You’re planning a passage through a narrow channel. Your chart shows 8 feet of depth at the entrance. The tide table predicts 2.5 feet at your crossing time. Simple math says you have 10.5 feet—plenty for your 5-foot draft, right?
Not necessarily. Those two numbers use different reference points.
Understanding tidal datums transforms how you interpret tide predictions and nautical charts. These standardized reference planes ensure mariners worldwide speak the same language when discussing water depths and tide heights. More importantly, they provide the safety margin that prevents groundings in unfamiliar waters.
In this guide, you’ll learn what tidal datums are, why Mean Lower Low Water (MLLW) dominates North American navigation, how to interpret tide predictions relative to chart depths, and when datum differences matter most for your planning. Whether you’re entering new harbors, crossing bars, or navigating shallow estuaries, mastering tidal datums ensures you maintain proper clearance under your keel.
What Are Tidal Datums?
Tidal datums are standardized reference levels that define how we measure ocean heights. Think of them as the zero line on a vertical ruler—every measurement of tide height or water depth needs a defined starting point. Without these standards, a “10-foot tide” in one location couldn’t be meaningfully compared to a “10-foot tide” elsewhere.
The Fundamental Concept
A tidal datum represents an average of tidal elevations over a specific period, typically 19 years. This National Tidal Datum Epoch (NTDE) captures the full range of astronomical tidal variations, including the 18.6-year lunar nodal cycle that affects tidal ranges. The current epoch runs from 1983 to 2001, though NOAA updates these periodically as new data becomes available.
These datums serve different purposes:
Chart Datums define the zero depth on nautical charts—the reference from which all charted depths are measured. Most modern charts use a low-water datum, ensuring charted depths represent near-worst-case conditions. When your chart shows 15 feet, you know you’ll have at least that much water under normal circumstances.
Tide Predictions reference specific datums when reporting heights. A prediction of “+3.2 feet” doesn’t mean 3.2 feet of water—it means 3.2 feet above a particular datum. Understanding which datum matters for safe navigation.
Geodetic Datums connect ocean levels to land elevations, allowing integration of tide predictions with terrestrial mapping and GPS coordinates. This becomes critical when planning approaches to bridges or evaluating flood risk.
Mean Lower Low Water (MLLW): The North American Standard
Mean Lower Low Water (MLLW) dominates navigation planning along North American coasts. Understanding why this datum was chosen—and what it represents—helps you interpret both charts and tide predictions correctly.
Why MLLW Exists
Most Pacific Coast locations experience mixed tides: two high tides and two low tides per day, but with different heights. One low tide reaches lower than the other. The Gulf Coast and much of the Atlantic also see this pattern, though less pronounced.
MLLW averages only the lower of each day’s two low tides over the 19-year epoch. This creates a conservative reference plane close to the lowest water levels you’ll typically encounter. When your chart shows “10 feet at MLLW,” you’ll usually find at least 10 feet of water—even at low tide—under normal meteorological conditions.
This contrasts with Mean Low Water (MLW), which averages all low tides. In areas with mixed tides, MLW sits higher than MLLW, sometimes by a foot or more. Using MLW as a chart datum would reduce the safety margin in Pacific Coast waters.
MLLW in Practice
Open Mariner Studio on your iPhone and tap any tide station along the West Coast. The predictions you see reference MLLW. A prediction of “+5.2 feet” means the water surface will be 5.2 feet above the MLLW datum. If your chart shows a charted depth of 12 feet (also relative to MLLW), the total water depth at that tide height would be approximately 17.2 feet.
The beauty of this system: both your chart and your tide predictions use the same datum. Add the predicted tide height to the charted depth for your total depth at that time, minus your draft, giving you clearance under your keel.
One critical detail: MLLW varies geographically. The MLLW plane in San Diego differs from the MLLW plane in Seattle—each represents the local average of lower low waters. This is why you can’t transfer tide predictions from one station to another without understanding the relationship between their respective datums.
When MLLW Falls Short
MLLW provides excellent margins under normal conditions, but it’s not the absolute lowest water level possible. Several factors can create water levels below MLLW:
Meteorological Effects: Strong offshore winds, high barometric pressure, and certain storm patterns can drive water levels significantly below predicted values. During extreme high-pressure systems, I’ve observed water levels 1-2 feet below MLLW predictions.
