Encoding OS Terrain 50 ASCII Grid data as Terrain-RGB tiles

Introduction

OS Terrain 50 is a digital terrain model of the landscape, designed for landscape visualisation and analysis over large areas. The ASCII Grid data (heighted points with regular 50 metre post spacing) can be downloaded here.

Prerequisites

1. Download/Install GDAL
   https://www.gdal.org/
2. Install rio-rgbify
   https://github.com/mapbox/rio-rgbify

Processing

# -- Extract ASCII grid files to output directory
find ~/Documents/Terrain50/zip -name "*.zip" -exec unzip {} "*.asc" -d ~/Documents/Terrain50/asc \;

# -- Build VRT (Virtual Dataset) for ASCII grid files
find ~/Documents/Terrain50/asc -name "*.asc" | sort > terrain50_filelist.txt
gdalbuildvrt -vrtnodata -9999 -input_file_list terrain50_filelist.txt terrain50.vrt

# -- Reproject VRT to Web Mercator and output as GeoTIFF
gdalwarp -s_srs EPSG:27700 -t_srs EPSG:3857 -srcnodata -9999 -dstnodata 0 -co COMPRESS=DEFLATE terrain50.vrt terrain50.tif

# -- Encode raster to Terrain-RGB mbtiles
rio rgbify -b -10000 -i 0.1 --max-z 9 --min-z 5 --format png terrain50.tif terrain50.mbtiles

MBUtil can subsequently be used to export the MBTiles output to a directory of (PNG) files.

Uploading the generated output to Mapbox Studio

The MBTiles format must be used if there is a requirement to upload the files to Mapbox.
Additionally the same encoding as Terrain-RGB must be used (with a base value of -10000 and an interval of 0.1).
See mapbox/rio-rgbify#19 for more detailed information.