Pathways, Boundaries, and Gateways
The real ocean has coastlines, shoals, traffic lanes, and restricted areas. Pathways, boundaries, and gateways teach the routing algorithm about the obstacles between you and your destination.
Pathways: Mandatory Waypoint Sequences
We introduced Pathways in the previous lesson as sequences of mandatory waypoints. Here we go deeper into how to build and use them effectively. A Pathway forces the routing algorithm through a specific sequence of positions — the router optimizes each leg individually but must pass through every point in order.
Create a Pathway through Routing → Pathways → Create Pathway. Add waypoints by clicking on the chart or selecting from your existing POIs. The order matters — the router calculates from start to Pathway point 1, then from point 1 to point 2, and so on to the destination. Each segment is independently weather-optimized.
A well-designed Pathway uses the minimum number of waypoints needed to keep the route in safe water. For a coastal passage, this typically means a waypoint at each significant headland or navigation constraint. Too many waypoints over-constrain the router — it can't optimize if every mile is pre-determined. Too few waypoints let the router cut corners through hazardous areas.
You can edit Pathways after creation by dragging waypoints, adding new ones, or removing unnecessary ones. After changing a Pathway, you must recalculate the Routing to see the effect. Comparing routes with different Pathway configurations is a powerful way to evaluate coastal options — does adding the extra offshore waypoint at that headland cost 2 hours or 8?
Place Pathway waypoints at the point where the constraint ends, not in the middle of the constraint. For a headland, place the waypoint at the seaward edge where it's safe to resume free routing — not at the headland's closest approach point.
What happens if you add too many waypoints to a Pathway?
Boundaries: Exclusion Zones
Boundaries are polygons drawn on the chart that the routing algorithm treats as impassable — like invisible walls. Where Pathways tell the router where it must go, Boundaries tell it where it cannot go. They're essential for keeping routes out of areas that don't appear as land on the chart: shallow waters, traffic separation schemes, military exercise areas, marine reserves, or simply areas you want to avoid for any reason.
Create a Boundary through Routing → Boundaries → Create Boundary. Click points on the chart to define a polygon — the router will not cross any edge of this polygon. You can create multiple Boundaries and they all apply simultaneously. Each Boundary can be toggled on and off, so you can maintain a library of exclusion zones and activate only the relevant ones for each route.
Common Boundary uses include: a polygon around a shallow area that your draft can't clear, a box around a TSS (Traffic Separation Scheme) that you'd rather avoid, an exclusion zone around a military exercise area with active NOTAM, or a zone around a lee shore that becomes dangerous in certain wind conditions. You can save Boundaries and reuse them across multiple routing calculations.
The routing algorithm handles Boundaries by treating the polygon edges as hard obstacles — functionally identical to coastline. The isochrones cannot expand through a Boundary. This sometimes produces routes with sharp course changes at Boundary edges, which is normal — the router is finding the fastest path around the obstacle.
Boundaries are only as good as your polygon placement. Draw them with adequate margin — a Boundary placed exactly on a shallow contour gives zero safety margin. Extend your exclusion zones well beyond the actual hazard to account for GPS accuracy, chart datum differences, and the coarseness of the routing algorithm's grid.
How does the routing algorithm treat a Boundary polygon?
Gateways: Forced Passage Points
A Gateway is a line segment that the route must cross — think of it as a gate or door that the route must pass through. Where a Pathway waypoint forces the route through a single point, a Gateway forces it through a line, giving the router freedom to choose exactly where along that line to cross. This is a more flexible constraint than a Pathway point.
Gateways are particularly useful for channel entrances, strait crossings, and passing between islands where you need the route to transit a general area but the exact crossing point should be optimized by the router. For example, a Gateway across the Strait of Gibraltar forces the route through the strait while letting the router choose the optimal crossing point and timing based on current and wind conditions.
Create a Gateway through Routing → Gateways → Create Gateway and draw a line across the passage the route must use. Like Pathways, Gateways are ordered — the route passes through them in sequence. Multiple Gateways can be combined with Pathways and Boundaries for complex constrained routing.
The power of Gateways becomes clear in multi-island routing. Instead of guessing which side of an island the route should pass and placing a Pathway waypoint there, draw a Gateway between the islands. The router evaluates all possible crossing points along the Gateway line and chooses the one that produces the fastest overall route given the forecast conditions.
Use Gateways instead of Pathway waypoints whenever the exact crossing point shouldn't be predetermined. A Pathway waypoint at a channel entrance fixes the route to one position. A Gateway across the channel entrance lets the router choose the optimal crossing point — often significantly better.
What is the key difference between a Gateway and a Pathway waypoint?
Combining Constraints for Real-World Routing
Real-world passages rarely need just one type of constraint. A coastal route from Maine to Bermuda, for example, might use Boundaries around Nantucket Shoals and Georges Bank (shallow and rough), a Pathway waypoint south of Cape Cod to stay clear of the shipping lanes, a Gateway across the Gulf Stream at its narrowest point, and wind speed limits to avoid the developing nor'easter forecast for Day 3.
The key is to layer constraints thoughtfully. Start with the minimum — just start point, end point, and wind limits. Run a routing and examine the result. If it crosses a hazard, add a Boundary. If it misses a required passage point, add a Pathway waypoint or Gateway. Each constraint you add reduces the router's freedom, so add only what's necessary and verify that each addition actually improves the route.
Iterative refinement is the professional approach. Run the route, examine it critically, add or adjust one constraint, re-run, and compare. Does the Boundary around Georges Bank add 4 hours to the route? That's probably worth it. Does the extra Pathway waypoint at Montauk add 18 hours? Maybe that constraint is too aggressive — try a Boundary instead, or move the waypoint further offshore.
Save your constraint sets for frequently sailed routes. A sailor who regularly passages from New England to the Caribbean can maintain a standard set of Boundaries and Pathway points that capture all the known hazards and required passages. Each routing then only requires fresh GRIB data and a departure time — the constraints are pre-configured.
Start with the fewest constraints possible and add more only as the route demands. Every constraint removes a degree of freedom from the optimizer. The best routes come from giving the algorithm maximum freedom while protecting against the specific known hazards along your passage.
What is the best approach to adding routing constraints?
Summary
Pathways force the route through mandatory waypoints in sequence — use the minimum number needed to keep the route in safe water without over-constraining the optimizer.
Boundaries are exclusion polygons that the router treats as impassable — draw them around shoals, TSSs, military areas, and any zone you want to avoid.
Gateways are line segments the route must cross — more flexible than Pathway waypoints for channel entrances and strait crossings because the router optimizes the crossing point.
Combine all three constraint types for complex real-world routing — Boundaries for hazards, Pathways for required waypoints, Gateways for flexible passage points.
Use iterative refinement: start with minimum constraints, examine the route, add constraints one at a time, and verify each addition improves the result.
Key Terms
- Boundary
- An exclusion polygon on the chart that the routing algorithm treats as impassable — routes cannot cross any edge of the polygon
- Gateway
- A line segment that the route must cross, giving the router freedom to optimize exactly where along the line to transit
- Iterative refinement
- The process of starting with minimum constraints, examining the route, and adding constraints one at a time to progressively improve the result
- TSS
- Traffic Separation Scheme — a shipping lane system that can be modeled as a Boundary exclusion zone to keep routes clear of commercial shipping corridors
- Constraint layering
- Combining Pathways, Boundaries, Gateways, and parameter limits to model real-world passage requirements while preserving routing optimization freedom