Saona Island Coastal Vertical Datums: Mean Sea Level, Chart Datum and Elevation Benchmarks

Introduction

Coastal surveying involves measuring both land elevations and underwater depths. These measurements are essential for preparing maps, designing coastal structures, conducting hydrographic surveys and understanding changes in shoreline conditions.

However, measuring a height is not simply a matter of recording a number. Surveyors must also identify the reference level from which that height is measured.

This reference is called a vertical datum.

For example, a coastal survey might report the elevation of a beach, while a hydrographic survey reports the depth of nearby seawater. Both measurements describe vertical positions, but they may use different reference surfaces.

If these measurements are combined without understanding their reference systems, the resulting map or engineering calculation may contain significant errors.

For Saona Island in the Dominican Republic, understanding coastal vertical datums is relevant to potential research involving shoreline mapping, beach elevation surveys, bathymetry, coastal infrastructure and sea-level monitoring.

This article explains the main vertical reference systems, how surveyors connect them and how a reliable coastal elevation framework could be developed for the island. It does not claim that a particular local vertical datum or benchmark network has already been established.

1. What Is a Vertical Datum?

A vertical datum is a defined reference surface used to express elevations or depths.

It provides a common starting point for vertical measurements.

For example:

• A beach elevation might be reported as 2.5 metres above a specified datum.
• A seabed location might be recorded as 8 metres below a chart datum.
• A coastal structure might have its foundation elevation referenced to a national geodetic vertical system.

Without a consistent reference, the numbers cannot be compared reliably.

A vertical datum is different from a horizontal datum. A horizontal datum defines the reference framework for positions such as latitude and longitude, while a vertical datum defines the reference for heights and depths.

NOAA explains that geodetic vertical datums are used to express land elevations, while water-level datums provide reference levels for marine measurements and nautical chart depths.

2. Main Types of Coastal Vertical Datums

Different vertical reference systems serve different purposes.

2.1 Mean Sea Level (MSL)

Mean Sea Level is an average sea-surface level calculated from water-level observations over a specified period.

It is often used as a reference in coastal mapping and land-elevation studies.

However, mean sea level is not a perfectly uniform surface around the world. Local sea level can vary because of ocean circulation, atmospheric conditions, tides, water density, coastal geography and vertical land movement.

The observation period must also be identified. A short-term mean sea level and a long-term mean sea level are not necessarily equivalent.

2.2 Mean High Water (MHW)

Mean High Water represents the average of observed high-water levels over a defined tidal datum epoch.

It can be useful for shoreline-related mapping and coastal management.

However, it should not automatically be treated as the exact boundary of the land and sea at every moment.

2.3 Mean Low Water (MLW)

Mean Low Water represents the average of low-water levels over a defined observation period.

It can help describe the recurring lower portion of the tidal range.

2.4 Mean Lower Low Water (MLLW)

Mean Lower Low Water is the average of the lower low-water levels during each tidal day over the specified datum epoch.

It is used as a chart reference in some countries, including the United States.

The chart datum selected for a particular region must be confirmed from the relevant hydrographic authority. MLLW should not automatically be assumed to be the official chart datum around Saona Island.

2.5 Mean Higher High Water (MHHW)

Mean Higher High Water represents the average of the higher high-water levels during each tidal day over a defined period.

It can be useful for describing tidal shoreline conditions and certain coastal mapping applications.

2.6 Chart Datum

Chart Datum is the vertical reference surface to which depths and drying heights on a nautical chart are referred.

It is selected according to hydrographic practice and the needs of safe navigation.

A chart datum is generally selected to provide a conservative reference for charted water depths, but its exact definition differs between regions.

For a Saona Island hydrographic project, the appropriate chart datum would need to be established from authoritative nautical chart information and local hydrographic requirements.

3. Geodetic Vertical Datums and Ellipsoidal Heights

Not all vertical reference systems are based on observed tides.

Geodetic surveying uses reference systems connected to the shape and gravity field of the Earth.

