CDMVT and Compass Error

**bensmiata** · March 17th, 2009, 08:55 PM

So I have always been confused about this, not the concept, just where to get CDMVT from. Compass Deviation Magnetic Variation True then use GET- Gyro East Error (Add) True . For example what compass do you use for Compass in CDMVT? Magnetic is obviously the standard compass and true is gotten from the difference between magnetic and variation adding east and subtracting west. Deviation is the error between the Compass and magnetic and variation is from the chart correct? I know these are basics, I just never asked the questions when I should have when initially learning it. Thank you.

**BlueNose** · March 18th, 2009, 03:30 AM

bensmiata,
It's a tool...

Example1:
How can you quickly tell if your gyro has shit the bed...CDMVT. If your Dev. if drastically +/- than what your Dev. table states...something is wrong then.

Example2:
TVMDC...Wow, I bet that just blew your mind

Hope that helps.

Cheers,
BlueNose

**Siberfire** · March 18th, 2009, 05:44 AM

So I have always been confused about this, not the concept, just where to get CDMVT from. Compass Deviation Magnetic Variation True then use GET- Gyro East Error (Add) True . For example what compass do you use for Compass in CDMVT? Magnetic is obviously the standard compass and true is gotten from the difference between magnetic and variation adding east and subtracting west. Deviation is the error between the Compass and magnetic and variation is from the chart correct? I know these are basics, I just never asked the questions when I should have when initially learning it. Thank you.

C- Compass on the boat. Normally PSC (per standard compass) one that is outside of the pilot house (to decrease potential deviation)

D- Deviation induced from the ship itself, the steel hull, electronics, etc. Deviation is different for each and every ship.

M- Magnetic is where the compass would point if there is no deviation at all from the ship.

Variation- Is the same for all ships in an area, it is found in the compass rose. It is the difference in degrees from the True north pole and the magnetic north pole.

True- The physical north pole. This is the most correct direction you can use.

GE - Gyro Error

G- Where the gyro points

CDMVT AE - Can Dead Men Vote Twice At Elections (add east)

The above is when you are correcting, from the least correct (compass) to most correct (true) you add any EAST variation and deviation. Subtract West.

If you cant tell I just spent the last 3 weeks studying this! Hope it helps.

**bensmiata** · March 18th, 2009, 10:49 AM

Originally Posted by Siberfire

C- Compass on the boat. Normally PSC (per standard compass) one that is outside of the pilot house (to decrease potential deviation)

D- Deviation induced from the ship itself, the steel hull, electronics, etc. Deviation is different for each and every ship.

M- Magnetic is where the compass would point if there is no deviation at all from the ship.

Variation- Is the same for all ships in an area, it is found in the compass rose. It is the difference in degrees from the True north pole and the magnetic north pole.

True- The physical north pole. This is the most correct direction you can use.

GE - Gyro Error

G- Where the gyro points

CDMVT AE - Can Dead Men Vote Twice At Elections (add east)

The above is when you are correcting, from the least correct (compass) to most correct (true) you add any EAST variation and deviation. Subtract West.

If you cant tell I just spent the last 3 weeks studying this! Hope it helps.

So from what I'm getting use a gyro repeater outside on a bridge wing for compass, magnetic for M and variation from the chart?

**Siberfire** · March 18th, 2009, 12:16 PM

LOL, the gyro is used to obtain a True heading. You just need to CORRECT it to True (that is, take into account Gyro Error, which should be known)

To get compass can either just look at your compass, but I wouldnt use it to chart with, you have to CORRECT the compass reading to get magnetic (you can use that) or go up to True using the local variation.

Read either Duttons: Navigation and Piloting or download Bowditch and take a look at the "compass" section.

**CMA_Decky** · March 18th, 2009, 01:34 PM

True Virgins Make Dull Companions, Add Whiskey (add west) :-)

**BlueNose** · March 18th, 2009, 03:58 PM

My bad, I thought you understood how calculate Can Dead Men Vote Twice and just wanted to know when to apply in real life situations.

You need to open up Pub 9 or 226.

**10talents** · March 18th, 2009, 11:42 PM

I like "True Virgins Make Dull Company" +Wiskey". Hey, my instructor was female what else can I say.

**jdcavo** · March 19th, 2009, 10:49 AM

Originally Posted by bensmiata

So from what I'm getting use a gyro repeater outside on a bridge wing for compass, magnetic for M and variation from the chart?

Maybe I should leave those compass error questions on the 3rd Mate exam for a while...

**Siberfire** · March 19th, 2009, 11:28 AM

Originally Posted by CMA_Decky

True Virgins Make Dull Companions, Add Whiskey (add west) :-)

LOL, that is so much better than our...

True Virgins Make Dull Companions, At Weddings

As a side note, my good friend a female cadet (read this as a smoking hot blonde) is also a big whiskey drinker. We will have fun with this one!

**Siberfire** · March 19th, 2009, 11:31 AM

Originally Posted by jdcavo

Maybe I should leave those compass error questions on the 3rd Mate exam for a while...

Bring them on! If you have ever met the nav instructor Bobby Mason at GLMA you know that we will know our corrections... or die of boredom trying!

**CMA_Decky** · March 19th, 2009, 06:06 PM

Originally Posted by jdcavo

Maybe I should leave those compass error questions on the 3rd Mate exam for a while...

Please do, I'd rather keep the nice fluffy correction questions on there then you guys get creative with new material.

**10talents** · March 20th, 2009, 12:35 AM

Originally Posted by jdcavo

You can always send us new material, if it's good, we'll use it. But don't just send us a question and an answer. The hard work is coming up 3 good wrong answers. Of course even giving us a question alone is more work than complaining.

I seem to have a boat load of these. It's like a tailbag full of crabs & seaweed when your trawling for shrimp.

**Capt. Nemo** · March 20th, 2009, 08:30 AM

Yes Mr. Cavo. By all means leave them on the test!

**New3M** · March 25th, 2009, 01:52 PM

Absolutely leave them on the 3M test - for the people that know what they're doing, those questions are easy gimmes.

For the people that don't know what they're doing, they're an easy way to show just that!

**fourdegreesc** · March 28th, 2009, 04:37 AM

Keep in mind too that Gyro is not error free either. There is usually a 1 or two, every now and then 3 degrees Gyro error. So don't automatically link Gyro to True.

Good luck,
-dennis