You can use Microstation do thin your point cloud. Attach las file to 3d dgn model, it will be converted to "pod" format. Then you can clip point cloud, decrease desnisty and export to external file (xyz, pod, and if running Descartes to las file). Then from the clliped and thinned point cloud you can create terrain with several methods. There is an add-in to Inroads SS2 to import las files. However in Inroads SS3-SS4 you can create terrain model directly form the point cloud, apply filters, etc. You can export terrain model to dtm to use it with Inroads SS2. Be sure your las file is classified and you can extract terrain class.