Two important concepts are orthometric height and ellipsoidal height.

Orthometric height

Orthometric height is the height of a point above a reference geoid, approximately corresponding to mean sea level.

It is commonly used in engineering, topographic mapping and construction surveying.

Ellipsoidal height

Ellipsoidal height is the distance measured along the normal to a mathematical reference ellipsoid.

GNSS equipment, including high-precision GPS receivers, commonly provides positions referenced to a geodetic reference frame and ellipsoidal heights.

An ellipsoidal height is not automatically the same as an elevation above mean sea level.

Geoid undulation

The geoid is a gravity-related reference surface used in physical geodesy.

The separation between the geoid and the reference ellipsoid is called geoid undulation.

A commonly used relationship is:

h = H + N

Where:

• h = ellipsoidal height
• H = orthometric height
• N = geoid undulation

Therefore:

H = h − N

This relationship allows surveyors to convert ellipsoidal heights into orthometric heights when a suitable geoid model and compatible reference systems are available.

The geoid model, coordinate reference frame and sign convention must be checked before applying the equation.

4. Why Are Coastal Vertical Datums Different?

A common misunderstanding is that mean sea level, chart datum and a geodetic vertical datum are interchangeable.

They are not.

Each represents a different reference concept.

Tidal datums are based on observed water-level behaviour over a defined period.

Geodetic vertical datums are reference systems used to express heights consistently across a larger geographic region.

Chart datums are selected for hydrographic charting and navigation.

Their relationships vary geographically.

NOAA explains that the relationship between tidal and geodetic datums depends on factors such as tidal range, bathymetry, topography, geoid variations and vertical land movement.

Therefore, a datum relationship established at one tide station should not simply be transferred to a distant coastline without appropriate analysis.

This is particularly important for island environments, where local coastal geometry and nearby water depths may influence tidal reference levels.

5. What Is a Tidal Benchmark?

A tidal benchmark is a permanent physical reference point with a documented elevation relationship to a nearby tide station or tidal datum.

It allows water-level measurements to be connected to stable land-based survey control.

A benchmark might be installed as a durable metal mark set into a suitable concrete structure or stable rock surface.

Its position and elevation should be documented so that future survey teams can recover and verify it.

Main purposes of tidal benchmarks

• Establish a consistent reference for water-level measurements.
• Connect tide-gauge readings to land elevations.
• Detect possible movement of monitoring equipment.
• Support long-term sea-level observation.
• Connect tidal and geodetic reference systems.

Multiple benchmarks are important because a single mark may be damaged, disturbed or displaced.

NOAA recommends connecting tidal benchmarks to more than one geodetic benchmark when establishing a reliable relationship between tidal and geodetic reference systems.

6. Connecting Land Elevations with Marine Depths

One of the main challenges in coastal engineering is combining topographic and bathymetric measurements.

Topographic surveys describe land elevations.

Bathymetric surveys describe underwater depths.

The two datasets may have different vertical references.

For example, a topographic survey could use an orthometric datum, while a nautical chart uses a local chart datum.

If the datasets are joined without a correct transformation, the seabed and beach may appear at incorrect relative elevations.

This can affect:

• Coastal terrain models.
• Shoreline mapping.
• Beach-profile analysis.
• Dredging calculations.
• Coastal flood modelling.
• Marine construction design.
• Navigation-related depth interpretation.

The International Hydrographic Organization's S-44 standard states that vertical reference frames must be specified and that connections between chart and land-survey vertical datums must be clearly determined and described.

7. How Are Vertical Datums Connected?

Surveyors use a combination of water-level observations and geodetic measurements to establish relationships between vertical reference systems.

Step 1: Establish a local water-level station

A tide gauge records water levels at a suitable coastal location.

The instrument should be connected to a stable local reference system.

Step 2: Install permanent benchmarks

Suitable physical marks are established near the monitoring station.

Their positions and elevations must be recorded and protected.

