which exceeds the mandatory threshold of 75% requested by the project. It is considered that this quality level is quite acceptable, owing to the difficulties stemming from the heterogeneity of data between the Swiss and French entities: vector layers having different meaning and showing mismatch across the border, and the SPOT mosaic being a composite of scenes from two years, resulting in differences in radiometry. The rugged topography in the Jura and pre-Alps also leads to extensive zones of shadows, which complicate the analysis. As an example of application of the CCSA04 map, land cover statistics were computed for the four entities. Finally, recommendations are presented towards an improvement of the map-making process (revised legend, compatible data across borders and better imagery), in the perspective of future applications such as change mapping for the computation of land cover trends.

Data credit(s): Stefan Schwarzer (UNEP/DEWA/GRID-Geneva), Karin Allenbach (UNEP/DEWA/GRID-Geneva), Jean-Michel Jaquet (UNEP/DEWA/GRID-Geneva)

GRID unit: Environment Modelling and Geoprocessing

UNEP region: Europe/Western Europe

UNEP priorities:  Environmental Governance

english version (2011, Size:3.28Mb)