Spring Tides: While MLLW accounts for average astronomical conditions, extreme spring tides during new and full moons may approach or slightly exceed MLLW’s lower bound—particularly during perigean spring tides when the moon is closest to Earth.
Long-Term Variations: The 18.6-year lunar nodal cycle creates slow oscillations in tidal ranges. During certain phases, low tides may consistently run slightly lower than the epoch-averaged MLLW.
For critical clearance situations—crossing bars, entering narrow channels, or transiting shoal waters with minimal margins—smart mariners add a safety factor beyond the MLLW-based calculation. A one-foot buffer addresses most meteorological variability; two feet provides substantial margin for worst-case conditions.
Other Important Tidal Datums
While MLLW dominates North American navigation, several other datums serve specific purposes or appear in international waters. Understanding these helps you interpret foreign charts, historical documents, and scientific data.
Mean Low Water (MLW)
Mean Low Water averages all low tides—both the lower low and the higher low in mixed tide areas. Along semidiurnal coasts (like much of the Atlantic, where both daily low tides reach similar levels), MLW and MLLW nearly coincide. On the Pacific Coast, MLW typically sits 0.5 to 1.5 feet above MLLW.
Many older charts and some international publications use MLW as the chart datum. When cruising foreign waters, always verify which datum your chart references. The difference matters when margins are tight.
Mean Sea Level (MSL)
Mean Sea Level represents the arithmetic mean of all hourly water heights over the tidal epoch—the true average ocean surface. MSL sits roughly midway between mean high water and mean low water, though not exactly due to tidal asymmetries.
MSL serves as the primary reference for geodetic surveying, flood mapping, and climate studies. It connects ocean heights to terrestrial elevations, allowing GPS coordinates to reference local water levels. When planning approaches under bridges, the bridge clearance is typically specified relative to a high-water datum, while your GPS position references MSL or a related geodetic datum.
In Mariner Studio, when you view a tide graph, the zero line represents MLLW, not MSL. This distinction matters when interpreting the vertical scale—positive predictions indicate water above MLLW, which may still be below MSL during lower tides.
Mean Higher High Water (MHHW)
Just as MLLW averages the lower of each day’s two low tides, MHHW averages the higher of each day’s two high tides. This datum defines clearances under bridges and power lines, and marks the typical upper limit of tidal action on shorelines.
When you see “Bridge clearance: 65 feet MHHW,” that means at the average higher high water level, you’ll have 65 feet of clearance under the bridge. During extreme spring tides or with onshore wind setup, water levels may exceed MHHW, reducing this clearance. Always check current predictions when transiting bridges with tight clearance margins.
Chart Datum (CD)
“Chart Datum” is a generic term for whatever reference plane a specific chart uses for depth measurements. On NOAA charts for U.S. waters, Chart Datum equals MLLW. British Admiralty charts traditionally used Lowest Astronomical Tide (LAT), while some nations use MLW or other local datums.
Always check your chart’s title block to confirm the chart datum. This information appears in the notes section, typically stating something like “Depths in meters below Mean Lower Low Water” or “Soundings in feet and fathoms referred to Mean Low Water Springs.”
When using electronic charts in Mariner Studio or other navigation apps, the datum information carries through from the underlying chart source—typically NOAA ENCs for U.S. waters, which use MLLW.
Lowest Astronomical Tide (LAT)
Used primarily by the United Kingdom and Commonwealth nations, LAT represents the lowest tide level that can occur under average meteorological conditions and any combination of astronomical conditions. It’s more conservative than MLLW—a lower reference plane that provides additional safety margin.
LAT accounts for the absolute lowest astronomical tides possible over the 18.6-year nodal cycle, while MLLW averages the daily lower low waters over that period. In areas with large tidal ranges, LAT may sit a foot or more below MLLW.
If you cruise internationally and encounter charts referencing LAT, treat this as an even safer chart datum than MLLW. Predicted tide heights above LAT will generally be greater than equivalent predictions above MLLW for the same physical water level.