Step 3: Measure benchmark elevations

Precise differential levelling or suitable GNSS surveying can be used to connect the benchmark network to geodetic control.

Step 4: Observe tidal water levels

A sufficiently long and reliable water-level record is required to estimate relevant tidal datums.

The observation period should match the intended datum definition and accepted methodology.

Step 5: Establish the datum relationship

The vertical difference between the local tidal datum and the geodetic reference is calculated using documented observations and survey measurements.

Step 6: Verify the connection

Independent benchmarks and repeated measurements help identify unstable marks and measurement errors.

Step 7: Document the results

The project should publish the datum name, reference epoch, benchmark descriptions, elevation values, uncertainties and transformation method.

This documentation allows future researchers to use the reference system correctly.

8. The Role of GNSS in Coastal Vertical Control

Global Navigation Satellite Systems, including GPS, are widely used for establishing survey positions.

High-precision GNSS methods can determine ellipsoidal heights relative to a specified geodetic reference frame.

These measurements can then be related to orthometric heights using an appropriate geoid model.

However, GNSS does not directly provide every vertical datum needed for coastal surveying.

A surveyor must identify:

• The GNSS reference frame.
• The coordinate epoch, where relevant.
• The ellipsoidal height reference.
• The geoid model.
• The required orthometric or tidal datum.
• The transformation uncertainty.

For accurate coastal work, GNSS measurements should be supported by suitable control surveys and documented vertical transformations.

9. Vertical Datum Transformations in Coastal Mapping

Vertical datum transformation is the process of converting elevations or depths from one reference surface to another.

For example, a project may need to combine:

• Beach elevations referenced to an orthometric datum.
• Water levels referenced to a tidal datum.
• Seabed depths referenced to a chart datum.
• GNSS observations referenced to an ellipsoid.

A transformation model can provide the necessary height differences where it has been developed and validated for the relevant region.

NOAA's VDatum is an example of a system designed to transform geospatial data between tidal, orthometric and ellipsoidal vertical datums.

Its coverage and supported transformations are region-specific. It should not be assumed that every transformation is available for Saona Island or the Dominican Republic.

For a local project, the relevant national geodetic and hydrographic authorities should be consulted to identify approved reference systems and transformation information.

10. Potential Applications Around Saona Island

A properly documented vertical reference framework could support several types of research and engineering work around Saona Island.

Coastal topographic mapping

Surveyors could record beach elevations, coastal slopes and landform changes using a consistent vertical reference.

Bathymetric surveying

A known chart datum connection would help researchers report underwater depths in a consistent and traceable way.

Beach-profile monitoring

Repeated surveys could compare beach elevations over time.

This would help distinguish actual changes in beach shape from differences caused by inconsistent survey references.

Coastal flood modelling

Flood models need compatible land elevations and water-level information.

A reliable vertical datum connection helps avoid artificial height differences between terrain data and water-level inputs.

Coastal infrastructure design

Designers of coastal structures need consistent elevation information for foundations, platform levels, drainage, access routes and other engineering components.

Sea-level monitoring

Long-term water-level observations become more useful when the tide gauge is connected to stable land-based benchmarks.

This connection helps researchers investigate whether observed water-level changes are associated with changes in the sea surface, movement of the land or both.

These are potential applications, not claims that a completed local benchmark network currently exists.

11. Common Vertical Datum Errors in Coastal Projects

Incorrect datum handling can create serious problems in surveying and engineering.

Mixing unrelated reference systems

Using beach elevations from one vertical datum and seabed depths from another without transformation can produce an inconsistent terrain model.

Assuming mean sea level is universal

Mean sea level is location-dependent and changes over time.

A value from a distant station should not be treated as a local reference without an appropriate connection.

Ignoring benchmark movement

A benchmark may be damaged or physically displaced.

Repeated checks are necessary to confirm its stability.

Using an unsuitable geoid model

A geoid model that is not compatible with the reference frame or geographic region may produce incorrect orthometric heights.

Failing to document the datum epoch

Tidal datums are based on specified observation periods.

