Saona Island Coastal Photogrammetry: 3D Beach Mapping, Drone Surveys and Shoreline Monitoring

Introduction

Coastal landscapes are constantly changing. Waves, tides, wind, rainfall and sediment movement can gradually reshape beaches and shorelines. Understanding these changes requires reliable information about the shape and elevation of the coastal surface.

On Saona Island in the Dominican Republic, coastal photogrammetry could provide a practical research method for documenting exposed beaches, coastal landforms and accessible shoreline areas. By taking overlapping photographs from a drone or another camera platform, surveyors can reconstruct three-dimensional surfaces and produce detailed coastal maps.

The method is known as Structure-from-Motion (SfM) photogrammetry. It uses computer vision and photogrammetric geometry to identify matching features in multiple photographs and calculate their spatial positions.

Unlike laser-based LiDAR, photogrammetry relies on image information rather than measuring the travel time of laser pulses. This makes it a useful complementary approach for coastal surveying, especially where repeated visual documentation and high-resolution surface models are required.

This guide explains the science, equipment, field workflow, data products, accuracy considerations and possible applications of coastal photogrammetry around Saona Island.

1. What Is Coastal Photogrammetry?

Photogrammetry is the science of obtaining measurements and spatial information from photographs.

A single photograph records a two-dimensional view. However, when the same object or landscape is photographed from multiple positions, overlapping images contain information about its three-dimensional geometry.

Specialised software identifies matching visual features across the photographs. It then estimates camera positions and orientations and reconstructs the relative three-dimensional arrangement of the observed surface.

In coastal applications, photographs may be collected using:

• Small unmanned aerial systems, commonly called drones.
• Aircraft-mounted cameras.
• Ground-based cameras.
• Handheld cameras.
• Fixed coastal camera systems.

The resulting data can be used to produce orthomosaics, three-dimensional point clouds, digital surface models and elevation maps.

NOAA and USGS describe photogrammetry and Structure-from-Motion as useful methods for producing three-dimensional models and coastal mapping products from overlapping imagery.

2. Understanding Structure-from-Motion (SfM)

Structure-from-Motion is a photogrammetric technique that reconstructs three-dimensional geometry from multiple images captured from different viewpoints.

The basic principle is straightforward: a feature visible in several photographs appears at slightly different positions within each image because the camera has moved.

By analysing these differences, software estimates the camera positions and the three-dimensional location of matching features.

The main processing stages are:

Feature detection

Software identifies distinctive image features, such as corners, edges, textures and patterns.

Feature matching

The same features are located in multiple photographs. Reliable matching is essential because incorrect matches can distort the reconstruction.

Camera alignment

The software estimates the relative position and orientation of each camera when the photographs were taken.

Sparse point-cloud generation

The initial reconstruction creates a collection of three-dimensional points representing matched features.

Dense reconstruction

Additional image information is used to generate a denser representation of the visible surface.

Georeferencing

Ground control points or suitable positioning information are used to place the reconstructed model into a real-world coordinate system.

The final model can then be used for mapping, measurement and comparison with later surveys.

The USGS coastal imagery processing workflow provides a detailed example of how SfM can be used to create coastal 3D products from aerial photographs.

3. Why Is Photogrammetry Useful for Beach Surveys?

Beaches are dynamic environments. Their surfaces may change after storms, strong waves, seasonal weather patterns or human activities.

Traditional surveying methods can provide accurate measurements, but covering a large beach repeatedly may require considerable field time.

Drone photogrammetry offers several potential advantages:

• Rapid image collection over accessible coastal areas.
• Detailed visual documentation of beach surfaces.
• Production of orthomosaic maps and elevation models.
• Repeat surveys using a consistent flight plan.
• Identification of visible erosion and deposition patterns.
• Integration with GIS and other coastal datasets.

The method is particularly useful when researchers need to document how the visible land surface changes over time.

A drone survey does not automatically provide a complete underwater terrain model. Ordinary aerial photogrammetry is primarily suited to visible surfaces. Underwater reconstruction requires suitable optical conditions and specialised image acquisition and processing.

4. Essential Equipment for a Coastal Drone Survey

A photogrammetry project needs more than a drone and a camera. The quality of the final model depends on the complete survey system.

Drone platform

A multirotor drone can be useful for small and medium-sized coastal survey areas because it can follow planned flight paths and maintain relatively low survey speeds.

