The GMI 10 and GMI 20 do have the ability to display the surface wind direction and speed, but will need more information to calculate than what the GWS 10 Wind Sensor sends to it.

The GWS 10 can only send the raw wind data like wind speed and direction. For the GMI 10 or GMI 20 to be able to calculate surface wind direction and speed, it will need to know the boat's speed and heading. The best way to add this information into the GMI 10 or GMI 20 is to connect it to a device that outputs GPS information over NMEA 0183 or NMEA 2000. If a GPS is not installed on the boat already, a GPS antenna can be used.

Garmin offers two GPS antennas that will provide the needed data:

The GPS antenna can read the speed of the boat based on the position changing when the boat is moving. The GPS antenna can also read the direction the boat is moving. However, when the boat is sitting still, the GPS antenna will not be able to provide a true heading. This is because the GPS does not know which way the boat is facing when the boat is stopped, therefore the GMI 10 and GMI 20 cannot provide an accurate reading. To correct this issue, a heading sensor can be connected to the GMI 10 NMEA 2000. Garmin offers a 9-axis heading sensor or a SteadyCast heading sensor.

With the Heading Sensor and the GPS antenna connected to the GMI 10or GMI 20, they will be able to calculate surface wind direction and speed as it will know the boats speed from the antenna, and the boats true heading from the heading sensor.