1Datum
a base elevation to which elevations or depths are referred.
2Water-level datum
a vertical datum used to determine heights or depths in marine applications.
3Sounding datum
the reference surface used for soundings in a particular hydrographic survey.
4Chart Datum (CD)
the reference surface to which depths shown on a nautical chart are referred.
519-year National Tidal Datum Epoch
used to calculate tidal datums from observed water levels, accounting for long-term tidal variations.
6Water-level datums
represent local plane elevations.
7Geodetic vertical datums
provide references for land elevations.
8Station Datum
a fixed base elevation established at a tide station.
9Highest Astronomical Tide (HAT)
the highest tide predicted to occur under normal meteorological conditions.
10Mean Higher High Water (MHHW)
the average height of the higher high water occurring each tidal day over the tidal epoch.
11Mean High Water (MHW)
the average of all daily high-water heights over the tidal epoch.
12Mean Sea Level (MSL)
the average sea level for all stages of the tide over a long period.
13Mean Low Water (MLW)
the average of all low-water levels over a long period.
14Mean Lower Low Water (MLLW)
the average of the lower of the two low waters occurring each day.
15Lowest Astronomical Tide (LAT)
the lowest predicted tide under normal meteorological conditions.
16Diurnal Tide Level (DTL)
the arithmetic mean of MHHW and MLLW.
17DTL = (MHHW + MLLW) / 2
Diurnal Tide Level (DTL)
18Mean Tide Level (MTL)
the arithmetic mean of MHW and MLW.
19MTL = (MHW + MLW) / 2
Mean Tide Level (MTL)
20Great Diurnal Range (GT)
the difference between MHHW and MLLW.
21GT = MHHW − MLLW
Great Diurnal Range (GT)
22Mean Range of Tide (MN)
the difference between MHW and MLW.
23MN = MHW – MLW
Mean Range of Tide (MN)
24Mean Diurnal High Water Inequality (DHQ)
represents the inequality between MHHW and MHW.
25DHQ = (MHHW − MHW) / 2
Mean Diurnal High Water Inequality (DHQ)
26Mean Diurnal Low Water Inequality (DLQ)
represents the inequality between MLW and MLLW.
27DLQ = (MLW − MLLW) / 2
Mean Diurnal Low Water Inequality (DLQ)
28Greenwich High Water Interval (HWI)
the average time, in hours, between the Moon's transit over the Greenwich meridian and the subsequent high water.
29Greenwich Low Water Interval (LWI)
the average time, in hours, between the Moon's transit over the Greenwich meridian and the subsequent low water.
30Seaward Boundary Line (SBL) Error
an error in the seaward boundary line caused by tide-gauge or datum error.
31e_SBL = e_TD × cot(α)
Where:
• e_SBL — error in the Seaward Boundary Line
• e_TD — tide-gauge or datum error
• α — beach slope angle
Seaward Boundary Line (SBL) Error Formula
32A
height of the RTK GPS antenna above the reference ellipsoid.
33RD
raw depth measured by the echosounder, calibrated to the static waterline.
34H
height of the RTK antenna above the static waterline.
35Tc
conventional tide correction.
37SEP
separation between Chart Datum and the Reference Ellipsoid.
39D
dynamic draft correction caused by vessel squat and settlement.
40Conventional Chart-Sounding Equation
Computes chart sounding using raw measured depth and the conventional tide correction.
41CS = RD + D − Tc
Where:
• CS — Chart Sounding
• RD — raw depth
• D — dynamic draft correction
• Tc — conventional tide correction
Conventional Chart-Sounding Equation
42RTK Tide Correction
Determines the RTK tide correction using antenna height, antenna height above the static waterline, dynamic draft correction, and datum separation.
43Tk = A − H + D − SEP
Where:
• Tk — RTK tide correction
• A — antenna height above the reference ellipsoid
• H — antenna height above the static waterline
• D — dynamic draft correction
• SEP — separation between Chart Datum and the Reference Ellipsoid
RTK Tide Correction
44RTK-Based Chart Sounding Equation
Computes chart sounding using RTK-based positioning and the separation between the reference ellipsoid and Chart Datum.
45CS = RD − A + H + SEP
Where:
• CS — Chart Sounding
• RD — raw depth
• A — antenna height above the reference ellipsoid
• H — antenna height above the static waterline
• SEP — separation between Chart Datum and the Reference Ellipsoid
RTK-Based Chart Sounding Equation