MSL (Mean Sea Level)

Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardized geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location.

MSL in Hydromagic

In Hydromagic, you can reduce your soundings to MSL by loading a tide file containing MSL tide values. There is also an option to calculate these values by entering tidal constituents for your area, using the EGM2008 geoid model or when you are in the United States, NOAA VDATUM.

Method #1 - MSL using a tide file

Loading a tide file is part of the process of creating filtered and corrected sounding files using the "Sounding Wizard". You can start this wizard by selecting the "Generate Soundings..." option from the "Tools" menu. Alternatively, you can right click the "Raw Data" folder in the "Project Explorer" and select the "Process Raw Data File(s)..." option from the popup menu.

Importing MSL tide files from ASCII

MSL tide files can be imported in the "Tide File Editor", which is part of the "Sounding Wizard", the tool which is used to convert your raw sounding data into a filtered and corrected sounding file.

The tide editor can be launched from the "Sounding Wizard".

To open the "Tide File Editor", select the "Use tide correction file" option, and click the "Editor..." button. The tide editor is now started using the selected tide file, and should look like this:

The "Tide File Editor" can be used to import or edit tide observation data.

If a file containing the tide values is in CSV (ASCII) or Microsoft Excel format you can import it into Hydromagic. Please note that you have to use Microsoft Excel to convert XLS or XLSX files into CSV format first. Other file formats are not supported. In these cases you have to enter all tide values for the period of your survey by hand. To start importing CSV files, click the "Import CSV" button (marked with a red rectangle in the image above).

CSV import tool

After the import tool has been started, select the file containing the tide observations or predictions first by clicking the "Browse..." button. After the file has been selected, click the "View..." button to inspect the format of the file.

Next, select the field separator, which would be in most cases, a comma. The date for the data can be read from the file, or selected manually in cases where the date is not present in the file. Select the field number for the date and date format when the date is in the file.

Use the next section of the window to select the column used for the time as well as the time format. When the time format used in the file is not supported, please contact support.

Finally you have to select the field that contains the tide level. For files in the United States, you probably have to select the units to "Feet". For most other countries, leave this field to "Meters".

The CSV (Comma Separated Values) import tool in the tide editor.

CSV tide file examples

Below are examples of the two most used formats to store tidal data as CSV:

  11:10:00 AM,1.446
  11:20:00 AM,1.470
  11:30:00 AM,1.497
  11:40:00 AM,1.522
  11:50:00 AM,1.544
  

The file above shows a tide file containing time stamp and tide level. In this case you have to select the date manually. Set the time field number to "1" and the time format to "HH:MM:SS PM". For the tide level, set the field to "2".

  08/09/2015 00:00:00 1.239
  08/09/2015 00:10:00 1.209
  08/09/2015 00:20:00 1.178
  08/09/2015 00:30:00 1.133
  08/09/2015 00:40:00 1.085
      

The file above shows a tide file containing date, time stamp and tide level. In this case the field separator is a space. you have to set the date field to "1" and select "DD/MM/YYYY" as date format. Set the time field number to "2" and the time format to "HH:MM:SS". For the tide level, set the field number to "3".

Saving the results

When settings are set according to the internal organization of the file, click "OK" to accept the settings. If everything went well and the file can be parsed, a graph and the tide values will be displayed.

Now click the "Save" button, enter a name for the tide value and click "OK" twice to save the file and close the dialog. You can now continue following the steps in the sounding wizard.

Method #2 - Using EGM2008

When you are going to perform your Hydromagic survey utilizing a GNSS RTK receiver with base station coverage or PPK post-processing, you can also choose to use real-time tide corrections by using the EGM2008 geoid model. To do so, you can use the EGM2008 utility to extract a part of the model covering the area you will be working in since the complete model is almost 1 Gigabyte in size. This process is very simple:

1. From the "Tools" menu, select the "EGM2008 Extractor..." option from the "External Tools..." sub menu to launch the EGM2008 extraction utility.
2. Download and, after unzipping the file, select the EGM2008 file from the application.
3. Enter the boundaries of your working area. It is safe to round the coordinates to whole degrees latitude and longitude.
4. Select the output file. The file should be stored in the "Geoids" folder under Hydromagic's "Program Data" folder (selected by default).
5. Click "OK" to start generate the file. When done, close the EGM2008 utility. You can now add the geoid model in Hydromagic.

The EGM2008 geoid conversion utility is shipped with Eye4Software Hydromagic.

Setting up the EGM2008 geoid in Hydromagic

To calculate the Mean Sea Level tide level in real-time, you have to instruct the software to use the EGM2008 geoid model when a RTK fixed 3D position is available. In addition, you also have to enter the absolute offset between your RTK antenna and the water surface, which can be entered in the Vessel Editor (or using the shortcut in the RTK settings tab).

To setup the parameter for RTK tides, launch the Hydromagic software and select the "Preferences..." option from the "Options" menu. Next click the "RTK" tab to switch to the RTK settings. When you are selecting the newly generated EGM2008 geoid file, click the "Select..." button to open the geoid model selection window. From this window, you can also modify, download and add new geoid models.

In the RTK tab, enable RTK tides and set the geoid model and antenna elevation.

To add our newly generated model, click the "Add" button and enter a descriptive name for the geoid model (MSL-EGM2008 in our example). In the second field you can select the country where the model is used. Finally, click the "Browse..." button and pick the geoid file from the file selection box. When done, you can click the "Save" button to store this new geoid model so it can be used directly from the software. After saving, click the "OK" button if you want to select the model into the RTK settings tab.

Create a geoid model definition in the Hydromagic geodetic database.

After adding and selecting the EGM2008 geoid model, make sure that you also enter the elevation of the GNSS antenna above the water surface. Please measure this distance as accurate as possible. In most cases you also want to reverse the sign of the calculated tide level or elevation. To do so, please see the paragraph "Sign Reversal" below. You can now record and process data using Mean Sea Level.

When you want to process the recorded raw data files using the "Sounding Wizard" using MSL tides, make sure to select "Use geoid model" option on the third page of the wizard. it is recommended to also always double check the selected geoid model and antenna height before moving to the next step.

From the "Sounding Wizard", select the "use geoid model" option.

Checking the results

You can check whether the tide corrections have been applied correctly by having a look at the sounding data. To do so, right click the sounding in the "Project Explorer" and select the "Edit Sounding..." option. The "Sounding Editor" dialog should appear, which shows the tide values in one of the columns:

Sounding editor showing tide values.

Sign reversal

When reducing depths relative to a tide level at sea, the convention is that the sign is to be reversed (The term above sea level generally refers to above mean sea level (AMSL) and not below sea level). This is different from measurements of river bottom elevations, which do not have a reversed sign. In case you have to reverse the sign on the map, in exported data or in the sounding editor, select the "Preferences..." option from the "Options" menu and select the "Map" tab.

In the map settings tab, locate the "Reverse sign of elevation values" check box and enable it. Please make sure to disable this option when you have a project where RTK tides are used (Inland).

The sign of elevation values can be reversed using the "Map" settings.