REDUCTION OF MEAN SEA LEVEL DEPTH BASED ON TIDE GAUGE DISTANCE-DEPENDENT AT SUNGAI DINDING, LUMUT

: Tidal correction is vital in shipborne bathymetric survey. This research uses two different tide gauge stations as tidal corrections to reduce the sounding depth. The tide corrections used are from the tidal observation at the survey site and the nearest tide gauge established by the Department of Survey and Mapping Malaysia (DSMM). The issue that may affect the results is the distance between the tide station used for corrections and the survey site. Are the results obtained by these two distinct tide corrections comparable? And if not, what are the cause for any discrepancies in the results? Thus, this research aims to assess the reduction of Mean Sea Level (MSL) depth using the bathymetric survey data at Sungai Dinding, Lumut relative to two different tide gauge with different distance to the survey site. The two distinct tidal corrections are observed and analysed separately. The tidal data are processed using HYDROpro software, followed by the analysis of the bathymetric plan and the computation of Root Mean Square Deviation (RMSD). The results of different tide corrections are then compared. Furthermore, the reliability of bathymetry data relative to two tide gauges at different distances is also evaluated. For various reasons, the tidal corrections shed new light on the accuracy of depth reduction in the Sungai Dinding area. Besides, using two different tide gauges: on-site tide gauge observation and the nearest DSMM tide gauge (Lumut), provide an insight into the reliability of bathymetry data by comparing the depth derived relative to two different tide gauges at different distances from the survey site. The findings are instructive for future depth reduction, as they will allow for more practical application of the tide corrections. Optimistically, this study will raise awareness of the importance of tide station location in the bathymetry survey area.


INTRODUCTION 1.1 Introduction
Tide is a critical component in most of the bathymetric survey application.It is because, tide is used in bathymetry data processing to determine the actual depth of the survey area.Various tide corrections are used to assess the depth reduction.The tide must be reduced to Mean Sea Level (MSL) or Lowest Astronomical Tide (LAT), depending on the purpose of the application.Generally, MSL is used as a sounding datum for engineering works.While for navigation purposes, the sounding datum is reduced to the LAT.Therefore, it is necessary to apply tide corrections when creating a bathymetric plan.
During hydrographic survey, the depth is measured from the water surface to the sea floor.However, the water surface of oceans, seas, rivers, and lakes are not static.It varies due to the meteorological, oceanographic, and tidal effects.Thus, this study focusses more on the tidal effect.The measured depth values of water level must be reduced to a specific vertical datum before constructing hydrographic plan or producing nautical chart.The values of tidal corrections must be determined to accomplish the depth reduction.Normally, the tidal corrections are computed based on the tide observation established near the survey site.A tide gauge (also known as a mareograph, marigraph, or sea-level recorder) is a device that measures the changes in sea level with respect to a vertical datum (Khare et al., 2019).
The tide gauge station used must be at the nearest hydrographic survey area.This is because tidal behaviour might differ at different places.According to Kim et al. (2022), tides do not follow the same patterns everywhere because the Earth's surface is not uniform.The shape of an ocean floor affects the range and frequency of tides.According to Awang et al. (2011) and Zapata et al. (2019), tidal readings are usually obtained at a tidal station established near the survey area.This method is applied to avoid the discrepancy of tidal behaviour in the survey area and the tidal station.
This study aims to assess the reduction of MSL depth based on tide gauge distance-dependent with the bathymetry data at Sungai Dinding, Lumut.The outcome may differ, and the results may be influenced by the differences in tidal corrections used.The nearest Department of Survey and Mapping Malaysia (DSMM) tide gauge station (Lumut) is located slightly far away from the survey area which is approximately more than 10km.Meanwhile, the other tide gauge station is less than 1km within the survey area (hereinafter, on-site tide station).Both data from these stations are used to derive tidal corrections in this study.The tidal data of two different years (2017 and 2018) are used in this study as the bathymetric survey is deployed during the stated years.HYDROpro software is used to process the depth reduction resulting from these tide corrections.The bathymetry plan for 2017 and 2018 with reduced sounding depth relative to in-situ tide gauge and DSMM tide gauge stations are generated.The establishment of the bathymetric plan is visualised using AutoCAD and Surfer software.Then, the Root Mean Square Deviation (RMSD) is calculated to evaluate the reliability of bathymetric data based on two different distance of each tide stations with the survey area.

