Hydrographic Survey PDF

409 CHAPTER 7 HYDROGRAPHIC PRACTICE 1. INTRODUCTION The execution of a surveying operation, from its inception to the final submission of the results, is a continuous process, all parts of which must be methodically...

409 CHAPTER 7 HYDROGRAPHIC PRACTICE 1. INTRODUCTION The execution of a surveying operation, from its inception to the final submission of the results, is a continuous process, all parts of which must be methodically and meticulously carried out if full value is to be gained from it. The most accurate and carefully gathered data will be wasted if not processed carefully and collated and rendered in a clear and understandable manner. All information must be gathered, validated, checked and rendered in a logical and uniform manner using clear unambiguous terms so that the data may be recovered and understood both immediately after the survey and in the future. The use of sophisticated computer systems and instrumentation in all areas has not lessened the surveyor’s responsibility. The need for rigorous quality management is as essential now as ever, but made the more difficult by the rapidly increasing volume and complexity of data gathered. The surveyor must understand the principles of the equipment he controls, be meticulous in ensuring that any data input are entirely correct and examine with care the data output before they are passed to the next stage. Only by these means will the data rendered be of the highest quality and be able to fulfil its needs until the task can be repeated perhaps decades, if not centuries, later. There is no substitute for practical experience where theoretical knowledge can be turned into practical skills; time spent in the field gathering data under the guidance of an experienced hydrographic surveyor will highlight the many difficulties and complex problems that need to be overcome to ensure the final product meets the initial requirement. The use and applications, to which highly complex sophisticated equipment can be best applied, can only be learnt whilst involved in the practical tasks of a hydrographic survey. The processes are discussed in more detail in the following sections. The points covered are not exhaustive, and the surveyor must use his judgement and experience to expand or contract the list as considered appropriate. This chapter will consider general principles applicable to any survey. 2. HYDROGRAPHIC SURVEY PLANNING As will be seen, survey planning is a complex process requiring considerable attention to detail, a flexible approach, good management and effective decision-making. If the planning is thorough, the chances are that the survey will be good too. 2.1 The Hydrographic Project Survey planning is the term used to cover the whole process of the development of a hydrographic project from its inception, its subsequent issue to a designated unit, the detailed planning within that unit of how to conduct the survey, and the final submission of data to the Hydrographic Office. Survey planning must, therefore, involve a coherent procedure which will consist of the following stages: a. survey requirement. b. preparation of a hydrographic survey specification. (To include a review of existing data.) C-13  410 c. issue to a designated unit. d. programme planning of that unit. e. assessment of the task within that unit. f. reconnaissance requirements. g. resource allocation. h. detailed survey planning. i. estimation of time required. j. final programme planning and approval. k. liaison with outside authorities. l. administration planning. m. daily planning. n. plans for compilation and checking of data. o. plans for rendering of data. The development of the survey requirement varies greatly from country to country. The final survey specification is assigned to a designated survey unit which has responsibility for the remaining planning requirements. A ‘designated unit’ might be a government-surveying vessel, an independent survey party working either in a chartered vessel or using boats, or a commercial company working under contract. Each nation will have its own planning process. 2.2 Assessment of the Survey Task Before detailed planning can take place, the surveyor must be very clear about the aim of the survey, and who will be the primary user of the information. In general every survey should cover the immediate needs of the user as well as benefiting others. Having studied the survey requirement, accompanying data and graphics of the area to be surveyed, the surveyor must first decide whether any additional data is required and propose any changes he considers necessary to the survey task. Once all the basic data is held and the limits and scale of the survey have been fixed, the main surveying tasks can be established. The specifications for the survey will be stated directly in the survey job specification, key amongst these should be the specified Order of the survey as defined IHO Publication S-44. The main task assessment points are as follows: a. establishment of geodetic control; b. method of positional control and calibration of navaids; c. sounding criteria including interlining policy; d. sonar search category; e. tidal datum and observations; f. wrecks and obstructions; g. seabed sampling; h. oceanographic observations; i. tidal stream observations; j. geophysical observations; k. coastline and topography; l. lights and buoys; C-13  411 m. sailing directions and views; n. radio Signals; o. ancillary observations (vertical photography, overfalls, measured distances, leading lines, shore magnetics, natural phenomena, etc); p. passage observations. Existing Data. The surveyor should be provided with copies of the largest scale published chart and all previous surveys to survey scale, together with their Reports of Survey (RoS). These should be studied carefully, together with the relevant sections of tidal data, sailing directions, aerial photographs and topographic maps. Resource Allocation. From the list of points in 2.2.5, and a study of previous data, the size of the task can be assessed. Detailed planning will take place after this process, but the initial study will reveal what resources are required to meet the task. The following list shows some of the considerations that should be made when planning resources: Weather predictions and sea state. These will affect the size of the vessel to be used for the task and the ability to use boats for inshore work and examinations. Size of the shallow water area. This will dictate how much boatwork is required. If boats are necessary, the time taken to complete the task will be highly dependent on sea state; 2.2.7.1 and 2 should, therefore, be considered together. Use of helicopters. It may be that the