Carrara handles SDTS files perfectly. Only not all area has coverage for free.

I think it does dems still also. Definatly Bryce still does dems. You have to get good at searching though as the usgs makes it next to impossible to find the free ones so you buy their pro sets. The EDU site all have links but finding them are hard. What you need is to find a student that already knows what the link is and PRESTO. Free Dems.