ouput on the spx5 will output the Fluxgate
heading data. You should look at the Garmin to see if you have the option to display that heading data on your unit. I have done this on Garmin 740 system and 4208 system. When you set the Garmin to display the heading this will lock your chart to the fluxgate compass
heading on the display.
You may notice that when you are dead in the water
i.e. at the dock
your chart will shift around because you have no GPS
heading data to orient the chart but when you feed the output from the spx5 NMEA to the Garmin input the heading data from the fluxgate will orient the chart.
Hope this helps