The epoch must be recorded so that the reference can be interpreted correctly.

Omitting uncertainty

Every measurement and transformation has uncertainty.

Survey reports should state the relevant uncertainty rather than presenting all elevations as exact values.

12. A Proposed Vertical Datum Framework for Saona Island

A future coastal survey programme could establish a systematic vertical reference framework through the following stages.

Stage 1: Review existing reference information

Identify available national geodetic control, hydrographic charts, tide-gauge information and published vertical reference documentation.

Stage 2: Select suitable monitoring locations

Choose locations where stable benchmarks and water-level monitoring equipment can be maintained.

Stage 3: Establish benchmark control

Install or verify permanent survey marks and connect them through precise levelling and GNSS observations.

Stage 4: Collect water-level data

Record water levels with reliable instruments and accurate timing.

Stage 5: Estimate relevant tidal datums

Use an appropriate observation period and validated analysis methods to establish the tidal reference levels required for the project.

Stage 6: Connect tidal and geodetic references

Determine the vertical relationship through documented surveying and water-level observations.

Stage 7: Develop transformation documentation

Prepare clear information describing the reference systems, benchmark elevations, transformation values and uncertainties.

Stage 8: Validate the framework

Conduct independent checks and repeat observations to identify inconsistencies or benchmark movement.

Stage 9: Apply the framework to mapping

Use the validated reference system for topographic surveys, bathymetric surveys, coastal modelling and infrastructure studies.

13. The Importance of Vertical Datum Consistency in Future Coastal Research

As coastal mapping becomes more integrated, researchers increasingly combine data from different instruments and platforms.

A single project may use satellite observations, GNSS surveys, drone-based elevation models, tide gauges, multibeam sonar and coastal numerical models.

These datasets are only directly comparable when their horizontal and vertical reference systems are properly understood.

A consistent vertical datum framework can improve:

• Data integration.
• Survey repeatability.
• Coastal elevation comparisons.
• Engineering design reliability.
• Long-term environmental monitoring.
• Interpretation of sea-level change.

For Saona Island, establishing a reliable relationship between land elevations and marine depths would be a valuable foundation for future coastal research.

Frequently Asked Questions

1. What is a coastal vertical datum?

It is a reference surface used to express land elevations and water depths in a consistent way.

2. Is mean sea level the same as chart datum?

No. Mean sea level is an average water-level reference, while chart datum is the reference surface used for charted depths and drying heights.

3. What is the difference between a tidal datum and a geodetic datum?

A tidal datum is defined from observed water-level behaviour. A geodetic vertical datum provides a reference framework for land elevations.

4. What is a tidal benchmark?

It is a permanent physical survey mark with a documented elevation relationship to a tide station or tidal datum.

5. Can GPS measure elevation above mean sea level directly?

GNSS commonly provides ellipsoidal heights. Converting these to orthometric or tidal heights requires suitable geoid information and datum transformations.

6. Why are vertical datums important in bathymetric surveying?

They provide the reference level for underwater depth measurements and allow bathymetric data to be compared with land elevation surveys.

7. Does Saona Island have a verified local vertical datum network?

This article does not establish that such a network exists. Local benchmarks, water-level observations and authoritative survey records would need to be investigated.

8. What happens if two surveys use different vertical datums?

Their elevation and depth values may not align. A validated transformation is needed before the datasets can be combined reliably.

Conclusion

Coastal vertical datums provide the reference framework needed to measure land elevations, sea levels and underwater depths consistently. Mean sea level, tidal datums, chart datums, orthometric heights and ellipsoidal heights each serve different purposes.

For Saona Island, a future vertical control project could connect coastal water-level observations with stable geodetic benchmarks. This would help researchers combine topographic and bathymetric data, improve coastal mapping and support engineering and environmental studies.

The most important principle is simple: every elevation and depth measurement must have a clearly identified vertical reference. Accurate surveying depends not only on the instrument used, but also on knowing exactly where zero is.