I needed to convert a DEM to a TIN for use with HEC-GeoRAS and I was so surprised how sensitive this tool was to the Z-tolerance factor which is supposed to be an optional parameter. I first left it at the default value which was 18 feet. Needless to say the difference in point elevations between the DEM and the TIN were in the order of 2-3 feet and the results were off. I tried a Z-tolerance of 0.01, thinking that would reduce the difference but that made it worse. The elevation differences between the DEM and the TIN were now in the order of 6-8 feet! I consider myself lucky as when I tried a Z-tolerance of 0.25, the TIN elevations were pretty close (no more than 0.5 ft difference). All other values for the Z-tolerance: 0.1, 1, 0.01, 5, ... yielded a TIN that was way off the DEM.
So how could such a process be so contingent upon the proper selection of a Z-tolerance, yet this parameter is specified as optional?? How come there's very little guidance on how to select this parameter? And isn't it supposed to be the maximum allowable error?? So if I specify that the max Z-tolerance is 0.1, shouldn't I expect that the max difference in elevation at any given point between the DEM and the TIN is 0.1 feet?
Thanks for helping understand this process.
Joe Farah
Civil Engineer
SAIC