SRTM Terrain Step-by-step

Note: The method below has been superseded by new tools built into Rail3D itself — see Digitising A Railway for the easy way to do it.

I’ve been trying to use srtm data with Mark’s coordinate tool.

This is what I’ve done:

  1. Downloaded the srtm GeoTIFF data for the Western Isles of Scotland
  2. Opened the GeoTIFF in MicroDem — Note: steps 2–11 are no longer necessary: you can now open the complete GeoTiff directly in Coordinate Tool
  3. Selected an area of interest (Mallaig > Fort William)
  4. Saved this area as a seperate file (save dem>md dem)
  5. Closed all open files and reopened my area file
  6. Info tells me this has a pitch of approx 50*92 m
  7. Used thin with a factor of 6 and saved result as a new md dem file
  8. Closed files and reopened my thinned area file
  9. Info tells me this has a ptich of approx 350*650 m
  10. Saved this as ascii xyz with Lat/Long
  11. Closed MicroDEM
  12. Opened Mark H’s coordinate tool
  13. I have a big digitiser image for this area - but Mark’s tool cannot open it - so I produced a new version a factor of 16 smaller: this is big enough to see the area I want to select. I had to hack the dgts file for the factor of 16, but I verified it still lines up with my layout.
  14. Opened this smaller map in the coordinate tool
  15. Used the os website to identify a datum point at grid reference nn 100 750
  16. Used the Rail3D digitiser to idetify layout coordinates of same point: 70750,92834
  17. Selected os grid system
  18. Entered these in coordinate tool as nn 10000 75000 and 70750 92834 Mark: I expected the values to be 100,000 and 750,000 not sure why I had to use 10,000 etc to get it to work 1
  19. Check: If I drag on the map, I get sensible filter limits in the coordinates filter box.
  20. Clicked “Open dem” and selected my thinned area xyz file: if you were opening an unmodified GeoTiff in Coordinate Tool, you would have to select the desired point spacing for the output, unless you really want the complete 3 arc-sec data
  21. It’s a good idea to use the “preview points” option in the coordinate tool to check the conversion - this is the whole set of points and shows good correlation between the converted points and the output (but you wouldn’t want to past the whole set into R3D)
  22. Ran convert dem which produced a set of terrain points ready to paste into R3D :-) Note, you need to be a little bit careful to make sure that when the tool saves the new points to a text file it does not overwrite your source xyz file. I had a bit of trouble until I realised what was happening
  23. Pasted the points into R3D and triangulated:
  24. I note that this method has produced a regular grid of points
    this doesn’t seem to be a problem as it seems to triangulate satisfactorily.
  25. In fact, the regular grid works to my advantage. If I view the terrain in the R3D digitiser,
    I can easily see where I have terrain points and I can easily select the next area I need in the coordinate tool. In fact I don’t need to worry about overlap, since if there is any overlap the pasted data will include exactly the same point and R3D will filter duplicate points out.

1 nn identifies a 100km square. The numeric part of os coordinates are usually quoted as if there were an imaginary decimal point at the 100km position, i.e. redundant trailing digits are dropped. So a 6-figure reference like nn 100 750 means “0.1 and 0.75 of the way across the 100km square labelled nn”, i.e. they are in hundreds of metres, and you have to add two zeros to each to get R3d values in metres. (↑)