National Network of Regional Coastal Monitoring Programmes

Topographic Surveys

A variety of techniques are employed to undertake monitoring of the coastline; when the coastal monitoring programmes started, levels and total stations were still in use, but were soon replaced with RTK GNSS (GPS) and, recently, increasingly by laser scanners. Topographic surveys generally cover beaches, dunes, cliffs, saltmarshes and coastal defence structures and aim to reach MLWS level where possible, while hydrographic surveys extend from the Mean Low Water contour to 1 km out to sea. When combined with hydrographic surveys, a seamless surface from land to sea can be produced.

RTK GNSS

RTK GNSS (or GPS) is the primary measurement tool and is employed in several ways. These include:

• Beach profiles, where surveyors measure at intervals and breaks of slope along a cross-shore transect


• Walkover surveys (also known as spot height surveys) where GNSS receivers are carried in a backpacks while a team of surveyors walks along the beach following the contours

Test points are measured at the beginning, during and at the end of the surveys to check that the survey fulfils the accuracy required by the Survey Specification.

Laser Scanning

Laser scanning is used to survey large areas of beaches and cliffs with a very high level of accuracy and detail, typically millions of data points in a 150 m radius from the instrument. Laser scanners can cover large areas quickly and the data can be used to generate detailed surface models for calculating beach volumes.

Topographic laser scanners cannot measure below water and are less effective over wet surfaces such as a mudflat recently uncovered by the tide, so in areas of beach close to MLWS or with standing water some walkover survey may be needed as well.

Laser scanning is particularly useful for monitoring cliffs since (as a remote sensing technique) data can be captured from areas which would be either dangerous or impossible to access on foot. The scanner is able to detect very small changes in ground movement and is especially useful where there might be the potential for landslips or cliff falls. Laser scanners are being used increasingly for surveys of coastal defence assets such as seawalls to provide a high definition baseline dataset for coastal engineering applications.

Terrestrial laser scanners can be either static or mobile types. Static scanners operate from a fixed position rotating through 360o collecting data of the surroundings. The scanner and targets are measured with GNSS enabling the resulting data to be co-ordinated to BNG. The process is repeated as the scanner is moved along the beach. In contrast, a mobile scanner scans only in the vertical plane but is usually mounted on an ATV, so is constantly capturing data as the ATV drives along the beach. The scanner gets its position from GNSS and movement is corrected for by an Inertial Motion Unit (IMU) within the instrument.

The mobile scanner can be mounted on a vessel, to capture areas which cannot be accessed from the beach cliffs or saltmarsh.

Unmanned Aerial Vehicles (UAVs)

UAVs or drones can be used to collect data for topographic modelling over large areas very quickly. There are two main types, fixed wing and multi-rotor with each type having advantages over the other. Fixed wing types are able to cover large areas quickly with good battery life though do require a relatively large area to operate from.

Multi-rotors can also cover large areas quickly though battery life can be limited but they have the advantage of being able to operate from very small spaces. Both types are very quick to deploy enabling survey data to be collected at very short notice and are able to reach areas otherwise not possible by other surveying means e.g. unstable cliffs and mud flats.

Multi-rotors are able to carry either a camera or a compact lidar unit whereas fixed wing are limited to downward looking cameras only.

Camera payloads are a lot cheaper than lidar and yield very precise point cloud data derived from photogrammetry achieving accuracies of +/- 0.05m or better. More recently UAV lidar systems are achieving similar accuracies to photogrammetric systems and are becoming competitively priced.

Key outputs include topographic models, point clouds, cross sections and orthophotos.