More slanted beams sometimes returns irregular soundings, since they are more refracted by sound velocity distribution, and their footprints become larger. If unsounded width is planned in advance, many supplement surveys will be required later when irregular soundings exist at the edge of the swath. Thus supplement surveys are much reduced. Fig.2 Attentions to multibeam echosoundings over cheracteristic features. In case we miss foreign bodies, adjacent swathes should be enough overlapped.
Same way is applicable to the case of wharf frontage.
When sonar head is permanently installed on the under-hull, changes of these biases are not expected to be large. It is recommended to set a fixed ' test area' around a specific topographic feature and to perform patch test in the same area every time. To avoid this, it is important to check outputs of motion sensors and remove such irregularities. As reflection intensity is not accessible in digital soundings, it is difficult to distinguish noises and floats from true bottom. In such cases, it is required to cover the object by enough beams of different angle from adjacent survey lines and to check if same feature appears.