*Or how to tell what direction to point the boat or which way is home.*

“Correcting” is converting a magnetic (M) direction to true (T). “Uncorrecting” is converting from true to a magnetic direction. “True” direction is relative to true north (north pole).

Variation is the failure of the compass to point to true north (90 Degrees North), this difference is due to the Magnetic North Pole being located in Canada. Variation is read from the compass rose on the chart. Variation is the same for all boats in the same area. Variation is the same for all directions in the same area.

Deviation is the failure of the compass to point to magnetic north due to local magnetic fields on a vessel. These fields may be caused by the engine, instruments or other electronic equipment, power wiring, tools or any other magnetic objects. Deviation is specific to an individual boat. Deviation changes with the boats heading. Deviation is read from the Deviation Table on the boat.

↓ T ↑ True (-) E ↓ V ↑ (-) W Variation ↓ M ↑ Magnetic (+) W ↓ D ↑ (+) E Deviation ↓ C ↑ Compass Tele-Vision Makes Dull Children add Wonder. The heading must be uncorrected to provide a compass reading for the helmsman to steer.

**Navigators Piloting and Charting Reference Card**

Reference card with help for correcting and uncorrecting a compass, speed / time formulas and other quick hints for navigators.

**Examples:**

True 347 237 160 070 ___ ___ ___ ___ Variation 011E 014W 012W 014W 013W 011W 012E 014W Magnetic 336 ___ ___ ___ 135 196 310 068 Deviation 005E 007E 010W 021W 012E 018E 007E 010W Compass 331 ___ ___ ___ ___ ___ ___ ___

**Examples 2:**

True ___ ___ ___ ___ 062 191 274 004 Variation 011E 013W 012E 010W ___ ___ ___ ___ Magnetic ___ ___ ___ ___ 057 179 290 354 Deviation 013E 009E 004W 006E ___ ___ ___ ___ Compass 025 114 265 358 045 185 276 010

**HELP HELP HELP!!!!!!!!!!!**

The skipper just ask me to give them a compass course to steer and when I ask for the “Deviation Table” for the boat they said there were no dirty magazines aboard the boat and furthermore their sex life was none of my business. And besides I can’t remember any of this stuff anyway. Now what the heck do I do??

The compass reading can in this case be taken directly from the chart using the magnetic Compass Rose if several cautions are used. The Deviation for a boat can be large enough that a course can have a significant error. This error can be enough that a course laid from Alki Point to the West Point buoy can be off enough that the boat would run aground east of the West Point Light House and not pass to the west of the buoy. The course can be checked by plotting a reciprocal and using a hand bearing compass to check from where you came from as well as the direction to the desired destination. Use the chart to keep track of the position of the boat on any other land marks. Using other types of position indicators such as GPS, water depth or radar.

Extreme caution needs to be used if visibility is impaired (such as fog or rain).

**Answers:**

True 347 237 160 070 122 185 322 054 Variation 011E 014W 012W 014W 013W 011W 012E 014W Magnetic 336 251 172 084 135 196 310 068 Deviation 005E 007E 010W 021W 012E 018E 007E 010W Compass 331 244 182 105 123 178 303 078

**Answers 2:**

True 049 110 273 354 062 191 274 004 Variation 011E 013W 012E 010W 005E 012E 016W 010E Magnetic 038 123 261 364 057 179 290 354 Deviation 013E 009E 004W 006E 012E 006W 014E 016W Compass 025 114 265 358 045 185 276 010Printer Friendly (pdf) Version

**Error Note: **The first group next to last question the True course should be 322 NOT the 298 shown in the PDF version. I have corrected the Table above and the PDF version. 09/14/2011 The new PDF file name is ( chart_02-b.pdf ) Sorry for the inconvenience.

Great site. Learned about it in the AUXNAV class last night.

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second to last answer on first group is wrong true course should be 322.

thanks

Andrew

Thanks Andrew – I have corrected the answer above but will not have time to correct the PDF version until later today.

Pingback: TVMDC – Practice – Correcting and un-correcting the compass | Boating Safety Tips, Tricks & Thoughts from Captnmike

how to un correct the compass?

Un Correcting is converting from True to Magnetic – I always write the formula the same way every time then work up or down – Un correcting for me is working down on TVMDC and up to correct

how to solve when true error is given

I am not sure what you mean by “True Error”

Variation error is caused by Magnetic North not being in the same place as True North.

Deviation error is caused by local magnetic fields on the boat.

When I am solving compass problems, I write down the TVMDC formula with T at the top – then I fill in the information that I have, then solve for the blanks. Exactly like solving for the blanks in the exercise problems.

I always solve in exactly the same way, working up or down for the answers. No shortcuts, no adding or subtracting just two numbers on a piece of scratch paper. Work the problems in the exact same way each time, this also makes it easy for someone else to check your work and you can also easily check your own work

thankyou