How to Use Tidal Datums in Navigation Planning
Understanding tidal datums conceptually helps, but applying this knowledge to real passage planning prevents groundings. Here’s how to integrate datum awareness into your navigation workflow.
Calculating Available Water Depth
The fundamental equation for depth calculation:
Total Depth = Charted Depth + Predicted Tide Height − Your Draft = Under-Keel Clearance
This works reliably when chart datum and tide prediction datum match—which they do for NOAA charts and NOAA tide predictions (both use MLLW).
Example: Crossing a Shallow Bar
You’re entering Yaquina Bay, Oregon. Your chart shows 10 feet MLLW at the bar. Your boat draws 5 feet. The tide prediction shows +2.8 feet at your crossing time.
- Charted depth: 10 feet (MLLW)
- Predicted height: +2.8 feet (above MLLW)
- Total depth: 10 + 2.8 = 12.8 feet
- Less your draft: 12.8 − 5 = 7.8 feet clearance
That’s adequate clearance for calm conditions, but marginal for large swells. Smart mariners would add a safety factor (discussed below) or wait for a higher tide.
When Datums Don’t Match
Occasionally you’ll encounter situations where your tide predictions and chart use different datums. This happens with:
- Foreign charts using different datums
- Older charts not yet updated to current standards
- Mixing tide data sources with different reference systems
If your chart uses MLW but your predictions reference MLLW, you need to convert between datums. NOAA provides datum conversion tables for U.S. stations, showing the difference between various datums at each location.
For the Pacific Northwest, MLLW typically sits 0.5 to 1.5 feet below MLW. If your chart references MLW and your prediction references MLLW:
Total Depth = Charted Depth (MLW) + Predicted Height (MLLW) − Datum Difference
This correction ensures you’re comparing equivalent reference planes. When in doubt, err conservative—assume the datum difference reduces your clearance rather than adding to it.
Adding Safety Margins
Even with matching datums, smart navigators add safety margins beyond the MLLW-based calculation. Several factors warrant additional clearance:
Meteorological Conditions (add 0.5 to 1 foot): High barometric pressure and offshore winds can depress water levels below predictions. I routinely add one foot for passages during high-pressure systems.
Swell and Wave Action (add 1 to 2 feet): In bar crossings or exposed entrances, wave troughs temporarily reduce water depth. A 6-foot swell creates 3-foot troughs below the mean water surface. Add half your anticipated swell height to maintain clearance in the troughs.
Bottom Type and Accuracy (add 0.5 feet): Charted depths represent surveys conducted over many years with varying accuracy. Shifting sand, accumulated silt, or survey tolerances mean your actual depth may vary from the charted value. Add margin for uncertainty, particularly in areas noted as “approximate.”
Soft Mud and Gradient (consider 0 to 1 foot): In areas with soft mud bottoms, your keel may penetrate somewhat without causing damage. However, don’t count on this—some “mud” is rock-hard clay. If you have local knowledge suggesting a soft bottom, you might accept slightly reduced clearance, but never assume without verification.
Personal Risk Tolerance: Conservative navigators maintain minimum 2-foot clearance at all times; aggressive navigators might accept 1 foot in calm conditions with favorable bottom. Your comfort level, boat construction, and financial risk tolerance determine your minimum acceptable clearance.
Using Mariner Studio’s Tide Favorites
When planning passages to unfamiliar locations, I add critical tide stations to my Favorites in Mariner Studio. This allows quick comparison of predictions across multiple stations along a route.
For a coastal passage from Seattle to Port Townsend, I monitor:
- Seattle (primary station) for departure timing
- Point No Point for mid-passage conditions
- Port Townsend for arrival window
- Admiralty Inlet entrance for current interaction with tides
By viewing all four stations’ predictions together, I identify the optimal departure time that provides favorable conditions at each waypoint. The ability to see hourly predictions for multiple locations simultaneously—all referenced to MLLW—allows complex routing decisions without manual calculations.
Each Favorite displays the full 7-day prediction, with color-coded visualization of high and low tides. Tap any station for detailed hourly predictions, then swipe to view upcoming days. For passage planning, I often screenshot these predictions and mark critical waypoint transit times for quick reference underway.