Research Area Identification
According to Tye (2020), Sungai Manjung is one of the major rivers in Perak.It was named after the district of Manjung, previously known as Dinding.Additionally, the river is known as the Dinding River or Sungai Dinding.The study area at Sungai Dinding, Lumut, with a scale of 1:26000, is shown in Figure 1.The study area includes Sungai Dinding and Lumut coastal areas.Table 1 demonstrates the geographical coordinates, the distance from the study area, and the data spans of the DSMM tide gauge station.

Data Acquisition
Hourly data from each tide gauge station and bathymetry data for year 2017 and 2018 are retrieved from the Hydrography Laboratory, Faculty of Built Environment and Surveying, Universiti Teknologi Malaysia from the Survey Camp data.The depth reduction process uses the hourly tidal data for one to two days-the latest updated and complete data for each station are shown in Table 1.

Data Processing
Tidal data are acquired and processed using HYDROpro software.In this software, the sounding depths are reduced and the final bathymetry data is generated.Then, the final bathymetric plans are produced using AutoCAD and Surfer software.

A) NavEdit
The NavEdit Tide Editor creates tide files used in the Depth Editor to reduce depth data for tidal effects.This editor allows tide data to be manually entered or by importing the data from ASCII files.In this study, the tide data is manually entered.Figure 2 shows the Tide Editor desktop interface, where the tidal data are plotted in the Tide Graph and the Tide Grid.The primary function of the Tide Editor is to create files consisting tidal data for a specific location, for instance, tide gauge station.The Depth Editor then uses the tide data to reduce the depth data to the local chart datum.Tide data can only be used in the Depth Editor if the tide file is created.

A) AutoCAD Software
In DOS Processing, the final bathymetry data can be exported into DXF file formats as illustrated in Figure 4.This DXF file is used in AutoCAD to perform final editing and produce the complete bathymetric plan.The complete bathymetry plan usually consists of legend, location plan, key plan, the north direction, and etc (See Figure 5).For the bathymetric plan in 2017, the scale used is 1:12000 in A4 size, while in 2018, the scale used is 1:8000.The bathymetric plan in this study uses the Geocentric Datum of Malaysia 2000 (GDM2000) as a reference datum and Rectified Skew Orthomorphic (RSO) Geocentric for map projection.Surfer software also has an ability to generate bathymetric plan.Nevertheless, the output is slightly different from AutoCAD.Surfer is used to interpolate the scattered bathymetric data into gridded data.In this study, Kriging is selected to generate a 3-Dimensional bathymetric plan.This is due to the flexibility of gridding method.Depending on the user-specified parameters, Kriging can either be an exact or a smoothing interpolator in Surfer.It incorporates anisotropy and underlying trends efficiently and naturally.Next, the grid spacing selected is based on the comparison of several values.The comparison is based on the smoothest map produced.Figure 6 shows the grid data set for the plot.Grid files are necessary for Surfer to create grid-based map types.Generally, the data files are randomly spaced files and these data must be converted into an evenly spaced grid before any other features in Surfer is used.Grid files are produced from East, North, and Depth data from the HYDROpro.A 3D surface clearly shows the terrains of river bed compared to a 2D surface (see Figures 7 and 8).

Data Evaluation
The RMSD is one of the statistical analyses used for data validation between the reduced sounding depth relative to the site tide gauge and the nearest DSMM tide gauge.It is often used in mathematical computations to assess the reliability of results.According to Glen (2021), the Root Mean Square Error (RMSE) measures how evenly distributed these residuals are.In other words, it indicates how robust the data is around the best fit line.The RMSD of the results can be used to determine the reliability of the bathymetry data between two different distance of tide gauge stations.The formula to calculate RMSD is identical to the RMSE formula and is shown in equation ( 1) (Ćalasan et al., 2020) where i is the variable value, X i is the DSMM tide gauge data, P i is the on-site tide gauge data, and n is the total number of data points.After obtaining the RMSD of reduced sounding depth relative to both tide gauge stations, these values are compared to the International Hydrographic Organization (IHO) S-44 Standard guidelines to determine the uncertainties of bathymetry.Generally, the IHO S-44 Standard aims to provide a set of standards for hydrographic surveys primarily used to compile navigational charts essential for navigation safety, particularly for LAT sounding datum.Although the reduced depth in this study is referred to MSL, the assessment is still corresponding to the IHO S-44 standard.This is due to the limited validation sources.The formula of Total Vertical Uncertainties (TVU) is used to calculate the maximum allowable vertical measurement uncertainty (International Hydrographic Organization Standards for Hydrographic Surveys, 6th edition).In order to calculate the maximum allowable TVU, the parameters "a" and "b", as well as the depth "d" must be included in the formula, as shown in equation 2: where a represents the portion of the uncertainty that does not vary with the depth, b is a coefficient, which represents the portion of the uncertainty that varies with the depth, and d is depth.Table 2 tabulates the minimum bathymetry standard.

