Electronic Navigational Charts
Electronic Navigational Charts (ENCs) are the foundation of BoatRoutes’ route generation system. These vector charts from NOAA provide the detailed coastline, depth, and hazard data we need to compute navigable paths.
What Are ENCs?
Section titled “What Are ENCs?”ENCs are digital vector charts published in the international S-57/S-100 standard. Unlike raster charts (which are images of traditional paper charts), ENCs contain structured data that can be queried, analyzed, and processed by software.
Each ENC contains features (objects like coastlines, depth areas, buoys) with attributes (properties like depth value, light character, name). This structure allows BoatRoutes to programmatically determine which areas are navigable and which are not.
How We Use ENCs
Section titled “How We Use ENCs”Building the Navigation Grid
Section titled “Building the Navigation Grid”We process NOAA ENCs into a high-resolution navigation grid — a raster grid where each cell is classified as:
- Navigable water — open water with sufficient depth
- Land — shoreline and above-water terrain
- Hazard — rocks, reefs, wrecks, and other obstructions
- Restricted — areas where vessel transit is prohibited or limited
The grid resolution is fine enough to capture narrow passages and small channels while being computationally efficient for path-finding across the entire coverage area.
Route Computation
Section titled “Route Computation”With the navigation grid in place, our Dijkstra path-finding algorithm computes routes between every pair of locations:
- Start and end points are placed on the grid at the coordinates of each marina, anchorage, or marine park
- Dijkstra search walks the grid cell by cell to find the lowest-cost path through navigable cells. The cost of each step accounts for distance and for favoring open water away from shore and charted obstructions.
- Passage handling — narrow passages like Deception Pass receive special treatment through portal waypoints that ensure the routing engine can navigate channels that might be only a few cells wide on the grid
- Variant generation — different weighting schemes produce the route variants
Depth Awareness
Section titled “Depth Awareness”ENC depth data (soundings and depth contours) inform which cells in the navigation grid are classified as navigable. We use a conservative minimum depth threshold to reduce the chance routes cross shallow areas.
ENC Coverage
Section titled “ENC Coverage”NOAA publishes ENCs covering all US navigable waters. BoatRoutes processes the charts covering:
- Puget Sound and adjacent waterways
- The San Juan Islands and surrounding straits
- The Strait of Juan de Fuca
- The Pacific coast of Washington
For British Columbia waters, where NOAA ENC coverage does not reach, we build the navigation grid from OpenStreetMap coastline data instead.
Chart Updates
Section titled “Chart Updates”NOAA updates ENCs regularly through:
- New editions — comprehensive updates to an entire chart
- Updates — incremental changes published through Notices to Mariners
When NOAA publishes updated ENCs, we reprocess the affected navigation grid cells and recompute routes as needed.
Limitations
Section titled “Limitations”ENC data is for planning, not real-time navigation. Always verify conditions using current charts and local knowledge. Specifically:
- Depths may change due to silting, dredging, or geological events
- New obstructions (sunken vessels, debris) may not yet appear on charts
- The navigation grid’s resolution means very small features might not be captured
- Charted depths assume MLLW — actual depths depend on the current tide