Keywords : overlapping

Overlapping

Overlapping zones can be problematic in a mosaic of bathymetric data as two or more depth values can contribute to a single location. But knowing the accuracy of data decreases with increasing beam angle, the DTM routines that indistinctly sum up all the contributing soundings would degrade (probabilistically speaking) a correct value by a more uncertain one. SonarScope uses a simple algorithm to overcome this problem: it uses the beam angle to decide which measurements, in the overlapping area, will be retained to make the DTM.

One distinguishes perfectly the left edge of overlapping areas which are of very poor quality.

Digital terrain model built by SonarScope: the center of the recovering zone is automatically found. This DTM has been created using only the least transmit beam angle for a given point in the overlaps.

Transmission angle mosaic (in absolute value): this image is the decision support for the line overlapping management, one clearly distinguishes places where lines moved together and those where lines went apart .

To manage the lines overlapping we could have chosen another metric as for example the acrosstrack distance. We chose the transmission angles because we know measurement accuracy is directly linked to this parameter. On flat seafloor, acrosstrack distances would give the same result, but on tilted seafloor our method is better.

Reflectivity mosaic: this image is realised according to the same principle than the DTM (it is the same functional module than the DTM creation, there is no distinction between "bathy processing chain" and "Reflectivity processing chain" with SonarScope.)

This image shows the advantage of the automatic management of lines overlapping to solve a different problem than the bathymetry measurement accuracy. The first image has been realised by averaging soundings coming from various lines, the second one has been produced by SonarScope ; it shows clearly a navigation gap issue.

The management of profiles overlapping only uses a sonars subset of the different profiles. SonarScope permits to create PingBeam geometry images only representing these sonars. The image on the left represents all the soundings of a profile, the image on the right represents the soundings used to realise the MNT. This can bring users, who spend their time to clean up bathymetric data, to gain precious times, because among every eliminated soundings, an important number of them are the overlapping zone.