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L'objectif de ce projet était de fournir des elements objectifs destinés à aider les décideurs dans la mise en place de mesures de conservation

This work is licensed under a Creative Commons Attribution 4.0 License (CC BY 4.0, https://creativecommons.org/licenses/by/4.0).

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The digitization of the Cassini maps and, in particular, of its road network, was achieved in a collaborative way using a shared PostgreSQL database and its spatial extension PostGIS. GIS editing tools such as Quantum GIS were used to remotely digitize the objects using a WMTS (Web Tile Map Service) layer provided by IGN as background. Details on the methods used to produce the georeferenced map are available on a dedicated website. This way, several operators have been able to digitize data simultaneously on the same database. In order to provide consistent data records, data specifications were proposed as a result of an important collaborative work. Nevertheless, as the specifications were enhanced during the digitization process, local variations in the capture of several attributes might be found (the attribute ‘bordered’ was added after a few months of digitization for instance). Further work will focus on the consistency of the data (both for attributes and geometries).An important aspect of the Cassini dataset is the fact that the Cassini map was not homogeneously drawn (different sheets might show different levels of detail as seen in Fig. 2) or conceived as a road map. Hence, one has to be careful when studying the road network extracted from it. Specifically, the road network inside most cities was not drawn in the map. An automatic process is therefore proposed to create so-called ‘fictive’ edges inside cities allowing to link all roads leading them. As shown in Fig. 3, a node representing the city is created at its centroid (or rather at the centroid of the geometry representing its boundary in the map) and edges are created to connect this node to the edges ending in the city. Furthermore, in order to speed up the digitizing process, some roads have been captured as continuous strokes rather than by topological road segments: some users digitized entire roads instead of stopping the capture at each road intersection. We therefore use the PostGIS topology engine to convert the digitized strokes into a topological network. This process uses a distance threshold to merge points closer than the given threshold and thus allows for the correction of minor shifts between points and a second threshold for to collect all nodes in the neighboorhood of a city. The thresholds used in the current export are 10 and 20 meters respectively. The digitized roads and cities are also provided in the export and the code for the topological export is available.

ISO 19115:2003/19139

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