10 



The Ocean 



lowest low water and on American charts it is the level of local mean low water. 

 Only the sea charts of tideless mediterranean seas relate their depths to mean sea-level 

 (e.g. the Baltic). Chart datum is nowhere the same as normal datum {NN) for carto- 

 graphical surveys on land but is generally lower. Since the tidal range varies from one 

 coastal station to another the chart datum forms an undulating surface which in 

 general falls as it approaches a coast. This fall is greatest in funnel-shaped bays where 

 the tidal range rapidly increases towards the inner end. On the open sea there are only 

 small differences between chart datum and mean sea-level. 



Chart datum must be taken into consideration in more accurate hydrographic cal- 

 culations. Raverstein (1886) first pointed out the importance of chart datum and 

 prepared two charts of a part of the English Channel. One of these showed isobaths 

 according to the sea chart (calculated from chart datum), and the other showed iso- 

 baths calculated from the surface of the geoid. These charts demonstrate clearly the 

 importance of considering a reference level. Figures 5 and 6 show two profiles of the 

 differences between mean sea-level and chart datum for a longitudinal section along 

 the Bristol Channel and for a cross-section of the Straits of Dover. For further informa- 

 tion on the often very complex question of chart datum see especially Horn (1944). 



B. THE THREE-DIMENSIONAL STRUCTURE OF THE OCEAN 



1. Methods of Recording Deep-sea Data 



The safety of shipping in coastal waters requires an accurate topographical survey 

 to considerably greater depths than the 12 m draught of the biggest ships, usually 

 down to 200 m. This is about the maximum depth at which soundings can be made 

 with any accuracy using a hand lead line. Soundings taken in this way can also be used 

 to measure the depth of water under a vessel anchored in shallow water and hence to 



13- 



8 10 12 14 16 18 20 22 24 2 4 6 8 10 12 

 I7-2II-34 hr 



Fig. 7. Tides determined by sounding from an anchored ship 

 (0 = 53° 55-9' N, A = 7= 52-2' E). 



determine the range of the tide at that point. This is the simplest method of determin- 

 ing the tidal range at a distance from the coast in adjacent seas that are not too deep 

 and along the continental shelves. The hemp lead line should have a piano wire trace 

 at the upper and lower ends. Soundings of this type can, with some practice, be de- 

 termined to within ±5 cm even for wave motion. Figure 7 gives an exam.ple of a tidal 

 cycle measured in this way at a station in the southern North Sea. 



