Question:
I downloaded LiDAR data for my county, and it is a DTM file. Can I use a DTM instead of DEM?

Answer:
Sometimes I've seen DTMs or digital terrain models represented within ArcGIS as TINs (triangular irregular networks) file structures or terrains (a large version of a TIN introduced a while back to handle big elevation data sets like LiDAR). I've been collecting Wisconsin LiDAR data sets and have seen some have a DTM folder with TINs in either Esri geodatabase or Autocad dwg CAD file formats. I think of DEMs or digital elevation models as regularly gridded sets of elevation values in some type of raster file format, like .img, .tif, or something else. The ACPF requires a DEM in raster gridded format, not a TIN. Since each HUC12 database is contained in a file geodatabase, the DEM should be added to this fgdb, and any output files will also be saved back to the same fgdb. If this suggested format is followed, rasters will always be in a file geodatabase raster format. We have done limited testing of writing the outputs to a folder (as a .tif, for example). It seems to work, but we have not thoroughly tested it. I’m not sure there’s much difference between a DTM and a DEM. A DEM is obtained from the “last return” points of the LiDAR collection, then those point elevations are converted into a gridded raster surface, which is the DEM. I believe we came across this before, and decided that a DTM is just fine, as long as it is in raster format. You will still see highways and ramps in a DEM or DTM.