The paper deals with the problem of modeling large-size terrain data sets. To this aim, we consider multi-resolution models based on triangle meshes. We analyze and compare two multi-resolution terrain models based on regular and irregular meshes. The two models are viewed as instances of a common multi-resolution model, that we call a multi-resolution triangle mesh. Our comparison takes into account the space requirements of the data structures implementing the two models as well their effectiveness in supporting the extraction of variable-resolution terrain representations.