unit has a helicopter available, but if not is one required for access to remote sites? Logistics. The endurance of the surveyor’s own resources will dictate fuel, water and stores requirements. Maintenance of equipment is another consideration. Manpower. The number and specialisation of personnel required to meet each task must be assessed. The following factors are also relevant: changes of personnel; mail and communications; leave and recreation; medical facilities ashore and afloat; shore support and transport; shore accommodation and monetary arrangements. Topography. This will dictate the resources required to access sites ashore. Detached Boat Camps - A detached boat camp may be ordered by survey specification in which case the planning list above should be followed. However, consideration should be given to detaching a boat to conduct inshore work and shoal investigations if a suitable harbour or sheltered mooring exists. Time spent lowering and hoisting boats is unproductive. Likely constraints on the conduct of the survey should also be investigated. Investigate what fishing activity is likely to affect survey progress together with the constraints imposed by danger, firing and practice areas, shipping lanes and marine traffic choke points. A field reconnaissance may be required to expedite the survey. See Section 3 C-13  412 2.3 Detailed Survey Planning Once the size and scope of the task has been assessed and the necessary resources required to conduct it decided upon, detailed survey planning can begin. A number of activities can be planned to run in parallel and a good surveyor will try to reduce the overall time required to achieve the aim. A comprehensive list of required actions is given in the following paragraphs, but it must be remembered that every survey will be different and additional items may have to be inserted, or listed ones deleted. 2.4 Horizontal Control The survey specification will detail the horizontal reference for the survey and list details of existing co- ordinated geodetic stations together with their descriptions if held: Decide how best to achieve the accuracy standards of horizontal control set out in survey specifications. The survey specification will detail the choice of navaid, more than one may be ordered. In rare cases, it may not be possible to achieve the stated standards with the navaids available and relaxations may have to be sought from the Hydrographic Office. Once the choice of navaids has been determined, their sites need to be chosen. Use whatever network analysis systems are available. Decide how best to co-ordinate new stations. Consider access to sites and any reconnaissance requirement. Decide how to power navaids and work out how often site replenishment will be required. Note the authorities that must be approached for permission to use the chosen sites, authorise frequency clearance and land boats and helicopters. This will include sites for shore marks if visual fixing methods are to be employed. Decide on how and where the chosen navaids will be calibrated and whether re-calibration will be necessary during the course of the survey. Most modern surveys use some form of GPS for control ashore and afloat. Where DGPS is used for control afloat it must be validated. In remote locations ashore, point positioning within 20cms should be achievable within 24 hours of data capture if data can be transmitted to the Hydrographic Office for comparison with the nearest ITRF monitoring site. Otherwise establishing a new station will require connection to an existing network. 2.5 Vertical control The survey specification will detail the datum to which soundings are to be reduced and its relation to existing land datums, a list of any existing benchmarks should also be provided. The following planning points should be considered: Decide where to observe tidal heights if not ordered in the survey specification. Decide on the siting of additional poles and gauges if required and plan the laying and recovery of offshore gauges if appropriate. Ensure that gauge and pole sites do not dry out at low water, plan additional gauges and poles if this is unavoidable. Decide how best to establish chart datum on the pole/gauge from existing benchmarks or from transfer of datum or observation and analysis. Plan to connect a newly established datum to a land based levelling system if applicable. C-13  413 Establish the nature of the tide and expected ranges, and the affect this will have on fieldwork. Where there is a possibility of tide poles or gauges drying out an additional pole or gauges should be planned to allow tidal data to be recorded throughout the survey. Decide whether co-tidal adjustments will be necessary. If they are, determine the tidal factors from tide tables and the appropriate co-tidal chart, or locally produce them from the best data available. Seek advice from the Hydrographic Office if necessary. 2.6 Tidal Streams Establish the expected maximum rate and direction of tidal stream within the survey area. Determine the requirement for full tidal stream observations and how online observations can be conducted. Identify charted overfalls, eddies and freshwater springs and plan to observe them. 2.7 Sounding The following general considerations should be taken into account when conducting initial planning: By examining the largest scale charts of survey area and previous surveys locate all critical or controlling depths and prepare sounding comparison overlay. Plan main survey line spacing, direction and sounding speed. For SBES, where possible, lines should be perpendicular to general direction of contours. Plan cross line direction, normally at right angles to main sonar and sound line direction, and plan to run these at the start of the survey as a crucial quality control measure. Estimate likely spatial or temporal changes in sound velocity regime and plan initial sound velocity probe coverage. Estimate sounding error budget and compare to the survey specification. Vessel speed is to be assessed for the expected range of depths in the survey area and the type of echo sounder in use. Compare vessel speed with speed required for towing sonar to determine optimum and maximum survey speed. During the planning of sonar and sounding lines, a list of all planned lines should be kept. For SBES surveys additional lines should be considered inside the 10m contour. Additional lines should be run parallel to a jetty or wharf. Lines of sounding should be planned and run along recommended tracks, leading lines, in possible anchorages and off headlands passed close to hand by vessels on normal passage. C-13  414 If using SBES for offshore surveys particular attention should be paid to the sounding of depths