The Hamburg Port Authority is among the first to have adopted a digital hydrographic survey sweeping configuration incorporating a position-fixing capability designed to meet required continuous positional accuracies of ±2m.
With the exception of a shore-based positional tracking station, all equipment is installed on the Authority's own survey vessel, Deepenschriewer 11. The equipment comprises a bottom-mapping system, a precision survey echosounder, gyrocompass, a laser range-azimuth position-fixing unit and, finally, a fully automated navigation and survey processing configuration for both pre- and post-survey operations.
Factors governing overall survey requirements and performance are discussed, and the various systems used are described.
When discussing survey requirements, it is, of course, the morphology of the bottom which should be considered first. Of equal importance, though, is the prescribed manner of positional determination, which is primarily dependent upon topographical considerations, as well as the economic factors affecting the rationalization of survey operations.
Example fair sheets (Fig 1(a) and b)) show an area where the River Elbe divides into a northern and southern arm. In Fig 1, (a) shows depth contour lines generated from cross-profile data while (b) illustrates the same area, but from a survey carried out with an Atlas Boma 20 sweeping system. Comparison of (a) and (b) shows clearly that assessment of morphology dictates whether or not cross-profile surveys can usefully determine a bottom structure.
A further typical practical example would be the Rive Rhine, near Bingen, where deepening or correction of the fairway can only be carried out with very precise blasting. Without the use of a sweeping system, it would be extremely costly and time-consuming to ascertain if and where boulders or shelves remained after blasting operations.
The morphological conditions of a sea bed or river bed, together with the current velocity of the water, may influence the design concept of a survey vessel. That is, whether the required ultrasonic transducer can be installed in booms for the vessel to move in the direction of ship's axis along the river, or whether transducers should be installed alongside the hull or in the keel in order for the vessel to move perpendicularly to its axis and across the river.
Fig. 1 Comparison between cross-profile method (a) and sweeping survey method (b). (available in full paper)
Given an average vessel speed of 4 m/s for longitudinal surveying and 2.5 m/s for cross-profile surveying, it would typically be assumed that a river area comprising a length of 25 km and a width of 100 m would have to be surveyed. The question of which of the following two survey methods would be the less time-consuming often arises.
A cross-profile survey with parallel track-distances of 30 m
A gapless sweeping survey with longitudinal tracks and 50 m sweeping width.
The answer is self-evident. A survey vessel equipped with a sweeping system is able to carry out the same survey operation in considerably shorter time than would be possible by cross-profile methods.