Data Analysis
The processed results from each tidal correction of HYDROpro software are evaluated based on the bathymetric plan.The results are analysed by interpreting the bathymetric plan in AutoCAD and Surfer, both in 2D and 3D surfaces.The bathymetric plans for 2017 and 2018 are also compared.In addition, the bathymetric data are compared based on the distance of tide measurement to the survey area.Since the DSMM tide gauge is slightly far from the survey area, the results might give more unreliable output compared to the onsite tide gauge station as the tidal behaviour are not be the same.The calculation of RMSD will facilitate in validating the DSMM tide gauge data.It determines whether DSMM tidal data can still be used as tidal corrections for the hydrographic survey.

Establishment of a Bathymetric Plan in AutoCAD
After processing the raw depth data at Sungai Dinding in Lumut, a bathymetric plan is created.Based on the bathymetric plan of 2017 reduced relative to the on-site tide gauge data, as shown in Figure 9, the plan with a scale of 1:12000 shows the highest depth of 15.55 metres and the lowest depth of 1.26 metres based on the colour gradient.However, the bathymetric plan of 2018 reduced relative to the on-site tide gauge data, the plan with a scale of 1:8000 shows the highest depth of 15.52 metres and the lowest depth of 2.66 metres based on the colour gradient, as shown in Figure 10.

Establishment of a Bathymetric Plan in Surfer
According to Golden Software (2017), robust 2D and 3D mapping, modelling, and analysis programs are designed to facilitate a deeper understanding of geospatial data.Surfer is a leading competitor in data modelling software.Surfer software can produce the bathymetric plan both on 2D and 3D surfaces.

Bathymetric Plan
The bathymetric plan shows the depth of each tide gauge data in 2017 and 2018.(See Figures 13 to 16).The major contour line is dark brown, where the line width is slightly thicker than the minor contour.The dark brown colour also indicates the lowest depth, while the dark blue indicates the highest depth.The interval of the major contour is 3 meters.

Comparison of Reduced Sounding Depth Relative to the On-site Tide Gauge
Table 3 shows the exact observation points between the 2 years of the bathymetric plan relative to the on-site tide gauge.Figure 21 compares reduced sounding depths relative to the on-site tide gauge in 2017 and 2018 with a scale of 1:7000.The depth of 48 points from both years are compared and the differences are calculated.The highest depth difference is 0.8 metres, while the lowest is 0 metres.Data in Table 2 are illustrated as indicates in Figure 22.Figures 23 and 24 illustrate the comparison between the depth observed in 2017 and 2018 relative to the on-site tide gauge using Surfer.

Comparison of Reduced Sounding Depth Relative to DSMM Tide Gauge
The exact observation points between the 2 years of the bathymetric plan relative to the DSMM Tide Gauge is shown in Table 4.The depth of 48 points is compared between the years and the difference are calculated.The highest depth difference is 0.86 metres, while the smallest is 0.01 metres.The river floor of 2017 and 2018 bathymetric plan relative to DSMM tide gauge is identical to the bathymetric plan relative to the on-site tide gauge.The difference in river floor between the 2 years is also due to the same causes.The sedimentation that affects the bathymetric plan relative to the on-site tide gauge is in the same way that it affects the bathymetric plan relative to the DSMM tide gauge.

Statistical Comparison of Reduced Sounding Depth in 2017
Table 5 and Figure 29 show the comparison of reduced sounding depth in 2017.The difference between sounding depths is calculated by comparing the depth at 48 points.The highest depth difference is 0.17 metres and the lowest is 0.04 metres.According to Zach (2021), the lower the value of RMSD, the better a model can "fit" to a dataset.The range of the dataset must be considered when determining whether the given RMSD value is "low" or not.In 2017, the RMSD between the reduced sounding depth relative to on-site and the reduced sounding depth relative to DSMM tide gauge is 0.1171m, as tabulated in Table 6.This RMSD value is relatively low.When refers to the IHO S-44 standard (TVU), the RMSD is acceptable as the value is within the tolerance in all Level Order (See Table 7).It implies that the model difference is reliable.Thus, the reduced sounding depth relative to the DSMM tide gauge still can be used.

