Yes, tons of confusion here, partly because of terminology, and partly due to misunderstanding of various pieces of equipment. Here's some of the confusion I'm reading:
- Some people are using the term "GPS" or "GPS map" to refer to a chart plotter. We can thank the automotive GPSs for this since on cars the GPS and charting (mapping) are a single unit and have come to be called a GPS. And to fuel the fire, many marine chart plotters now include an integrated GPS sensor too.
- We would talk across each other much less is we referred to our chart plotter and our GPS as separate devices/functions.
- A GPS gives position, of course, but it also give course over ground (COG) and speed over ground (SOG).
- COG and heading (HDG) are different. They differ by the effects of current, wind, etc.
- It hasn't come up yet, but SOG is different than STW (speed through water) for the same reason that COG and HDG are different.
- It's a little unclear what an "electronic" compass is. It probably means a fluxgate or rate compass.
- A rate compass and a flux gate compass are magnetic heading devices, though solid state. They report magnetic heading (HDG), and sometimes rate of turn (ROT). Some even report pitch and roll.
- A Sat compass is both a GPS and a heading sensor. It reports all the usual GPS data (COG, SOG, position), plus it reports all the usual heading sensor data like HDG, ROT, and pitch/roll.
- A Sat Compass includes 2 or more GPS receivers, and uses them to compute the heading. It is very accurate - more accurate than the best rate compass. All the sat compasses I'm familiar with also include a rate compass that is used to stabilize the GPS derived heading, and provide very rapid and accurate changes to heading. It also preserves heading through GPS outages like when passing under a bridge.
- A Sat Compass can report true heading where a fluxgate or rate compass can only report magnetic heading. In many, if not most cases, this doesn't matter. Other devices will often know and share the local variation so true heading can be computed and displayed if desired. But this matters when you get into commercial equipment, several of which require true heading (HDT) directly from the heading device. Class A AIS is an example, along with any IMO certified radar. These devices require a Sat Compass or a Gyro.
- A GPS can not report meaningful COG when there is no movement, or little movement.
- A heading sensor can report HDG regardless of boat movement.
- Every chart plotter that I have seen can display both COG and HDG. They can display it numerically in a data box, and graphically as part of the boat icon. Some display two different colored vectors emerging from the bow of the icon, one showing COG and the other showing HDG. Others show COG with a vector, and skew the boat icon itself to represent the heading. When you are in a strong cross current these can be significantly different. I have seen a difference of 15-20 degrees on a number of occasions.
- Auto pilots work much better with a heading sensor vs GPS COG. In fact, I think they all REQUIRE a heading sensor and will not work on COG.
- Radar overlay on a chart plotter often requires HDG. So consumer plotters will use COG, but HDG works much better, and some plotters will only provide overlay if HDG is available.
- ARPA/MARPA on a radar also requires HDG.
- You do not NEED a heading sensor to navigate. GPS will work fine. But lots of things will work much better if you have a heading sensor, and some things will only work if you have a heading sensor.
Sorry for being pedantic here. I know lots of people understand all this, but there is also clearly some misunderstanding about what the various devices do and what they enable.
