What is True North? Why do we have true North and What is it really - Well as you know the earth is round and the paper charts are flat - So when the chart makers want to lay out a map of the earth they have to start some where - So they choose a point to start from - TRUE NORTH - So there are many different chart makers and they are just as many different start points or DATUMS - The current U.S. standard is NAD 83. NOAA is in the process of changing from the North American Datum 1927 (NAD 27) to the newer World Geodetic System 1984 (WGS 84) standard. NOAA charts using the WGS 84 standard are listed as NAD 83. Be aware that the differences between NAD 27 and NAD 83 paper charts may range from 100 feet in Maine to 1,500 feet in Hawaii.
Ok so what! Well this brings us to GPS's and Compasses - If you are using a paper chart and you are taking Lat and Long of of it and plugging them into your GPS you best make sure that you are using the same DATUM as the paper chart - So how does this effect a compass a compass points to Magnetic North - as we know the magnetic north is different than true north and as we just discussed true north is whereever the chart company wanted to start. For this discussion we will stay in the northern hemisphere. When you are in Boston lets say the declination is about 16 degrees west for a particualar DATUM - as you move down the coast it gets less - about the time you hit the end of Florida it is about 0 declination - Declination is the difference from Magnetic north to the datum of the chart you happen to be using -
Now for the GPS - and I will qualify this with a standard GPS no electronic compass! A GPS gets its position data from satellites once locked-in it knows where it is however does not know where it is going or what the headings are to particular points. Once the GPS is moved and it acquires a new location it then calculates the heading from where it was to where it is - Never use a GPS heading from a stand still - It is the calculation that makes the GPS NOT real time but some moment after real time! Yes the GPS can tell you the heading to and object by knowing its current position and the objects position but from a stand still the GPS can't point you to it - So what to do! I am standing still in the water I have a buoy to the harbor as a waypoint I ask the GPS to get me a heading it calculates the heading, now I need to point the boat in that direction - Since the GPS is giving me a magnetic heading to the buoy I just look at my REAL TIME instrument my compass and point the boat to the magnetic heading the GPS gave me using the compass - Now this is assuming that you have the correct DATUMS and the GPS is set to give magnetic headings - I start moving baot using a correctly calibrated compass heading and some time later the GPS will plot my direction to the buoy and verifying my compass heading - Hence now I have two instruments telling me where I am heading - While staying with the compass heading I notice that the GPS is showing me that I am going off course - This is due to wind and current so you make a slight adjustment to the compass heading to compensate for the drift. While boating on a clear day you can compensate for current and wind drift with a compass, in a fog it is almost impossible because you have NO reference point. GPS adusts for wind and current drift -
Now those of us that are pilots know all about compasses in planes it is pretty simple 90% of the time they lie - Most smaller planes do have a compass however usually never use them for anything but straight and level non accelerated flying we use what is called a directional gyro - Why? - Well if you think boaters have it bad pilots have it worst - Flying with the compass is quite different from using the directional gyro. The compass has several inherent errors relating to turn, speed and geographic latitude. The compass should only be "read" in level un-accelerated flight. The swinging and dipping of the compass during a turn or acceleration due to changes in speed or direction is a physical phenomenon caused when the north seeking end of the compass dives toward the north magnetic pole. The higher the latitude the greater the dipping tendency. Turns from a northerly heading lag behind the turn; turns from a southerly heading lead the turn. When the card is banked the compass dips to the low side of the turn. ANDS is the mnemonic for acceleration errors. In a shallow 360 turn the compass is most accurate at 90 and 270 degrees. In a smooth turn from a south heading the rate shown on the compass exceeds that of the actual turn at a diminishing rate until at 90 or 270 degrees. So as you can see the compass in a plane is about useless to come to a direct compass heading - yes it can be used and pilots practice using it all the time - however is take allot of experience to use a compasses as the main and only directional instrument in a plane - Pilots are trained to use several instruments to verify what the plane is actually doing these are called secondaries - For example if you where to make a 10 degree change in direction a pilot would make compass turns using the turn coordinator and time. At 3 degrees a second a turn of two minutes is 360 degrees, one minute 180 degrees, 30 seconds 90 degrees and 10 seconds 30 degrees. A normal count of 1, 2, 3, (4) will be close to 10 degrees. Then it would be verified - against the other instruments -
Well I know this is boring it was something I happened to want to say now that many of us are hitting the water for the new season -
Mike -
