Elevation Pipeline¶

Last updated: June 2026

This document explains how Crestr enriches the pathfinding graph with elevation data and uses it to produce realistic hiking times and route costs.

Overview¶

The elevation pipeline is a one-time preprocessing step that:

1. Attaches accurate elevation values to every node in the graph.

2. Calculates realistic travel times for every edge using an enhanced version of Naismith's Rule.

3. Adjusts costs based on terrain difficulty, surface type, and trail visibility.

The result is a better_path_graph.pkl file that the routing engine uses for A* pathfinding.

Process Steps¶

1. Node Elevation Assignment¶

• All node coordinates (stored in Web Mercator) are converted to WGS84.
• A digital elevation model (DEM) raster (Cumbria-Elevation-File.tif) is sampled at every node location using rasterio.
• Elevation values (in metres) are attached directly to each node as the elev attribute.
• NoData values are replaced with 0.

2. Edge Enrichment¶

For every edge in the graph, the pipeline performs the following calculations:

Horizontal Distance Correction¶

• Accounts for Web Mercator distortion using the cosine of the midpoint latitude.

Elevation Difference & Slope¶

• Computes the vertical gain/loss and slope ratio between the two nodes.

Naismith's Rule (Enhanced)¶

Calculates realistic travel time by combining:

• Flat walking time based on adjusted speed
• Additional time for ascent (+10m = +1 minute)
• Additional time for descent (+7.5m = +1 minute)

Walking speed is dynamically adjusted according to slope steepness:

• less than 5°: 1.4 m/s (gentle)
• 5–12°: 1.1 m/s (moderate)
• 12–25°: 0.8 m/s (steep)
• over 25°: 0.5 m/s (extreme / scrambling)

Terrain Difficulty Multiplier¶

Further adjusts cost using OSM tags:

• sac_scale
• trail_visibility
• surface

3. Final Cost Assignment¶

Each edge receives a cost value in seconds (used directly by the A* algorithm).

Additional attributes stored for filtering and debugging:

• slope
• terrain_factor
• ascent / descent attributes (used in the assinging of costs to edges)

Key Files¶

• Cumbria-Elevation-File.tif – Source DEM raster
• Pathfinding/new_path_graph.pkl – Input graph
• Pathfinding/better_path_graph.pkl – Output enriched graph

Design Decisions¶

Why Naismith's Rule?¶

• It is a well-established hiking-specific model that balances distance and elevation.
• Our version improves accuracy by incorporating slope-dependent walking speed and terrain multipliers.

Precomputation vs Runtime¶

• All elevation work is done once during preprocessing. This keeps routing queries extremely fast.

Accuracy Trade-offs¶

• The pipeline uses a single DEM file. Future improvements could include higher-resolution rasters or dynamic fetching for other regions.

Future Enhancements¶

• Support for multiple regional DEMs
• Integration of more advanced cost models (e.g. Tobler's Hiking Function)
• Real-time slope-based filtering options in the UI