OUTFLOW PRESERVATION OF THE HYDROGRAPHIC NETWORK ON THE RELIEF IN MAP GENERALISATION

J. Gaffuri

IGN France, COGIT Laboratory, Saint Mande, France

julien.gaffuri@ign.fr

In the recent years, some research in map generalisation techniques using the agent paradigm have allowed to progress significantly toward automation. Our challenge is to introduce “field objects” in an existing agent based generalization model designed for buildings and networks generalisation. Field objects are objects defined everywhere, such as the relief and the land use cover. They compose the background of the map, on which other discrete objects are displayed. We aim to treat altogether discrete objects and fields during the generalization process. Many relationships exist between fields and objects (especially buildings and networks) and should be preserved.

In previous works, we propose an agent based method to make the field objects deformable. This deformation method is applied to the preservation of the elevation value of buildings: a building is able to deform the relief during its own generalisation in order to preserve its elevation value.

In this paper, we present a new application of our model to networks, and especially to the hydrographic network outflow. Our purpose is to preserve the consistency between the hydrographic network and the relief.

First, a method to explicitly characterize the outflow relationship between the relief and the hydrographic network is presented. We propose to use, for each segment composing the network, the value of the difference between the orientation of the segment, and the orientation of the slope under the segment: the network perfectly flows down when this value is null for each segment composing the network.

In a second part, this measure is used to apply our deformation method to both the hydrographic network and the relief in order to preserve the outflow relationship. The principles are:

- To decompose the network and the relief into constrained parts to make it deformable. The network is decomposed into a graph structure composed of connected segments and the field into a TIN.

- To constrain the relief and the hydrographic network to flow down: the orientation difference between the segments of the network and the slope is constrained to be null.

- To obtain a deformation of the network and the relief using an agent based convergence method. We obtain an average position of each point composing the hydrographic network and the relief in order to preserve their shape and their outflow constraint.

Finally, we discuss some results of the implementation of this method, and give some future possible improvements.