Fixed-wing platforms may be more suitable for larger areas where longer flight endurance is important.

Camera

The camera should produce sharp, high-resolution images with consistent exposure and reliable image metadata.

Important characteristics include:

• Sensor resolution.
• Lens focal length.
• Shutter performance.
• Image sharpness.
• Exposure control.
• Lens distortion characteristics.

GNSS equipment

A survey-grade Global Navigation Satellite System receiver can be used to measure ground control points and independent checkpoints.

Ground control targets

Clearly visible targets can be placed at surveyed locations. These help connect the image reconstruction to known real-world coordinates.

Processing computer and software

Photogrammetry software processes the photographs, estimates camera geometry, creates point clouds and generates mapping products.

Field accessories

Additional equipment may include spare batteries, memory cards, measuring tools, survey markers, safety equipment and weather-monitoring instruments.

5. Planning the Drone Flight

A successful survey begins with a carefully prepared flight plan.

Define the survey boundary

The project team first defines the area that needs to be mapped. This may include the dry beach, dunes, coastal vegetation and other visible landforms.

Select flight altitude

Flight altitude affects ground sampling distance, image coverage and the level of detail captured by the camera.

Lower flights generally capture more detail over a smaller area, while higher flights cover larger areas with less image detail.

The correct altitude depends on the required output resolution, camera specifications, terrain and applicable flight restrictions.

Set image overlap

Photogrammetry requires sufficient overlap between consecutive photographs and adjacent flight lines.

Forward overlap refers to the shared area between successive images along a flight line. Side overlap refers to the shared area between neighbouring flight lines.

Adequate overlap allows the software to identify common features from multiple viewpoints. The exact percentages should be selected according to terrain, camera and project requirements rather than treated as universal values.

Plan lighting and weather conditions

Strong shadows, rapidly changing cloud cover, haze and moving vegetation can complicate image matching.

Consistent lighting and suitable wind conditions help produce more reliable image sets.

Check operational requirements

Before any flight, the project team must check the applicable aviation rules, permissions, protected-area restrictions and safety requirements. Flight operations should not disturb wildlife, visitors or sensitive habitats.

6. Ground Control Points and Survey Accuracy

Ground Control Points (GCPs) are surveyed locations that can be identified clearly in the photographs.

They help constrain the photogrammetric model to known real-world coordinates.

A typical GCP workflow includes:

1. Select stable and visible locations.
2. Place suitable targets where permitted.
3. Measure target coordinates using survey-grade GNSS or another suitable surveying method.
4. Record the coordinate reference system and measurement information.
5. Identify the same targets in the photographs.
6. Use the points during photogrammetric processing.

The distribution of control points matters. A group of points concentrated in one corner of the survey may not adequately control the entire model.

Independent checkpoints should also be measured. These are not used to fit the model; instead, they provide a separate way to evaluate its accuracy.

USGS guidance on uncrewed aircraft imagery calibration emphasises the importance of accurate ground control and independent assessment. Ground control quality should be appropriate to the accuracy required from the final product.

7. Important Photogrammetry Outputs

A coastal photogrammetry project can produce several useful spatial datasets.

Orthomosaic

An orthomosaic is a geometrically corrected image assembled from multiple photographs.

Unlike an ordinary aerial photograph, it is designed to provide a consistent map-like view with reduced perspective distortion.

It can support visual interpretation, coastal feature mapping and GIS analysis.

Three-dimensional point cloud

A point cloud is a collection of reconstructed 3D points.

Each point represents an estimated location on the visible surface. The points can be coloured using image information to create a realistic representation of the surveyed area.

Digital Surface Model

A Digital Surface Model (DSM) represents the elevation of the visible surface, including features such as vegetation and structures.

Digital Terrain Model

A Digital Terrain Model (DTM) attempts to represent the underlying ground surface after removing above-ground objects where the data allow reliable classification.

Coastal cross-sections

Cross-sections show elevation changes along selected lines across a beach or coastal feature.

They are useful for comparing beach slopes and identifying changes in the shape of the surveyed surface.

Volume calculations

If two compatible surface models are available, software can estimate the difference in surface volume between survey dates.

Such calculations may help researchers assess sediment gains and losses, but their reliability depends on survey alignment, surface quality and uncertainty.

8. Measuring Beach Change Over Time

One of the most valuable applications of coastal photogrammetry is repeat monitoring.

A single survey provides a snapshot of the beach. Repeated surveys can reveal how the surface changes.

