WebGIS and virtual globes allow DTMs distribution and three dimensional representations to the Web users' community. In these applications, the database storage size represents a critical point.

DTMs are obtained by some sampling or interpolation on the raw observations and typically are stored and distributed by data based models, like for example regular grids. A new approach to store and transmit DTMs is presented. The idea is to use multi-resolution bilinear spline functions to interpolate the observations and to model the terrain. More in detail, the algorithm performs the following actions.

1) The spatial distribution of the observations is investigated. Where few data are available, few levels of splines are activated while more levels are activated where the raw observations are denser: each new level corresponds to an halving of the spline support with respect to the previous level.

2) After the selection of the spline functions to be activated, the relevant coefficients are estimated by interpolating the observations. The interpolation is computed by batch least squares.

3) Finally, the estimated coefficients of the splines are stored.

The model guarantees a local resolution consistent with the data density and can be defined analytical, because the coefficients of a given function are stored instead of a set of heights.

The approach is discussed and compared with the traditional techniques to interpolate, store and transmit DTMs, considering accuracy and storage requirements. It is also compared with another multi-resolution technique. The research has been funded by the INTERREG HELI-DEM (Helvetia Italy Digital Elevation Model) project.