Definition
Mean High Water (MHW) represents the average elevation of the highest water level occurring daily during a complete tidal cycle, established through systematic observation over the National Tidal Datum Epoch of 19 years. This fundamental tidal datum serves as the legal and practical boundary between private and public land in coastal jurisdictions throughout North America and internationally. The MHW elevation is expressed relative to a local or national vertical datum, such as the North American Vertical Datum of 1988 (NAVD 88), and is critical for determining the extent of navigable waters, riparian rights, and coastal property ownership.
In surveying practice, MHW functions as a primary reference point for establishing tidal boundaries and conducting hydrographic surveys. The National Oceanic and Atmospheric Administration (NOAA), through its Center for Operational Oceanographic Products and Services (CO-OPS), maintains and publishes official MHW datums for thousands of tidal stations across the United States and territories.
Technical Details
Calculation Methodology
MHW is computed through rigorous statistical analysis of continuous or near-continuous water level observations. The calculation process involves:
1. Data Collection: Automated tide gauges record water elevations at regular intervals (typically every 6 minutes) throughout the 19-year epoch. 2. Daily Maximum Extraction: The highest water level for each tidal day is identified. 3. Averaging: These daily maxima are arithmetically averaged across the entire 19-year period. 4. Datum Establishment: The resulting mean elevation is established as the MHW datum for that location.
According to NOAA's Tidal Datum Computation Manual and aligned with IHO (International Hydrographic Organization) standards, this methodology ensures consistency and reproducibility across monitoring stations.
Tidal Datum Hierarchy
MHW occupies a specific position within the complete tidal datum framework. Related datums include:
The vertical relationship between these datums varies by location based on tidal characteristics. Surveyors must understand these relationships when transitioning between datasets or updating historical surveys.
Standards and References
The establishment and application of MHW are governed by:
Applications in Surveying
Coastal Property Boundary Determination
In most U.S. states and many international jurisdictions, MHW represents the legal boundary between private upland property and public tidal lands. Surveyors are frequently engaged to locate this boundary for property disputes, development approvals, and title clarification. Accurate MHW determination requires understanding both the current tidal datum and any historical variations that may affect deed interpretation.
Hydrographic Survey Reference
Hydrographic surveyors use MHW as the primary vertical reference for charting and navigational surveys. Soundings are typically reduced to MLLW for navigation charts, but surface features and tide-dependent navigation corridors reference MHW. Integration of [GNSS](/glossary/gnss-global-navigation-satellite-system) positioning with tidal corrections requires precise knowledge of MHW elevations and their relationship to ellipsoidal heights.
Regulatory and Environmental Assessment
Environmental agencies utilize MHW to:
Real-Time Kinematic (RTK) Surveying
When performing [RTK](/glossary/rtk-real-time-kinematic) surveys in tidal zones, surveyors must account for the current water level relative to MHW. Modern RTK systems from manufacturers like [Leica Geosystems](/companies/leica-geosystems) and [Trimble](/companies/trimble) integrate tide prediction software to provide real-time corrections and ensure vertical accuracy for tidal boundary work.
Related Concepts
Tidal Datum Conversion
Surveyors frequently encounter the need to convert between MHW and other vertical references. NOAA provides tidal bench marks and published relationships between MHW and NAVD 88 at each tidal station. Alternatively, [Total Stations](/instruments/total-station) can be used to establish local vertical control tied to permanent tidal benchmarks.
Epoch Considerations
The current 19-year epoch runs from 1983-2001 for most U.S. stations. NOAA recalculates MHW periodically to account for sea level rise and subsidence. Surveyors working with historical surveys must identify which epoch was used, as MHW may shift several tenths of feet between epochs at dynamic coastal locations.
Tidal Benchmarks
Physical monuments, typically bronze or stainless steel tablets set in bedrock or stable structures, mark the elevation of MHW and other datums. These benchmarks serve as the ground truth for verifying field observations and tying local surveys to the National Spatial Reference System (NSRS).
Practical Examples
Example 1: Waterfront Development Survey
A surveyor is tasked with locating the MHW line for a proposed marina development in Portland, Maine. The project area falls within NOAA's Portland tidal station jurisdiction. The surveyor:
1. Obtains the published MHW elevation (3.28 feet above NAVD 88) from NOAA CO-OPS 2. Establishes a local control network using [RTK GNSS](/glossary/rtk-real-time-kinematic) referenced to NAVD 88 3. Locates nearby tidal benchmarks to verify the vertical datum 4. Performs cross-shore profile surveys to identify where the topography intersects the MHW elevation 5. Marks the MHW line on the final survey plat with appropriate notation of date, epoch, and survey method
Example 2: Property Boundary Dispute Resolution
A coastal property owner disputes the tidal boundary location after a survey reveals erosion has altered the shoreline. The surveyor researches:
1. The original deed description and its reference to MHW 2. Historical surveys and photographs to establish the baseline MHW line from the date of original conveyance 3. Current MHW elevation and its relationship to the property's topography 4. Evidence of accretion or erosion affecting the boundary
This analysis determines whether the boundary moves with the natural migration of the shoreline or remains fixed at its original location—a critical distinction under common law principles.
Example 3: Hydrographic Survey Integration
A hydrographic surveyor conducting a bathymetric survey of an estuary references all depths to MLLW per chart standards, but must also provide MHW elevations for navigation warnings and shallow-draft vessel guidance. Using NOAA's tidal predictions and real-time water level data, the surveyor converts MLLW soundings to MHW-referenced depths for specific tide stages.
Frequently Asked Questions
Q: What is MHW - Mean High Water?
Mean High Water (MHW) is the average elevation of the highest daily tidal water level measured over a 19-year National Tidal Datum Epoch. It serves as the legal boundary between private upland and public tidal lands in coastal surveys and property determinations.
Q: When is MHW - Mean High Water used?
MHW is used when determining coastal property boundaries, conducting hydrographic surveys, assessing environmental jurisdiction (wetlands), designing coastal structures, and interpreting historical property deeds. Surveyors reference MHW whenever tidal boundaries or tide-dependent features must be located or documented.
Q: How accurate is MHW - Mean High Water?
MHW accuracy depends on the length and quality of tidal observations. NOAA's published MHW values at primary tidal stations typically have uncertainties of ±0.1 to 0.3 feet (±3 to 9 centimeters). Accuracy degrades at secondary stations or in areas with significant subsidence or sea level rise trends between epoch calculations.