For example, a research team could survey a selected beach section at regular intervals and compare the resulting digital elevation models.

A basic elevation difference can be written as:

Δz = z₂ − z₁

Where:

• Δz = change in surface elevation.
• z₁ = elevation measured during the first survey.
• z₂ = elevation measured during the later survey.

Positive values indicate a surface that is higher in the later model, while negative values indicate a lower surface.

However, the difference map must be interpreted carefully. Apparent changes may result from measurement errors, poor alignment, different water levels, vegetation changes or differences in image quality.

A reliable change-detection study should report uncertainty and avoid interpreting very small elevation differences as real coastal change unless they exceed the combined measurement uncertainty.

9. Potential Applications Around Saona Island

Coastal photogrammetry could support several research activities around Saona Island if appropriate surveys are carried out.

Beach profile monitoring

Repeated drone surveys could document the shape of accessible beach surfaces.

Researchers could extract cross-sections and compare their elevation profiles between survey dates.

Shoreline feature mapping

Orthomosaic imagery can help identify visible shoreline features and provide a consistent base map for coastal analysis.

The shoreline indicator must be defined carefully. A visible wet-dry boundary, vegetation line and tidal waterline are not necessarily the same feature.

Coastal erosion and deposition studies

Comparing repeated surface models may help identify where visible land surfaces have lost or gained material.

Field observations and suitable sediment measurements are needed to interpret the processes responsible for these changes.

Coastal vegetation mapping

High-resolution imagery may help map visible vegetation boundaries and broad land-cover patterns.

Multispectral imagery may provide additional information about vegetation characteristics, but image classification should be validated using field observations.

Coastal infrastructure documentation

Photogrammetric models may help document visible paths, buildings, access routes and other coastal infrastructure.

Such models can support preliminary planning, inspection and visual records. Detailed engineering design still requires appropriate survey control and site investigations.

Environmental monitoring

Repeated image surveys may document changes in exposed coastal landforms and help researchers identify locations that require further ecological investigation.

Photogrammetry alone cannot confirm species presence, habitat health or ecological condition.

10. Can Drone Photogrammetry Map Shallow Underwater Areas?

Photogrammetry can sometimes be used to reconstruct visible shallow-water seabed surfaces, but underwater mapping is more challenging than mapping dry land.

The technique depends on the camera receiving useful information from the seabed through the water.

Several conditions affect performance:

• Water clarity.
• Surface waves and ripples.
• Refraction at the air-water interface.
• Sunlight reflection and glare.
• Seabed texture and visible features.
• Water depth and image resolution.

Clear water, calm conditions and distinct seabed features are favourable. Turbid water, breaking waves and uniform sandy surfaces can make reconstruction difficult.

NOAA's guidance for nearshore bathymetric mapping using small uncrewed aircraft and SfM identifies these environmental limitations and recommends suitable camera positioning and high-quality trajectory information.

Therefore, a Saona Island beach photogrammetry project should distinguish between reliable land-surface measurements and any attempted underwater reconstruction. Sonar or other appropriate bathymetric techniques may be needed where optical imagery cannot provide dependable depth information.

11. Photogrammetry Compared with LiDAR

Photogrammetry and LiDAR can both produce three-dimensional coastal datasets, but they use different measurement principles.

Photogrammetry reconstructs geometry from overlapping images. It can provide detailed colour imagery and surface models, but it depends on image texture, lighting and reliable image matching.

LiDAR measures distances using laser pulses. It can capture surfaces with less dependence on visible photographic texture, although its performance also depends on sensor characteristics, surface conditions and the survey environment.

For exposed beaches and accessible coastal land, drone photogrammetry may provide an efficient option for repeated visual and topographic surveys.

For projects requiring different surface characteristics, vegetation penetration or suitable shallow-water laser measurements, LiDAR may offer advantages.

The choice should be based on project objectives, required accuracy, environmental conditions, equipment and cost.

12. Common Errors in Coastal Photogrammetry

A technically successful flight can still produce an unreliable model if the survey or processing is poorly controlled.

Common problems include:

Insufficient image overlap: Too little shared image coverage can cause alignment failures or weak reconstruction.

Blurred photographs: Motion blur reduces the quality of image matching.

Poor ground control: Incorrect or unevenly distributed control points can distort the final model.

Changing water surfaces: Moving waves and reflections create features that do not remain stable between images.

