Determining Survey Frequency and Resolution01/01/1970 |
| BSH concept and realisation |
| Survey vessel Wega was recently deployed in the Weser estuary, where it performed a high-resolution survey of a 52 km2 area, in accordance with Order 1 of S44. The sea-bottom was surveyed intensively during an extended wreck search, but this area will again be part of next year’s programme. This is no exception. German coastal waters are re-surveyed at regular intervals by the Federal Maritime and Hydrographic Agency of Germany (BSH). |
| Thomas Dehling, Federal Maritime and Hydrographic Agency, Rostock, Germany |
The BSH uses a surveying concept that defines quality requirements such as the frequency and resolution of surveys. In order to keep abreast of recent developments and make more efficient use of available surveying time, the concept has just been reviewed and updated. The development of an effective survey plan taking into account existing framework conditions is described here.
Revised Task Concept Compared to the old basic concept there is now much higher differentiation among sea areas to be surveyed, according to particular quality requirements. This means the areas no longer have a rectangular shape. The transformation has been made in two steps. In the first, areas were defined to which ident-ical resolution and accuracy criteria (including Order according to IHO S44) were applicable. In the second step areas with identical repetition frequencies were defined, taking into account parameters ‘variability of the sea bottom’ and ‘up-to-dateness’. Of course, the need for up-to-date survey data also depends on the quality requirements defined in the first step. The two steps are thus not independent of each other. Both area patterns were then overlaid and areas that were too small or had an unfavourable shape were smoothed toward the higher-quality levels. The result was a compilation of areas subject to identical quality requirements. For each of these the survey ship-time was estimated from the total length of sounding lines. The total sum thus obtained was compared to the BSH surveying capacity which, not surprisingly, was found to be too small. The process was thus optimised by iteration. The outcome can be seen in the following two graphs for the North Sea and Baltic Sea. Figure 2 (in the magazine) shows graphic representation of the basic survey concepts for the Baltic Sea (excerpt). Since both survey pattern and definition of survey quality for individual areas have been optimised to meet present-day requirements it has been possible to enhance the efficiency of surveying and to accommodate the mandatory additional surveys in the area of the main shipping routes. However, in order to achieve a genuine reduction in overall workload it will be necessary also to modernise existing surveying equipment and to use alternative survey methods. Modernisation is underway, and state-of-the-art, high-speed side-scan sonar have been purchased. The ongoing modernisation of multi-beam echosounding equipment will be continued. A particularly suitable alternative method for German waters is laser scanning, which is being used in large-scale surveys of Wadden Sea areas that fall dry at low water. The data obtained are integrated with sea survey data. Other methods are being reviewed. The draft of the basic concept has been completed this year and submitted to the Federal Ministry of Transport for approval, which means that the concept here presented is a preliminary version. The survey pattern has to be adjusted continually to the natural and anthropogenic changes taking place in the survey area. As long as this is without major impact on overall survey requirements it is covered by approval for the basic concept. Annual Survey Plan How does the BSH develop its annual survey plan from the basic concept? For each survey in a particular area, the year when re-survey is due follows from the basic survey concept. Every year in autumn the survey requirements for the following year resulting from the basic concept are reviewed, taking into account possible modifications in survey pattern and survey quality requirements. Also included in the review are uncompleted tasks from the preceding year and tasks that have been rescheduled to an earlier date. In addition to these internal tasks, special customer requirements are included. Major customers are asked each year to state their requirements. A joint meeting is scheduled every year in November to discuss internal and external requirements and determine priorities. By the end of the year a plan of tasks is prepared and distributed to our customers and, of course, our vessels. This approach ensures that tasks not part of the basic concept can also be included in the annual plan. The plan lists survey tasks, required survey quality and any special features to be considered. Working Together Looking at the German-Danish boundary in the Baltic Sea one notices at once the unfavourable shape of survey areas close to the boundary. The fact that the shape of maritime boundaries does not follow morphological features is quite obvious in the Kadetrenden. In a bilateral arrangement between Denmark and Germany, morphologically continuous area responsibilities have been agreed for the mandatory survey tasks. The areas to be surveyed were allocated in such a way that practicable shapes were obtained. This means that Denmark’s surveys also cover some areas in the German Exclusive Economic Zone (blue in Figure 4), and vice versa (green in Figure 4). The total size of swapped areas is identical. On the basis of this arrangement Germany has been allocated the task of performing a complete survey of the Cadet Channel (as in 1997). This survey task is scheduled for 2007. |
| Biography of the author Thomas Dehling is a geodesist and head of Hydrographic Surveying at the Federal Maritime and Hydrographic Agency of Germany (BSH) since 1999. |
|
| Archive |
| Archive > March 2006, Volume 10, Number 2 > Determining Survey Frequency and Resolution |
|
