Eye4Software Hydromagic

  Hydrographic Survey Software


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Using RTK tide corrections

Using a RTK receiver in fixed mode, you have X, Y and Z coordinates with an high accuracy ( < 2 cm ).
Using the Z coordinate (also known as ellipsoidal height), you can calculate the current tide level in real-time,
making it no longer necessary to have a tide gauge connected to the software, or to correct the sounding afterwards.


Requirements

In order to calculate tide levels in real-time, you need a RTK receiver with a valid correction signal.
This correction signal can be provided by a GSM / GPRS ( NTRIP ) service provider, UHF base station or your own base station on a known location.

NOTE: When your RTK receiver does not have a built in cellular modem to make connection to the internet, but your computer does have an active internet connection, you can use the 'Hydromagic NTRIP Plugin' to feed RTCM correction data into your RTK receiver.

You also need a local geoid model. This is a file containing geoid - ellipsoid separation values for the area or country you are performing your survey.

Hydromagic ships with a couple of geoid models, including Belgium, the Netherlands and North America.
Additional geoid models can be downloaded free of charge from the "geoid download page".

If no geoid model is available for your area, please contact support for assistance.

To perform the calculation, you also need to know the exact distance between the receiving element of your RTK antenna and the water surface.


RTK receiver output

To retrieve the ellipsoidal height from your RTK receiver, it should have an NMEA0183 data output, supporting either the $GPGGA or $PTNL,GGK sentence as shown in the examples below:

$PTNL,GGK,453049.0,0,3728.455440850,N,12215.253291068,W,3,9,2.0,EHT35.7424,M*
Trimble proprietary message for RTK position

$GPGGA,042629.00,3242.86076286,S,15133.15384408,E,4,12,1.1,6.573,M,26.799,M,1.0,0000*51
Standard NMEA0183 sentence for GPS or RTK fix data

When the Trimble proprietary NMEA0183 message is used, the ellipsoidal height is provided directly in the sentence, in the example above it is in the ellipsoidal height (10) "EHT35.7424" field.

For standard NMEA0183, the ellipsoidal height is calculated from the M.S.L. (9) height and geoidal separation (11) field. In this case the ellipsoidal height is calculated by using the formula below: In the example above, the ellipsoidal height is 6.573 + 26.799 = 33.372 meter.

Ellipsoidal Height = M.S.L. Height + Geoidal Separation
Formula for calculating ellipsoidal height from NMEA0183 GGA sentence

Please not that the M.S.L. ( Mean Sea Level ) height value returned by the GPS is calculated using the global EGM96 geoid model, and isn't accurate enough to use for your surveys. This is why you have to provide a local geoid model instead.


RTK configuration

In order to configure Hydromagic to use RTK tides, open the preferences dialog by selecting "Preferences..." from the "Options" menu.

To view the RTK configuration, click the "RTK" tab. You should see following dialog:

RTK Configuration

To enable tide calculation, you have to check the "Use Real Time Kinematic (RTK) for real time tide calculation" box.
When checked, you can alter the other configuration options.

First you have to enter the height of the antenna above the water surface.
Make sure to measure this distance as accurate as possible, an error in this measurement is directly converted to an error in the tide level.
Secondly, select the geoid model used in your area by clicking the "Select..." button.

Select geoid model

For some area's it is possible to select a vertical reference, like NAVD88, NVGD29, M.S.L. etc.
Please note that this setting is ignored for some (most) countries.

When finished configuring, just click "OK" to apply the settings.


Downloading additional geoid models

When selecting a geoid from the list, you may have noticed that some models are displayed in 'red' while others are displayed in 'black'.
Geoid models that are displayed in 'black' are already installed because they were shipped with the Hydromagic installer.

When you need to use a geoid model that is displayed in 'red', you have to download it first.
To do so, right-click the model and select the 'Download and Install' option from the popup menu:

Download geoid model

` After downloading and installing the geoid model should be displayed in 'black'. If not there was a problem with the download.
You might want to retry another time, or download and install the geoid model manually, which is explained on the "geoid download page".


Advanced Settings

Normally you do not have to use these settings. In some cases they are needed though to get a good result on the height readings.
When your height measurement is far off, please try these settings to see whether they fix the error.

Another cause of height measurement errors is selecting a missing geoid model.
Always check whether the geoid model used is installed (see previous chapter).

Ignore geoid model and use MSL field from GPS directly

Some newer RTK rovers have the ability to load a geoid model internally. This means that the orthometric height is already calculated by the rover.
In these cases the orthometric height is returned in the MSL field of the NMEA0183 GGA sentence. If this is the case, select this option.

Treat ellipsoidal height as orthometric height

This options should be used when a total station is used which outputs XYZ data in a pseudo NMEA0183 GGA sentence.
In this case no geoid model will be applied and the ellipsoid height field is used as orthometric height directly.


Testing RTK and geoid calculations

In order to test RTK tide calculations, connect and power on your RTK receiver and make sure you have a valid RTK fix.

The next step is to check the values in the "Navigation Data" window.
If this view isn't visible, you can activate it by selecting "Navigation Data" from the "View" menu.

RTK tide values

You have to check the following:



Start recording

When the conditions above match, you can just start a new recording and proceed with your survey.
When generating a sounding from your raw data files, you will be able to correct the tide settings if needed.