Véronique Cuq

Adequacy of pseudo-direct georeferencing of terrestrial laser scanning data for coastal landscape surveying against indirect georeferencing

ABSTRACT The georeferencing process is crucial to the accuracy of terrestrial laser scanner (TLS) data, in particular in the context of diachronic studies relying on multi-temporal surveys. The use of Ground Control Points in the georeferencing process can however be complex when confronted with the practical constraints of coastal surveying. A simple and quick alternative method called “pseudo-direct georeferencing” is proposed in the present paper. This method involves internal inclinometers to measure roll and pitch angles and a centimetric GPS to measure the position of the TLS center and the position of one backsight target. When assessing the transformational uncertainty by using a set of independent ground validation points for both classical indirect and proposed pseudo-direct methods, we respectively obtain root mean square errors of 4.4 cm for the indirect method and 3.8 cm for the pseudo-direct method.

High resolution DEM derived from thermal infrared images: Example of Aber Benoit (France)

Intertidal DEM have a growing interest for ecological, geological and land development purposes. However, due to the characteristics of this environment, DEM are not easy to obtain and frequently not good enough in resolution to be interpreted. In this study, waterline method has been used to build a high-resolution DEM. This technique is basically based on the accurate localization of the boundary between water and sand. This limit being an iso-height line, multitemporal detection can be used to derive a DEM. Because of the distinct physical characteristics of both sand and water, this limit is clearly visible on thermal infrared images. We tested this method on a beach of Aber Benoit (France) by recording the shoreline movements during a whole tide rise with a thermal infrared camera with simultaneous check of the water height (DGPS on board a boat) to provide an absolute calibration. The reconstructed DEM, validated by an independent DGPS profile, has a mean resolution of 2 cm and accounts for details smaller than a centimeter.

Equilibrium modeling of the beach profile on a macrotidal embayed low tide terrace beach

Eleven-year long time series of monthly beach profile surveys and hourly incident wave conditions are analyzed for a macrotidal Low Tide Terrace beach. The lower intertidal zone of the beach has a pluriannual cycle, whereas the upper beach profile has a predominantly seasonal cycle. An equilibrium model is applied to study the variation of the contour elevation positions in the intertidal zone as a function of the wave energy, wave power, and water level. When forcing the model with wave energy, the predictive ability of the equilibrium model is around 60% in the upper intertidal zone but decreases to 40% in the lower intertidal zone. Using wave power increases the predictive ability up to 70% in both the upper and lower intertidal zones. However, changes around the inflection point are not well predicted. The equilibrium model is then extended to take into account the effects of the tide level. The initial results do not show an increase in the predictive capacity of the model, but do allow the model free parameters to represent more accurately the values expected in a macrotidal environment. This allows comparing the empirical model calibration in different tidal environment. The interpretation of the model free parameter variation across the intertidal zone highlights the behavior of the different zones along the intertidal beach profile. This contributes to a global interpretation of the four model parameters for beaches with different tidal ranges, and therefore to a global model applicable at a wide variety sites.