2.測量準備と補正(Preparation & Calibration)


Q. RTK GPSによる潮位補正2(A Discussion of RTK TIDES)[英語]


We have been having a lot of internal discussions regarding the computation of RTK Tide Corrections and their comparison to Measured Tide Corrections. This article presents our current thinking on the subject and illustrates how we are computing and applying the corrections.

1. First – The Basics

The figure to the right shows all of the components needed to compute the Chart Sounding (CS = the distance the bottom is beneath the Chart Datum).

Using conventional survey techniques we have always added the various corrections to the measured depth from the echosounder to obtain the chart sounding. In this example, we have calibrated our echosounder to the surface. The formula would be:

CS = B + T1 + D
CS = 30 + (-10) + 0 = 20.0

where:  T1 = Conventional Tide Corr.
             B = Measured Sounding
             D = Dynamic Draft Meas.

The basics behind the computation of RTK Tide is that we can use the z-value of our GPS antenna to determine the tide correction in real time.


Assuming the vessel is not pitching and rolling (which adds another level of difficulty), we can compute the RTK Tide Correction (T2) as follows:

T2 = -K + N – A + H – D
T2 = -4 + 9 – 22 + 7 – 0 = -10.0

where:  K = Height of the Geoid Above the Chart Datum
                N = Height of the Geoid Above the Ellipsoid Reference
                A = Height of the RTK Antenna Above the Ellipsoid Reference
                H = Height of the RTK Antenna Above the Boat Origin Point
                D = Dynamic Draft Measurement

The K component comes from an KTD (Kinematic Tidal Datum) file created by the user.

The N component is actually the height of the Geoid Above the Ellipsoid (as read from the Geoid99 model in real time) plus an orthometric correction specified in the Geodetic Parameters program. If the user does not have access to a geoidal model of their area, they should create the KTD file so that it contains K - N values. This would represent the height of the chart datum above the reference ellipsoid.

The A component comes from our RTK system. This is provided anywhere from 1Hz to 10Hz by different GPS systems and broadcast as a part of a GGA or GGK or other message. Every time the KINEMATIC.DLL device driver receives a position update, it computes the new position, along with a new RTK Tide value.

The H component is the static height of the RTK Antenna above the water line. In order to maximize accuracy, this measurement should be taken at the same time you calibrate your echosounder to the surface. In theory, this should be measured to the same point (boat origin = static waterline) that you are using to calibrate your echosounder. In actual practice, it’s not practical to measure the antenna height out in the middle of the channel when you are doing a bar check. It is suggested that you measure the antenna height when the vessel is at the dock and place a mark on the hull to denote the static waterline. Then make an adjustment to the antenna height when you calibrate the echosounder by noting the change in height of the waterline relative to the mark.

The D value represents the vertical movement of the transducer in the water column and is called the Dynamic Draft Measurement. This movement can be caused by a lot of reasons. As you take on more fuel, the vessel can sit lower in the water. Due to squat and settlement, the vessel can have a drastically different position in the water when it is moving that when it is stationary. Some vessels, such as hopper dredges, have pressure transducers that allow us to constantly measure the current draft. Other survey vessels use the DRAFTTABLE.DLL that allows them to construct a table that is used to assign the draft based on the vessel speed. Other survey vessels assume the draft to be a constant and ignore any dynamic effects.

2. Underway – Dynamic Draft Sensor

Now we are driving are vessel over the same bottom that was used in the first example. The tide is unchanged, but due to squat and settlement, the vessel is sitting 1.0 lower in the water column.

Let’s first examine the values that have changed.

A = 21
D = 1
B = 29

All other values are the same.

Computing the RTK Tide:

T2 = -K + N – A + H – D
T2 = -4 + 9 – 21 + 7 – 1 = -10.0

This is correct, as the tide has not changed. Our chart sounding is:

CS = B + T2 + D
CS = 29 + (-10) + 1 = 20.0

In this case, the RTK Tide continues to equal the traditional tide correction (T2 = T1).

3. Underway – No Dynamic Draft Sensor

Now let’s look at the last case again, but this time we do not have any dynamic correction to the draft. We set it for 0.0 (calibrating the echosounder to the surface) and left it there.

Now, the values are the same as in the previous example, except:

D = 0.0

When we compute the RTK Tide (T2):

T2 = -K + N – A + H – D
T2 = -4 + 9 – 21 + 7 – 0 = -9.0

Using the RTK Tide to compute the chart sounding:

CS = B + T2 + D
CS = 29 + (-9) + 0 = 20.0

We still get the correct answer! What you must note (and something I have only recently learned) is that the RTK Tide correction that is computed in this case is not equivalent to the conventional tide correction (T2 ≠ T1).

If we used the conventional approach in this example, the computed chart sounding would be:

CS = B + T1 + D
CS = 29 + (-10) + 0 = 19.

In this case, the RTK Tide result is more accurate than the conventional result. RTK Tides has compensated for the squat and settlement without any measurement of the change in draft. This compensation is built into the RTK Tide value, resulting in it no longer being equivalent to the conventional tide value.


The important points of this case are:

When using RTK Tides, you should always calibrate your echosounder to the static waterline.
You should measure the height of the GPS antenna above the water line at the same time you calibrate the echosounder. As a minimum, adjust the GPS antenna height based on the change in draft from when you measured the original antenna height to how the vessel sits during the echosounder calibration.
RTK Tides will generate an accurate chart sounding, whether you apply a real time draft correction or not.
If you apply a real time draft correction, the routine in the KINEMATIC.DLL subtracts the dynamic draft correction in order to compute the ‘true’ tide correction. Absent a dynamic draft correction, the RTK Tide method will still result in an accurate chart sounding, but the user must realize that the RTK Tides values and conventional tide values will not be equal.
If your vessel is prone to squat and settlement or sits differently due to fuel loading, you may want to consider the RTK Tide approach being superior to the conventional tide approach.

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