Statistical Comparison of Reduced Sounding Depth in2018
In 2018, Table 8 and Figure 30 show the comparison of reduced sounding depth.The depth at 48 points is used to calculate the difference between the sounding depths.The highest depth difference is 0.18 metres, while the lowest is 0.05 metres.In 2018, the RMSD values between the reduced sounding depth relative to the on-site and DSMM tide gauge is 0.1129m, as shown in Table 9.This RMSD value is relatively low.Referring to the IHO S-44 standard guidelines (TVU), the RMSD is acceptable because the value is within the tolerance standard in all Level Order, as shown in Table 10.It implies that the model difference is reliable.Thus, the reduced sounding depth relative to the DSMM tide gauge can still be used.

CONCLUSION
Tidal corrections based on the on-site and DSMM tide gauge are used to reduce the bathymetry data at Sungai Dinding.It reveals comparable results in bathymetric plans with low RMSD values at cm-level.These findings imply that the DSMM tide gauge at Lumut is reliable for tidal correction in this hydrographic survey area.This study provides guidelines of the selection of tide gauge stations for hydrographic surveys.Furthermore, comparing the distance between tide gauges while using the two tide gauge stations, on-site and the nearest DSMM tide gauge provides insights to the reliability of bathymetry data.In conclusion, this study will raise awareness of the importance of station location in the bathymetry survey area.

Figure 2 .
Figure 2. The Tide Grid and Tide Graph display B) Disk Operating System (DOS) ProcessingNavEdit can export a variety of file formats using the Export command.Then, the exported file can be used in DOS Processing to process the bathymetry data, as shown in Figure3.

Figure 3 .
Figure 3.The interface of DOS Processing while editing the bathymetry data

Figure 4 .
Figure 4.The DXF file formats

Figure 9 .
Figure 9. Bathymetric plan relative to the on-site tide gauge in 2017 plotted using AutoCAD

Figure 11 .
Figure 11.Bathymetric plan relative to the DSMM tide gauge in 2017 plotted using AutoCAD

Figure 12 .
Figure 12.Bathymetric plan relative to the DSMM tide gauge in 2018 plotted AutoCAD

Figure 13 .
Figure 13.Bathymetric plan relative to the on-site tide gauge in 2017 generated using Surfer

Figure 15 .Figure 16 .
Figure 15.Bathymetric plan relative to the DSMM tide gauge in 2017 generated using Surfer

Figure 17 .
Figure 17.A 3D bathymetric plan relative to the on-site tide gauge in 2017

Figure 21 .Figure 22 .Figure 23 .Figure 24 .
Figure 21.The bathymetric plan plotted using AutoCAD.a) The bathymetric plan in 2017 and b) the bathymetric plan in 2018 Figure 25 illustrates the comparison of reduced sounding depths relative to the DSMM tide gauge in 2017 and 2018 with a scale of 1:7000.The data tabulated in Table 4 is interpreted in Figure 26.Figures 27 and Figure 28 illustrate the comparison between the depth observed in 2017 and 2018 relative to the DSMM tide gauge.

Figure 25 .Figure 26 .Figure 27 .Figure 28 .
Figure 25.The bathymetric plan plotted using AutoCAD.a) The bathymetric plan in 2017 and b) the bathymetric plan in 2018

Figure 29 .
Figure 29.The comparison between depth relative to the on-site and DSMM tide gauge in 2017

Table 1 .
The coordinates and data period of the DSMM tide gauge station(Hao, 2021)

Table 2 .
Minimum bathymetry standard (International Hydrographic Organization Standards for Hydrographic Surveys, 6th edition)

Table 3 .
Depth comparison between 2017 and 2018 relative to the on-site tide gauge

Table 4 .
Depth comparison between 2017 and 2018 relative to DSMM tide gauge

Table 5 .
Depth comparison of reduced sounding depth relative to on-site and DSMM tide gauge in 2017

Table 6 .
RMSD between the reduced sounding depth relative to on-site and DSMM tide gauge in 2017

Table 8 .
Depth comparison between the reduced sounding depth relative to on-site and DSMM tide gauge in 2018

Table 9 .
RMSD between the reduced sounding depth relative to on-site and DSMM tide gauge in 2018 Figure 30.The comparison of depth relative to the on-site and DSMM tide gauge in 2018