Zill

Generates graphs from OpenStreetMap data optimized for elevation-based routing.

Structure

The library is split into two sub-libraries that can be used independently:

• zillconv - Converts OSM data into a .zl file. Requires PROJ and expat as a system dependency.
• zill - Loads the .zl file and runs graph algorithms on it. Has no dependencies.

.zl is a custom binary format to store the graph data in a compact way. It does not follow the OSM data model of nodes and ways directly; only nodes are stored that are either part of an intersection or mark the beginning or end of a way. Intermediate nodes that are only part of a single way are not stored. The file is split up into three parts (view graph/shared.zig for the exact layout):

• Header: Contains the Zill version the file was generated with, as well as the number of nodes and edges.
• Nodes: Each node store the OSM node ID, latitude, longitude, and elevation.
• Edges: Each edge (u,v) stores the indices of nodes u and v (not the OSM ID directly). It additionally stores the total elevation gain, loss, and distance from u to v. The elevation and distances are accumulated to include intermediate nodes along the original OSM ways, allowing for accurate route elevation calculations, but result in incorrect map routes when converting the nodes of a network path into a GPX file.

Note

Releases page includes pre-generated .zl files.

Graph Algorithms

There are two variants. You can either find:

• the maximum elevation under a given distance --max-distance
• the minimum distance required to reach a point --max-elevation higher than the start (can be very slow and find extremely long distances if there are no close-by climbs)

Usage: zill [options]

Options:
  -i, --input-path      Path to the input graph file. Default: data/graph.zl
  -d, --max-distance    Maximum total distance (in m) path to compute. Default: null
  -e, --max-elevation   Maximum total elevation path (in m) to compute. Default: null
  -r, --max-radius      Maximum radius (in m) to consider for starting nodes. Default: 10_000
      --lat             Latitude of the starting point. Default: 47.38300076849868
      --lon             Longitude of the starting point. Default: 8.539661719099556
  -t, --max-threads     Maximum number of threads to use. Default: 1
  -h, --help            Show this help message

zig build -Doptimize=ReleaseFast run -- -i switzerland_walkable.zl -d 1000

Convert - Generate .zl file from OSM data

If you want to generate your own .zl file, you can use the zillconv library. It depends on PROJ to convert the coordinates from WGS84 to Swiss coordinate system (EPSG:21781) and expat to parse the OSM XML data. End goal would be to fully remove these system dependencies and build everything with the Zig build system.

Generating graph.zl

Requires proj.db for now.

PROJ_DATA="/usr/share/proj" zig build zillconv

Elevation Map

Requires GDAL to convert the coarse elevation data from SwissTopo into a GeoTIFF.

1. Download coarse elevation data from SwissTopo.
2. Convert the .asc into .tif.

curl -f -o /tmp/dhm25.zip "https://cms.geo.admin.ch/ogd/topography/DHM25_MM_ASCII_GRID.zip"
unzip -q -j /tmp/dhm25.zip -d /tmp/dhm25

gdal_translate \
    -a_srs EPSG:21781 \
    -a_nodata -9999 \
    -co TILED=YES \
    -co COMPRESS=DEFLATE \
    -co PREDICTOR=2 \
    -co BIGTIFF=IF_SAFER \
    /tmp/dhm25/dhm25_grid_raster.asc \
    data/switzerland_dhm25.tif

OSM Extraction

https://download.geofabrik.de/europe/switzerland.html