^or "as-built" surveys, twice the vertical inaccuracy is the contour interval (see Equation 4-2, page 4-12). ^Average water depth of mapped area. bThe contour interval should never be less than twice the vertical inaccuracy (see Equation 4-2, page 4-12). Line Spacing. A major consideration in selecting the survey line spacing is the scale of the final bathymetric chart produced. Survey line spacing of less than 1/2 inch, at the final scale, is rarely of value and a generally recommended value is 1 inch at the final mapping scale. Therefore, at a 1:1,000 map scale, the minimum survey line spacing (0.5 inch) should be 500 inches or 42 feet. This close line spacing requires greater precision of horizontal and vertical control. This would not be an appropriate line spacing for either a civilian (standard positioning services) GPS survey (accuracy = ±150 feet) or a deep water survey using a fathometer with a footprint on the order of 40 feet. At a 1:5,000 scale, the recommended minimum survey line spacing (0.5 inch) is 208 feet, which is marginal for civilian GPS since, in general, it is desired that line spacing be two times navigational inaccuracy.

43.3.2 Fix Interval. Select the fix intervals along the survey line to provide four to five fixes per survey line interval when using a grid pattern or select the fix interval at 0.25 inch at final map scale when conducting a reconnaissance survey. Line Direction. In general, survey line direction should be perpendicular to the bottom slope and tie lines should be parallel to the bottom slope. The exceptions will be route surveys and as-built construction surveys.

Route surveys are arranged with survey lines parallel to the proposed route, and tie lines perpendicular to the route. It is important in route surveys to have adequate areal coverage, 50 percent greater than the proposed lane is recommended.

As-built surveys are laid out relative to the axis of the project, with little or no excess areal coverage.

The following convention is recommended for line naming (Figures 4-3 and 4-4 show this convention):

• East-West (magnetic headings of 45° to 135° and 225° to 315°) line numbers are even (2,4,6... 102,202,302...) increasing to the East.

• North-South (magnetic headings of 3150 to 045° and 135° to 225°) line numbers are odd (1,3,5...101,201,301...) increasing to the North.

• Blocks of line numbers are assigned to areas, i.e., 1 - 499 area A, 500 - 999 Area B, etc.

• Fix numbers should be sequential by area, restarting the unit digit at 1 each new line and increasing to the north and east (see Figure 4-4). If using an integrated navigation system that allows it, fixes should be equally spaced (distance) rather than time based.


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