In this paper we introduce a new compression technique for the connectivity of a Triangulated Irregular Network (TIN) by using Delaunay triangulation. The strategy in this paper is based on that most of 2.5 dimensional terrain connection are very similar to plane-projected Delaunay triangulation of their vertex set. We only need to keep the vertex coordinates and a few edges that are not included in the Delaunay edges. We present an efficient encoding method for the set of edges using vertex reordering and general bracketing algorithm. Also we propose progressive visualization method for web-based GIS.In the experiments, the method was examined on several TIN data with various resolutions, which were generated by five typical terrain simplification algorithms. The connectivity information of common terrain data could be compressed by 0.23 bits per vertex.