## 2 Hydrographic Survey Planning  
### Regulations Regarding Planning of Hydrographic Survey  
There are certain international standards which lay out detailed guidelines for planning hydrographic work. One of these is the standard issued by the International Hydrographic Organization (IHO), entitled _\"S-44 IHO Standards for Hydrographic Surveys\"_[9], which provides information on the distance between the main lines for four orders of survey and defines the nominal distances between check lines for a multibeam echosounder (MBES) and a single beam echo sounder (SBES). Table 1 shows that no distance between neighbouring measurement profiles has been established for Special Order and Order 1a. However, a full seafloor search is required. This is only possible with a MBES. The maximum distances between sounding profiles are defined for the other two orders of survey (1b and 2). Therefore, it can be surmised that these two orders of survey apply to hydrographic work with the use of SBES. For Order 1b, the maximum line spacing should be equal to three times the average depth of the sounded area or 25 m (whichever number is greater), whereas for order 2, the distance is equal to four times the mean measured depth [9]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Order** & **Special** & **1a** & **1b** & **2** \\ \hline \multirow{6}{*}{Description of areas} & & Areas shallower & Areas shallower & \\  & & than 100 m where & than 100 m where & Areas generally \\  & & under-keel & under-keel & under-keel & deeper than 100 m \\  & Areas where & clearance is less & clearance is not & where a general \\  & under-keel & critical but features & considered to be an & description of the \\  & clearance is critical & of concern to & issue for the type of & seafloor is \\  & & surface shipping & surface shipping & considered \\  & & may exist & expected to transit & adequate \\  & & & the area & \\ \hline Full seafloor search & Required & Required & Not required & Not required \\ \hline \multirow{2}{*}{Recommended maximum line spacing} & Not defined as full & Not defined as full & 3\(\times\) average depth & \\  & seafloor search is & seafloor search is & or 25 m, whichever & 4\(\times\) average depth \\ \cline{1-1}  & required & required & is greater & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of selected requirements for hydrographic measurements [9]. The IHO S-44 standard also defines the minimum distances between control profiles, which are determined perpendicularly to the main profiles to control the measurements. The distance should not be greater than 15 times the distance between the main lines in hydrographic measurements with a SBES. On the other hand, control measurements with MBES should be conducted where coverage for the neighbouring swaths show considerable depth errors [14; 15]. The majority of the other standards concerning hydrographic survey refer to the requirements laid down in the IHO S-44 standards regarding the planning of hydrographic surveys. However, there are differences in the requirements for determining the distance between sounding lines. For example, the standard issued by the American National Oceanic and Atmospheric Administration (NOAA) entitled \"_NOS Hydrographic Surveys Specifications and Deliverables_\" [16] provides information on the permissible distance between sounding profiles using a SBES. It should not be smaller than 3-5 m. The standard also defines the distances between neighbouring swath profiles using a MBES; however, the overlap zone between the neighbouring swaths are not determined. The only condition that has to be met in hydrographic work done with a MBES is that the seafloor must be fully covered by the measurements.