Hydrographic and Topographic Surveying
This domain combines water-surface and bottom surveying with topographic surface modeling. DC questions lean on tidal datums, sounding reduction, and volume or contour accuracy.
Hydrographic surveying
- Soundings are reduced to a chart datum, commonly MLLW, by applying tide, transducer draft, and sound-velocity corrections. The water surface is not an ellipsoid or a fixed elevation.
- IHO S-44 defines survey orders (Exclusive, Special, 1a, 1b, 2) with different accuracy and feature-detection requirements.
- Single-beam, multibeam, side-scan sonar, and ADCP each answer different questions: bathymetry, object detection, or current measurement.
- In DC, the Potomac and Anacostia are tidal; MHW affects the landward boundary, while navigation and dredging involve USACE permits.
Topographic surveying
- Breaklines control the TIN and the contours; spot elevations alone cannot represent a curb, wall, or ditch.
- Cross sections and average end area or prismoidal methods compute volumes.
- FEMA floodplain mapping and DC floodplain review depend on accurate ground and structures.
Worked example
A sounding is taken at 14:32 with a measured depth of 12.4 ft. The tide at that time is +1.8 ft above MLLW, and transducer draft is already included in the measured depth. Reduced depth = 12.4 + 1.8 = 14.2 ft at MLLW. If the tide were falling, the sign would reverse; a tide below MLLW would reduce the depth below the measured value.
Volume example: two cross sections of 120 sf and 180 sf, 50 ft apart, give (120 + 180)/2 × 50 = 7,500 cf, or 278 cy.
Traps that cost marks
- Applying the tide correction with the wrong sign.
- Forgetting transducer draft or sound-velocity correction.
- Using ellipsoid or orthometric heights for a water surface.
- Interpolating contours across breaklines instead of honouring them.
- Computing volumes without stripping or without consistent end areas.