Understanding NMEA Data Strings in Hydrographic Surveying

In hydrographic surveying, seamless communication between sensors and software is vital. Most marine electronics use the NMEA 0183 standard to transmit data. In this post, we will discuss the most critical NMEA strings used for positioning and depth.

1. What is NMEA 0183?

NMEA (National Marine Electronics Association) 0183 is a combined electrical and data specification for communication between marine electronics such as GNSS receivers and Echo Sounders.

2. Critical Data Strings for Surveyors

• $GPGGA: This is the primary string for 3D positioning data (Latitude, Longitude, and Altitude).

• $GPVTG: Provides Track Made Good and Ground Speed—essential for maintaining constant survey speed.

• $GPZDA: Delivers UTC time and date for precise data synchronization.

Figure 1: High-precision GNSS receivers like the Trimble SPS generate the NMEA strings required for accurate mapping.*

• 3. Hardware Interfacing and COM Ports

To receive these strings in HYPACK, your computer must be connected to the sensors via RS232 Serial ports. Understanding the Baud Rate (usually 4800, 9600, or 38400) is key to a successful connection.

Figure 2: Proper cable management and port configuration ensure that NMEA data strings reach the acquisition software without delay.*

4. Troubleshooting Data Flow

If you don't see data in your software, check the following:

1. Wiring: Ensure the TX and RX pins are correctly assigned.

2. Baud Rate: The sensor and the software must "talk" at the same speed.

3. Data Interval: Set your GNSS output to 1Hz or 5Hz depending on your survey requirements.

Conclusion

Mastering NMEA data strings is the first step toward becoming a technical hydrographer. In our next post, we will cover Common Errors in Single Beam Surveys and how to avoid them.