USGS NED DEMs are offered in two sometimes three different resolutions: 1 arc sec, 1/3 arc sec and, for some areas, also 1/9 arc sec.
All NED DEMs are terrestrial, the elevation should identify terrain height, not roofs or tree tops. They are also available in GeoTIFF format with floating point vertical resolution, that is, resolution is in fractions of a meter, not in integral meters.
1/9 arc sec NED DEMs are supported in TransDEM from version 2.1. They are the optimum for the Trainz 5 m grid.
The series of DEMs for this comparison shows an area at the Potomac, near Cumberland, WV, a classic coal and steel district.
Click on the thumbnails for full size images.
The first samples depict an area of 6 x 4km.
We start with a low resolution of 3 arc sec. This first image is not yet NED, but SRTM. It's the standard resolution for DEMs available worldwide. 3 arc sec at this latitude gives a 60 x 90m DEM, shown as a 60m raster in TransDEM. Workable, but needs refinement for rail, road, and water.
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Srtm3-td.jpg)
Next is the first NED, in 1 arc sec, shown in TransDEM as a 25m raster. Now you get an idea where the river flows and may even sense some cuttings or embankments. If you have a slow internet connection or plan a larger route and do not want to deal with too much data, go for 1 arc sec.
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Ned1-td.jpg)
Then we have 1/3 arc sec, shown here as a 10m grid. This is the nominal grid of Trainz terrain before TS2009. Cuttings and embankments appear much sharper.
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Ned3-td.jpg)
Finally 1/9 arc, shown as a 3 m grid. A perfect source for the Trainz 5m grid, introduced with TS2009. For this image, however, the difference in detail is not fully apparent at the current zoom level.
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Ned9-td.jpg)
The last two images are taken at a greater zoom level and show a smaller area of only 3 x 2km. This gives a better idea of the DEM detail for the high-res rasters.
The 1/3 arc sec DEM again (10m):
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Ned3-2-td.jpg)
And finally 1/9 arc sec (3m) in its full glory:
![Image](http://img.transdem.de/albums/userpics/10001/thumb_Ned9-2-td.jpg)
There is a catch, though. With each level, 3 -> 1 -> 1/3 -> 1/9 arc sec, the amount of memory needed does increase by factor 9! The same area at 1/9 arc sec resolution will take up 81 times the space needed for 1 arc sec or 729 times the space needed for 3 arc sec.