(a) Positions of sounding lines (A, B) are plotted on chart; (b) Soundings are plotted as profiles (A, B) at 40:1 vertical exaggeration; (c) Features shown on profiles (A, B) are sketched on chart along tracks; (d) After all available sounding profiles are sketched the remaining unsounded areas are filled in by extrapolating and interpolating trends observed in a succession of profiles.
The echo soundings made by research vessels fall into three classes: (1) precision soundings (accuracy better than 1 fathom in 3000); (2) nonprecision soundings obtained by research vessels using commercial echo sounders with control or close check on time standard; (3) poor to bad soundings made with commercial echo sounders without timing control or adequate checks. Most of the soundings used in this paper fall into the first two categories. In Figure 2 the Precision Depth Recorder (PDR) sounding tracks are shown. In Figure 3 the good but nonprecision tracks are shown. The soundings of the third class are not shown. All tracks used in the preparation of the diagram are shown in Plate 21. Most of the sounding lines were located by standard dead-reckoning procedures from astronomical fixes. Errors of a few miles are probably common. Position errors do not seriously affect the work described here since we are dealing largely with texture read from profiles and plotted on a small-scale sheet.
In addition to the sounding tracks shown in the control chart, spot depths shown on U. S. Hydrographic Office charts HO 0955a, 0955b, 0956a, 0956b, and 5487 and on feuille A-1 of the Carte Générale Bathymétrique des Océans (1935) were used where profiles were lacking. Along the east coast of the United States the Coast and Geodetic Survey soundings published by Veatch and Smith (1939) were used for the continental shelf and slope. Other important sources of published soundings include Hill (1956), De Andrade (1937), Dietrich (1939), Wüst (1940a), Emery (1950), and Tolstoy (1951).
The land areas of the diagram were sketched to the same rigid vertical scale as that used for the deep sea. Elevations for the United States were taken from United States Geological Survey and Army Map Service quadrangle maps; elevations for Europe and Africa are from Bartholomew maps; and elevations for the islands from United States Navy Hydrographic Office charts.
EXAGGERATED PROFILES: The profiles plotted at 40:1 vertical exaggeration are the basis for the topography sketched on the physiographic diagram. A selection of these profiles is reproduced in Plates 22, 24, 25, and 27, and in Figure 45. All profiles from precision soundings were originally plotted at a vertical scale of 2 inches equals 1000 fathoms and a horizontal scale of 2 inches equals 40 miles. Nonprecision soundings were plotted at scales of 1 inch equals 1000 fathoms and 1 inch equals 40 miles. In a typical area 40 to 60 soundings were plotted for each 60 miles of profile. The points were connected and then qualitatively checked against the original echogram. Although all the larger features are represented on these profiles, features of less than a mile in width may be missed. The small scale of the physiographic diagram excluded the possibility of portraying most of the features less than 3-6 miles in width and less than 20 fathoms in height.
Detailed study of the small-scale features less than 2 or 3 miles in width is best accomplished by a study of the original echograms. The PDR records are ideal for this purpose.
Figure 2.—Precision depth recorder (PDR) sounding lines obtained by research vessels