UNITS OF DEPTH AND SLOPE
On the profiles and echograms the vertical scale is in units of echo-sounding time rather than in units of true depth. In other words, all depths are calculated under the assumption that the vertical sound velocity is 800 fathoms per second. Considering that the sound travels to the bottom and back, the calculation is based on 400 fathoms per second of lapsed time.
Since the average vertical velocity is, within the area covered, always slightly less than 800 fathoms per second, the true depth is always slightly greater than the "echo-time depth" as expressed in "nominal fathoms". Figure 5 shows the range of corrections which must be applied in various parts of the area. The spot depths indicated on the physiographic diagram are in units of true depth corrected according to Matthews' tables (1939) for regional variations in the average vertical sounding velocity.
Figure 5.—Sound-velocity corrections for echo soundings
Add correction to uncorrected echo sounding to obtain true depth. Curves I and II are representative of North Atlantic 17°-50°N. exclusive of the Grand Banks region. Curves III and IV are representative of the deep-water areas near the Grand Banks. Curves are based on Matthews (1939) and are for use only where assumed sounding velocity is 800 fathoms/second. All soundings mentioned in the text are uncorrected for sound velocity.
Figure 6.—Conversion diagram for degrees, per cent grade, feet per statute mile, and gradient
(a) Values of gradient from 1:10-1:1500; (b) Values of gradient from 1:100-1:5000
The inclination of the bottom is given as the tangent of the angle between the sloping plane and the horizontal expressed as a ratio of integers. These ratios are referred to as gradients. In Figure 6 slope values expressed in degrees, per cent grade, feet per statute mile, and gradient are compared. With a few exceptions all profiles are represented with a 40:1 vertical exaggeration. To facilitate judging the magnitude of slopes on these profiles, Figure 7 shows various gradient ratios at a 40:1 vertical exaggeration. Slope corrections have not been made to the soundings. Except in special cases such corrections would make insignificant changes in the 40:1 profile.
All distances are given in nautical or geographical miles (1 nautical mile = 6080 feet).
Figure 7.—Gradients from 1:5 to 1:8000 shown at a 40:1 vertical exaggeration
Most profiles reproduced in this paper are at 40:1 vertical exaggeration. This template is provided to aid the reader in judging slopes on these exaggerated profiles.
Figure 8.—Outline of submarine topography
Line 1, first-order features of the crust; line 2, major topographic features of the ocean; line 3, categories of provinces and super-provinces; line 4, provinces; line 5, sub-provinces and other important features.