mrgisa wrote:
when I open up the Canadian .DEM I could put in a different number? You suggested 10 a long time ago, but what should I put there to be accurate? I think the numbers 23 x 17 showed but I can't remember...
When importing lat/long encoded DEMs TransDEM creates a default value for the metric raster which will usually be on the conservative side data-wise, i.e. trying not to lose detail.
Until TransDEM 2.0 the suggested raster width was a multiple of 10 m. With TransDEM 2.0 and support for the 5m Trainz grid, the higher resolution DEMs will be rounded to a multiple of 5m. Actual resolution in metres of all lat/long DEMs depends on latitude. (Closer to the equator a resolution rectangle will become more of a square.) If at a certain latitude the DEM transforms to a 23 x 17 m rectangle, TransDEM will now suggest 15m. That's better than the lower value of the rectangle.
In former versions of TransDEM the suggested value in this case would have been 20m. To be on the safe side - not losing detail - you would have manually changed the value to 10m. With newer TransDEM versions and the Canadian DEMs that's no longer necessary or recommended. (see note below)
For the US 1/9 arc sec DEM TransDEM will suggest 5m. If the major purpose of processing the DEM is a Trainz route, leave it at 5m as suggested. However, to get the most of viewing the DEM or using it for other purposes, you may want to set to 3 m, which is closer to the nominal resolution, at the expense to eating even more memory.
Note: There is a slight error in this way of processing the DEM. For true lossless transformation, the re-sampling raster width for the 23 x 17 m rectangle would have to be 8.5 m. This is what the Nyquist–Shannon sampling theorem says. However, I have never noticed "noise" in the transformed DEMs. Noise would be the result of subsampling.