Adding a Buffer to Tiles
Most tiled elevation datasets have an overlap: a 1 pixel overlap lets you interpolate locations between pixels at the edge, and some datasets (like those produced by USGS) have larger datasets that give more flexibility with interpolation and resampling.
However not all datasets come with an overlap, which makes it impossible to interpolate locations within 1 pixel of the edge of a tile without opening multiple files. Because Open Topo Data only opens one file for each location, it may return
null for these border locations in non-overlapping datasets.
This isn't a problem for everyone: if your tiles are large enough, only a very small fraction of pixels lie on the edges so you may never notice issues.
But to fix this problem you can add a buffer to the tile files. I'll use EU-DEM as an example, as the tiles by default have no overlap.
Start by downloading the 27
.TIF files into
./data/eudem. You'll also need to install the gdal commandline tools.
Next we'll make a VRT file for the rasters
mkdir ./data/eudem-vrt cd ./data/eudem-vrt gdalbuildvrt -tr 25 25 -tap -te 0 0 8000000 6000000 -co VRT_SHARED_SOURCE=0 eudem.vrt ../data/eudem/*.TIF cd ../../
te options in the above command ensure that slices from the VRT will use the exact values and grid of the source rasters.
Now we could actually stop here: if we add the
eudem-vrt as a single-file dataset, Open Topo Data will handle the boundaries just fine. But for a slight performance improvement we can slice the VRT back into it's original tiles with an overlap. The following code will make put buffered tiles into
import os from glob import glob import subprocess import rasterio # Prepare paths. input_pattern = 'data/eudem/*.TIF' input_paths = sorted(glob(input_pattern)) assert input_paths vrt_path = 'data/eudem-vrt/eudem.vrt' output_dir = 'data/eudem-buffered/' os.makedirs(output_dir, exist_ok=True) # EU-DEM specific options. tile_size = 1_000_000 buffer_size = 25 for input_path in input_paths: # Get tile bounds. with raster.open(input_path) as f: bottom = int(f.bounds.bottom) left = int(f.bounds.left) # For EU-DEM only: round this partial tile down to the nearest tile_size. if left == 943750: left = 0 # New tile name in SRTM format. output_name = 'N' + str(bottom).zfill(7) + 'E' + str(left).zfill(7) + '.TIF' output_path = os.path.join(output_dir, output_name) # New bounds. xmin = left - buffer_size xmax = left + tile_size + buffer_size ymin = bottom - buffer_size ymax = bottom + tile_size + buffer_size # EU-DEM tiles don't cover negative locations. xmin = max(0, xmin) ymin = max(0, ymin) # Do the transformation. cmd = [ '!gdal_translate', '-a_srs', 'EPSG:3035', # EU-DEM crs. '-co', 'NUM_THREADS=ALL_CPUS', '-co', 'COMPRESS=DEFLATE', '-co', 'BIGTIFF=YES', '--config', 'GDAL_CACHEMAX','512', '-projwin', xmin, ymax, xmax, ymin, vrt_path, output_path , ] r = subprocess.run(cmd) r.check_returncode()
These new files can be used in Open Topo Data with the following
datasets: - name: eudem25m path: data/eudem-buffered/ filename_epsg: 3035 filename_tile_size: 1000000