Planning Around Datum Uncertainty
In some coastal areas—particularly estuaries, bays, and inland waters distant from primary tide stations—datum elevations may be less precisely defined. NOAA marks these stations as “subordinate,” providing time and height adjustments relative to a primary reference station.
When using subordinate station predictions:
- Understand they’re less accurate than primary station data
- Add extra safety margin for prediction uncertainty
- Recognize local factors (wind, river flow, barometric pressure) may have magnified effects
- Use multiple subordinate stations to triangulate conditions
For critical clearance situations, time your passage based on the nearest primary station rather than relying solely on a distant subordinate station’s adjusted predictions. The additional conservatism provides peace of mind when margins are tight.
Real-World Applications
Understanding tidal datums becomes critical in these common scenarios:
Bar Crossings
River and harbor entrance bars often show minimal charted depths—sometimes only 6 to 10 feet MLLW. Combined with swell and wave action, these entrances demand precise tide timing and datum understanding.
Columbia River Bar Example:
The Columbia River bar is North America’s most dangerous, with numerous groundings and capsizings. Charts show highly variable depths across different transit routes, with some channels marked as shallow as 8 feet MLLW.
A skipper planning a bar crossing checks tide predictions:
- Predicted height: +5.2 feet (above MLLW) at planned transit time
- Charted depth at entrance: 10 feet MLLW
- Total depth: 15.2 feet
- Boat draft: 5 feet
- Calculated clearance: 10.2 feet
But here’s where experience matters: the Columbia River bar regularly sees 8 to 12-foot swells. Those swells create troughs 4 to 6 feet below the mean water surface. Suddenly the 10.2-foot clearance becomes 4 to 6 feet in wave troughs—completely inadequate.
Smart skippers add the swell height to their clearance requirement: with 10-foot swells, they want at least 15 feet of clearance to maintain 5 feet in the troughs. This means waiting for a predicted height of +10 feet or more—possible only during higher high tides.
Understanding that both the chart and predictions reference MLLW allows this calculation. If they referenced different datums, an additional conversion would be necessary.
Shallow Harbor Navigation
Many popular cruising destinations—like the Bahamas, parts of the Florida Keys, and numerous Pacific Northwest anchorages—have charted depths of only 6 to 12 feet MLLW. For boats drawing 4 to 6 feet, tidal timing becomes essential.
In these areas, skippers often plan movements around higher tides, arriving during rising water rather than falling. The difference between +2 feet and +4 feet can determine whether you clear a bar or wait another 6 hours.
Mariner Studio’s hourly predictions let you identify optimal transit windows. For a harbor with 8 feet charted depth and a 5-foot draft boat, you need at least 5 feet of tide to maintain minimal clearance. Scanning the 7-day prediction quickly identifies which days offer suitable high tides—particularly important in neap tide periods when tidal ranges are reduced.
Bridge and Overhead Clearance
While less common, tidal datums also affect vertical clearance under bridges. Bridge clearances are specified relative to MHHW—the opposite end of the tidal range from MLLW.
A bridge marked “clearance 40 feet MHHW” provides 40 feet of vertical space when the water is at Mean Higher High Water. During lower tides (including MLLW), the water surface drops, increasing clearance. During extreme spring tides or with meteorological setup, water may exceed MHHW, reducing clearance below the stated value.
For masted sailboats navigating fixed bridges, calculate clearance accounting for current tide height relative to MHHW:
Available Clearance = Stated Clearance + (MHHW − Current Height)
If the bridge offers 50 feet at MHHW, and the current tide is −1.5 feet relative to MHHW (i.e., 1.5 feet below MHHW), your available clearance is 50 + 1.5 = 51.5 feet.
Most bridge clearances are generous, but in areas with extreme tides (like Puget Sound’s 10+ foot ranges), the difference between MHHW and MLLW can be substantial. Sail through during low tide for maximum clearance.
Drying Heights and Uncovering Features
Some chart features—rocks, reefs, mudflats—are marked with drying heights above chart datum. A notation like “(+2)” or “dries 2 feet” means that feature rises 2 feet above MLLW.
When the predicted tide is below +2 feet, that feature is exposed. When predictions exceed +2 feet, it’s submerged—and potentially hazardous. Understanding that these drying heights reference MLLW helps you identify when such features become obstacles.
