This article outlines the process I've developed for creating highly detailed mesh scenery of New Zealand's Southern Alps for the X-Plane Flight Simulator, using the best freely available digital elevation data.

1. Obtain DEM Data

Get DEM data from https://koordinates.com/publisher/university-of-otago-national-school-of-surveying/data/

The data avilable here is 15m DEM of New Zealand.

2. Merge DEM into single file

Convert TIF files into a single vrt file https://gis.stackexchange.com/questions/48910/avoiding-reprojection-gaps-when-merging-dem-rasters Please pay attention to rendering artifacts, these can be verified using hill shading.

gdaldem hillshade input.tif.vrt output.png

gdal_translate -scale input.tif.vrt output.png

I put all the tif dem files into one folder and then created them using:

gdalbuildvrt -r cubic tile.tif.vrt *.tif

Unfortunately this was producing artifacts. https://gis.stackexchange.com/questions/119057/merging-dem-rasters has some answers. I can use a different sampling method mentioned on: https://www.gdal.org/gdalbuildvrt.html and this solves the issue of squares appearing in the middle, but there are still artifacts along the boundaries between the tifs.

Eventually I found out about the gdal_fillnodata.py script. This only operates on tif files, not the vrt. Initially it didn't work because the values used for the edges were -nan but the values for no data were -inf, which meant that the fillnodata script was only operating on the original -inf null data values and not the newly created gaps.

I edited the vrt file to change what the NoDataValue is from -inf to -nan:

  <GeoTransform>  1.6861088038099999e+02,  1.5699496274303981e-04,  0.0000000000000000e+00, -4.1465068702600000e+01,  0.0000000000000000e+00, -1.5699496271755233e-04</GeoTransform>
  <VRTRasterBand dataType="Float32" band="1">
    <NoDataValue>-inf</NoDataValue>
    <ColorInterp>Gray</ColorInterp>
    <ComplexSource resampling="cubic">
      <SourceFilename relativeToVRT="1">17-greymouth-15m-dem-nzsosdem-v10.tif</SourceFilename>

Got changed to:

  <GeoTransform>  1.6861088038099999e+02,  1.5699496274303981e-04,  0.0000000000000000e+00, -4.1465068702600000e+01,  0.0000000000000000e+00, -1.5699496271755233e-04</GeoTransform>
  <VRTRasterBand dataType="Float32" band="1">
    <NoDataValue>-nan</NoDataValue>
    <ColorInterp>Gray</ColorInterp>
    <ComplexSource resampling="cubic">
      <SourceFilename relativeToVRT="1">17-greymouth-15m-dem-nzsosdem-v10.tif</SourceFilename>

Now I can render out as a tif to use in the fillnodata:

gdal_translate tile.tif.vrt tile.tif
gdal_fillnodata.py -md 10 tile.tif tile_filled.tif

And then convert the -nan value to 0.0 so that Ortho4XP can deal with the file (it complains about NaNs):

gdal_calc.py -A tile_filled.tif --outfile=tile_filled_zeroed.tif --calc="nan_to_num(A)"

Finally this results in a good quality tile_filled_zeroed.tif file to use with Ortho4XP.

3. Create Scenery with Ortho4XP

If you've already created a tile you'll need to re-run the first two steps, first having set the custom dem data file to the final tif from the previous step. I like the curvature_tol setting to be 1.0 for this data set, it seems like a good compromise between quality, and number of triangles, resulting in around 3.5 million on a mountainous land mass. I've also only currently got 15gb of ram which dies with many more than this.