Understanding Tide Overlays
The tide overlay shows predicted tide heights along your route at your chosen departure time. This helps you assess water depths and identify areas where low tides could reduce clearance under your keel.
What You See
Section titled “What You See”When the tide overlay is active, the route line on the map is color-coded based on predicted tide heights at each point. The color mapping shows:
- Deeper blues — higher tide levels, meaning more water depth
- Lighter shades — lower tide levels, meaning less water depth
A legend on the map shows the exact color-to-height mapping for the current predictions.
How It Works
Section titled “How It Works”BoatRoutes uses NOAA tide prediction data from nearby tide stations to estimate the tide height at each point along your route. Here is the process:
- Station selection — For each segment of the route, we identify the nearest NOAA tide station
- Time calculation — Based on your departure time and boat speed, we estimate when you will reach each point along the route
- Height prediction — We look up the predicted tide height at the nearest station for that estimated arrival time
- Interpolation — Between stations, we interpolate to create a smooth prediction along the route
Reading Tide Heights
Section titled “Reading Tide Heights”Tide heights in US waters are referenced to MLLW (Mean Lower Low Water), which is the average of the lower of the two daily low tides. This means:
- A tide height of 0.0 ft is an average low tide — charted depths assume this water level
- Positive values (e.g., +8.2 ft) mean there is more water than charted depths indicate
- Negative values (e.g., -1.5 ft) mean there is less water than charted depths — extra caution is needed
In Canadian waters, heights are referenced to Chart Datum, which is similar to MLLW.
When Tides Matter Most
Section titled “When Tides Matter Most”Tide heights are most important when:
- Navigating shallow areas — Harbors, river mouths, and channels with charted depths under 10 feet
- Crossing bars — River and harbor entrances where shoaling reduces depths
- Anchoring — Ensuring adequate depth at low tide, plus enough scope for your anchor rode
- Entering marinas — Some marina basins have draft restrictions at low tide
- Always add your boat’s draft to the minimum depth you need. If your boat draws 5 feet and the chart shows 8 feet at MLLW, a -2 ft tide would leave only 1 foot of clearance.
- In the Pacific Northwest, the largest tide ranges occur during spring tides (around new and full moons). Tidal ranges of 12-14 feet are common in parts of Puget Sound.
- Change your departure time to see how tide levels shift. The difference between high and low tide can be dramatic — often 10 feet or more in our coverage area.
- Use the tide overlay in combination with the chart base map, which shows depth soundings.
Related
Section titled “Related”- Current Overlays
- Tide & Current Stations
- Marine Navigation Glossary — see entries for MLLW, Datum, Ebb, Flood