Vegetation movement: Wind-blown leaves and grasses may produce inconsistent matching.

Inconsistent camera settings: Rapid exposure or focus changes can reduce image consistency.

Incorrect coordinate references: Mixing coordinate systems or vertical references can create misleading elevation comparisons.

Unreported uncertainty: A model should not be presented as survey-grade simply because it looks detailed.

These issues should be addressed through field planning, quality checks and transparent reporting.

13. A Proposed Coastal Photogrammetry Project for Saona Island

A future research project could use the following framework.

Phase 1 – Site selection

Select accessible coastal sections with clearly defined survey boundaries. Consider visitor activity, vegetation, terrain, environmental sensitivity and operational permissions.

Phase 2 – Baseline survey

Collect overlapping aerial photographs under suitable weather and lighting conditions. Establish ground control and independent checkpoints.

Phase 3 – Photogrammetric processing

Align photographs, estimate camera positions, generate point clouds and produce orthomosaics and elevation models.

Phase 4 – Accuracy assessment

Compare independent checkpoint measurements with the reconstructed model. Review horizontal and vertical errors, image quality and areas of incomplete coverage.

Phase 5 – Repeat monitoring

Repeat the survey using a consistent methodology and suitable reference control. Compare the resulting surface models to assess measurable changes.

Phase 6 – Coastal interpretation

Use the validated data to examine beach profiles, visible shoreline features, surface changes and possible sediment movement.

This is a proposed research framework. It does not claim that a particular drone photogrammetry monitoring programme has already been established on Saona Island.

14. The Future of Drone-Based Coastal Mapping

Improvements in cameras, GNSS positioning, processing software and automated image analysis continue to expand the possibilities of drone-based mapping.

Potential developments include:

• More efficient automated flight planning.
• Improved camera calibration.
• Real-time image quality checks.
• Integration with GIS platforms.
• Automated shoreline feature extraction.
• Artificial intelligence-assisted land-cover classification.
• Comparison with satellite, sonar and LiDAR datasets.
• More accessible repeat-survey workflows.

These developments may make coastal monitoring more practical for universities, research teams and environmental organisations.

However, automated processing does not eliminate the need for good field data, independent validation and responsible interpretation.

Frequently Asked Questions

1. What is coastal photogrammetry?

Coastal photogrammetry is the use of photographs to measure and reconstruct coastal surfaces, producing maps, point clouds and three-dimensional elevation models.

2. What is Structure-from-Motion?

Structure-from-Motion is a technique that uses overlapping photographs taken from different viewpoints to estimate camera positions and reconstruct three-dimensional geometry.

3. Can a drone create a 3D beach model?

Yes. With suitable image overlap, camera calibration, positioning and processing, drone photographs can be used to generate three-dimensional models of exposed beach surfaces.

4. Why are Ground Control Points important?

Ground Control Points connect the reconstructed model to known real-world coordinates and help constrain its position and scale.

5. Is drone photogrammetry accurate enough for coastal engineering?

It can be suitable for some coastal mapping and monitoring tasks when properly designed, controlled and validated. The required accuracy depends on the engineering application.

6. Can photogrammetry measure underwater depth?

It can sometimes reconstruct shallow visible seabed surfaces under favourable optical conditions. It is not a universal replacement for sonar or other hydrographic survey methods.

7. How often should a beach be surveyed?

The frequency depends on the research objective. Surveys may be planned before and after major events or at regular intervals, provided that the timing and environmental conditions are documented.

8. Is coastal photogrammetry already being used on Saona Island?

This article does not verify a specific local programme. Any existing survey coverage or monitoring project should be confirmed through authoritative project records or local organisations.

Conclusion

Coastal photogrammetry offers a practical way to transform overlapping photographs into detailed maps and three-dimensional models of exposed coastal landscapes.

Through Structure-from-Motion processing, drone imagery can support beach profile analysis, shoreline feature mapping, surface comparison and coastal research. Ground control, independent checkpoints and careful quality assessment are essential if the resulting data are to support reliable measurements.

For Saona Island, a properly designed photogrammetry programme could provide useful baseline information and help researchers document changes in accessible beach environments over time.

The method is most effective when used as part of a broader coastal mapping strategy that combines image-based surveying with GNSS measurements, field observations, LiDAR or sonar where appropriate.

By turning photographs into measurable 3D surfaces, coastal photogrammetry makes it possible to study beach landscapes in greater detail and compare how they change over time.