This article describes a new hierarchical model for representing a terrain. The model, called a Hierarchical Triangulated Irregular Network (HTIN), is a method for compression of spatial data and representation of a topographic surface at successively finer levels of detail. A HTIN is a hierarchy of triangle-based surface approximations, where each node, except for the root, is a triangulated irregular network refining a triangle face belonging to its parent in the hierarchy. In this paper we present an encoding structure for a HTIN and we describe an algorithm for its construction. 1 I n t r o d u c t i o n In the last few years hierarchical surface models have been developed to describe terrains at different levels of resolution. Variable-resolution surface models provide a da ta compression mechanism which allows a reduct ion of the number of points needed to describe a surface: fewer points, for instance, should be used to represent large surface regions of almost constant slope. Such models are also used for robot navigat ion on a terrain, since a coarse map of the terrain is necessary to plan an initial pa th for the robot, while a more detailed description is required to plan local motions.