Drying grids (extensive mudflats that uncover at low tide) are common in areas like Alaska, the Pacific Northwest, and parts of New England. Anchorages in these areas may offer ample depth at high tide but completely dry out at low tide. Check drying heights before anchoring to ensure your boat remains afloat throughout the tidal cycle.
Common Questions About Tidal Datums
Q: Why don’t all countries use the same tidal datum?
Different datums evolved from different navigational traditions and local tidal characteristics. The United Kingdom developed LAT for maximum safety margin in areas with large tidal ranges. The United States adopted MLLW as a practical compromise providing good safety margins while remaining close to typical low-water conditions. Countries with primarily semidiurnal tides (where both daily low tides are similar) often use MLW since the distinction between MLW and MLLW is minimal. International efforts are underway to standardize datums, but complete global harmonization remains years away. When cruising internationally, always verify your chart’s datum.
Q: How often do tidal datums get updated?
NOAA updates the National Tidal Datum Epoch approximately every 20-30 years to account for long-term sea level changes and improved measurement techniques. The current epoch (1983-2001) will eventually be superseded by a newer 19-year period. When this happens, all charted depths and tide predictions reference the new datum values. In practice, these updates cause minimal change in most locations—typically inches rather than feet—but in areas with rapid sea level change, the shifts can be more significant. Updated charts note their datum epoch in the title block.
Q: If MLLW is an average, can water levels go lower?
Yes. MLLW represents the average of lower low waters over 19 years, but individual low tides can—and do—fall below this level. Extreme spring tides during perigee (when the moon is closest to Earth) may slightly exceed MLLW’s bounds. More commonly, meteorological factors depress water levels: high barometric pressure, strong offshore winds, and certain atmospheric setups can drive water 1-2 feet below predicted values. This is why conservative navigators add safety margins beyond the MLLW-based calculation, particularly during high-pressure systems or offshore wind conditions.
Q: What does a negative tide prediction mean?
A negative tide prediction indicates the water surface will be below MLLW at that time. For example, “−0.8 feet” means the water will be 0.8 feet lower than the MLLW datum. This occurs during extreme low tides, particularly spring tides combined with favorable meteorological conditions. Negative predictions are common in areas with large tidal ranges and significantly reduce the water depth available. For navigation planning, subtract the absolute value of negative predictions from charted depths: if the chart shows 10 feet MLLW and the prediction is −1.2 feet, your total depth is only 8.8 feet—potentially critical information for boats with deeper drafts.
Q: Can I compare tide predictions from different stations directly?
Only if both stations reference the same datum and you account for the different tidal characteristics of each location. Two stations both using MLLW allow direct comparison of predicted heights, but remember that MLLW itself represents a different physical elevation at each station. A +5-foot tide in San Diego is not the same absolute ocean height as a +5-foot tide in San Francisco—each is 5 feet above its local MLLW datum. For navigation, what matters is the relationship between the predicted height and the charted depth at your specific location, not absolute elevation comparisons between stations. Never apply one station’s predictions to a different location’s chart without proper secondary station corrections.
The Science Behind Tidal Datums
Understanding why tidal datums are calculated the way they are deepens your appreciation for their practical value.
The 19-Year Tidal Epoch
Why 19 years specifically? This period captures the major astronomical cycles affecting tides:
The 18.6-year lunar nodal cycle creates slow variations in tidal ranges. The moon’s orbit is tilted relative to Earth’s equator, and this tilt oscillates over 18.6 years. During maximum tilt, tidal ranges increase; during minimum tilt, ranges decrease. A 19-year epoch captures this full cycle plus additional margin.
The 8.85-year lunar apsidal cycle affects how close the moon approaches Earth, influencing tidal strength. Perigean tides (occurring when the moon is closest) are stronger than apogean tides (when most distant).
The 4.53-year lunar phase cycle combines the synodic month (29.53 days between new moons) with the anomalistic month (27.55 days between perigees). When new/full moons coincide with lunar perigee, you get perigean spring tides—the year’s highest tides.
By averaging over 19 years, the tidal epoch smooths out these variations, creating datums that represent typical conditions rather than extremes. This provides a stable reference despite the continuous variation in astronomical forcing.
