TransDEM Forum

So I wanted to move this off of the Canada source forum since I'm likely the source of this issue.

I went to import the ESRI Ascii DEM and it looked very strange. Not what I was to expect. The Co-Ordinates seemed natural but the heights were two tone (I believe green and orange). The values though correct at one area of the tone were off the charts in another.

There were no errors when importing and I left the default 15M UTM when prompted.

Going to dig into this a little further and see if I missed something.

Any ideas, please advise.

Thanks

Sean

Well as I type this I'm trying to import 2GB of raw data When I georeferenced what I was seeing it was part of the map with no data (United States away from the border). The heights are likely correct.
Not sure what the resolution is as it's still trying to build. How big?

2 degrees W to E, 1 degree N to S. No my route isn't that big but I guess I'll explore it anyways if it survives

Thanks

Sean

Do not try to load enormous amounts of data before you made yourself familiar with a few fundamentals, like the "flat earth" challenge, aka map projection.

Go for 0.75 arc sec and just pick a small rectangle, e.g. one of the prefabricated ones.

ESRI ASCII grid comes with two files: The actual data in the .asc file and the meta data in the .prj file. TransDEM will look for and read that .prj file. The data itself is in Plate Carrée pseudo projection, which means the axes are longitude and latitude, but 0.75 arc sec longitude only equals 0.75 arc sec latitude at the equator. TransDEM will come up with an equivalent in metres for the UTM projection, e.g. 15 x 23 m, and suggest 15 m as the new uniform grid width. That should do nicely. It will then convert the entire DEM from Plate Carrée to UTM for the current UTM zone, based on the given DEM longitude.

UTM has Cartesian and metric axes, 1m easting equals 1m northing. But it is also a projection that produces the resulting flat map (which Trainz will require). The projection is Transverse Mercator and like all projections for medium to large scale, it is valid only for a limited range of the earth's surface. More precisely and mathematical, UTM is "conformal", i.e. it preserves angles locally, but not "equidistant" or "equal area". No projection can fulfil all criteria. (Think of the orange peel. You cannot flatten it without tearing it). In UTM it's called a zone. There will be a new zone every 6 degrees longitude. As long as you stay in the same zone, you will not notice any distortion. It means, within that zone, the projection is near enough "equidistant"/"equal area", with a certain tolerable error. You can also cross into adjacent zones and it will still be accurate enough. But there is a limit. You will not be able to build a Canadian transcontinental railroad with the UTM projection as single route.

Yeah everything is going nicely now. My original plan was to bring in small segments suggested in your first route.

Someone was originally nice enough to provide some DEMS for me created by TransDem to see it's potential. It was enough to hook me on purchasing it for myself. So I was able to merge much smaller routes into what I was looking for. I was able to make quite a bit of headway last night and will set an original limit. I know from his experience trying to do too many UTM's at once can also lead to issues.

The manuals are very well done BTW and have been a great help in self diagnosing issues.

Thanks again for all the support and this great software.

Sean