Computing Datums from Observations
NOAA calculates tidal datums from water level gauges that record ocean height every six minutes. Over 19 years, that’s approximately 1.7 million measurements per station. Sophisticated algorithms analyze these observations to:
- Identify and remove outliers caused by storms or tsunamis
- Apply harmonic analysis to separate tidal constituents
- Filter meteorological effects from astronomical patterns
- Calculate mean values for various datum definitions
- Determine relationships between different datums
The result: a precise definition of MLLW, MHHW, MSL, and other datums specific to that location. These values are published in NOAA’s tide tables and form the basis for both tide predictions and chart depth references.
Why Local Datums Vary
MLLW in San Diego is not the same absolute elevation as MLLW in Seattle. Each represents a local average because tidal characteristics vary geographically:
Tidal Range: Areas with larger tidal ranges (like the Pacific Northwest) have greater separation between high and low water datums than areas with smaller ranges (like the Gulf Coast).
Tidal Type: Mixed tide areas (with unequal daily high and low tides) show greater difference between MLLW and MLW than semidiurnal areas (with two similar low tides daily).
Bathymetry: Underwater topography affects tidal amplitudes and asymmetries, influencing how high and low waters average out.
Geographic Features: Bays, estuaries, and inland waters may experience amplified or dampened tides compared to the open coast, shifting datum levels.
This geographic variation in datums is why you can’t transfer predictions from one station to another. Each station’s predictions must reference its own local datums, derived from observations at that specific location.
Related Features & Learning
Understanding tidal datums enhances your use of several Mariner Studio features and connects to broader navigation knowledge:
Mastering Tide Predictions with Favorites – Learn how to monitor multiple tide stations simultaneously, all referencing MLLW for consistent planning across your route.
Spring vs Neap Tides: Planning Your Passages – Discover how spring and neap cycles affect tide ranges relative to MLLW, and why spring tides create both your best and worst clearance conditions.
How to Read Tide Tables Like a Pro – Master the art of interpreting NOAA tide tables, including datum references, secondary station corrections, and how to apply predictions to navigation.
Safe Bar Crossing: A Complete Guide – Apply your datum knowledge to one of the most critical navigation decisions: timing bar crossings with adequate depth and favorable conditions.
Understanding Chart Symbols and Depths – Learn how charted depths reference datums, what drying heights mean, and how to interpret the full range of depth information on nautical charts.
Conclusion
Tidal datums transform from abstract reference planes to essential navigation tools once you understand their practical application. MLLW’s role as both chart datum and tide prediction reference creates a consistent framework for depth calculations—as long as you remember both measurements use the same zero point.
The next time you plan a passage through shallow waters, pause before doing the simple math. Verify your chart datum matches your prediction datum. Add appropriate safety margins for meteorological conditions, swell, and uncertainty. Consider whether negative tides or drying heights affect your route. And remember: MLLW provides excellent clearance under normal conditions, but it’s not an absolute minimum—conditions can and do push water levels lower.
Open Mariner Studio and examine your favorite tide stations with fresh eyes. That “+3.2 feet” prediction now carries more meaning—you know it’s 3.2 feet above the carefully calculated average of 19 years of lower low waters at that exact location. You understand how to add it to charted depths for total water depth. You appreciate the conservative safety margin built into the system, and you know when to add even more margin for challenging conditions.
Tidal datum knowledge doesn’t just prevent groundings—it opens new cruising opportunities by giving you confidence to explore waters you might have avoided, knowing precisely when adequate depth exists and when it doesn’t.
Key Takeaway
Tidal datums provide the reference framework for all depth calculations in navigation. Understanding that MLLW serves as both chart datum and tide prediction reference for North American waters allows you to accurately calculate available water depth by adding predicted tide heights to charted depths. This knowledge, combined with appropriate safety margins, forms the foundation of safe shallow-water navigation.
Have questions about tidal datums or specific situations where datum interpretation gets complex? Share your experiences in the comments below—datum calculations can be tricky, and we learn from each other’s real-world examples.
Download Mariner Studio for iOS to access NOAA tide predictions, monitor multiple stations with Favorites, and plan your passages with confidence. All tide predictions in Mariner Studio reference MLLW for consistent depth calculations.