Declination Adjustments, Conflicting Answers

[icave]

I know there has already been some discussion regarding declination adjustments (http://www.forums.caves.org/viewtopic.php?f=26&t=6943), but I have a pretty specific question. Has anyone else noticed that the declination adjastments given by Compass, Geographic Magnetic Calculator, and NOAA don't always agree. I have a cave that we have been working in on and off since 2001, so declination adjustments are a must, espically when I compare to a map done in the 1980s of a nearby cave.

My thoughts are to just use the declination adjustments from NOAA and ignore the rest. What do the rest of you do?

Thanks,
Mike Spencer

[George Dasher]

I can only comment on NOAA's numbers, and they seem correct.

[John Lovaas]

Mike-

How much of a difference are you getting? The geomagnetic model that Larry Fish is using in COMPASS may not be the "up to the minute" model that is generating the data at the NOAA site. I also noticed that changing the datum in COMPASS will shift things a few hundredths of a degree.

[Roppelcaver]

The numbers available on NOAA and others are calculated values based upon location. I have found that the actual value may be quite different due to other variables in play; and the difference between calculated and measured is not even consistent.

In the absence of other data, these sites are fine, but measured data is always better.

[George Dasher]

Jim: How much difference are you getting? And how are you shooting to true north?

I've always thought that NOAA's site is pretty accurate, even if they are using a computer program to estimate where a randomly moving location is located at any given moment in time (and smoothing out all the little movements...). That should work good for past locations, but there is a certain amount of computer guessing for future locations.

But the bottom line is that I feel their declinations are more than good enough for caving applications. And probably most all other applications.

[Roppelcaver]

I wrote a long answer and it vanished.

Now I will be brief.

We use a compass course corrected to true north via a star shot. This has given us a means to correct for declination over a long period of time (30+ years). The difference between calculated (NOAA) versus measured (course) has ranged from -1.5 to +1.5 degrees. I have experimented in the data and found that using measured versus calculated resulted in significantly more accurate cave data (looking at variance data of traverses in WALLS), by a factor of four.

NOAA values are fine most of the time, but results are even better when augmented by a compass course to correct for compass variations. The couse also helps weed out technique problems in reading, rogue metal on readers that affect the readings, and broken compasses (sticky cards).

Jimb

[George Dasher]

I don't know that I can make any more relevant comments...

I have to admit that measured data (or field data) is most always better than data calculated or taken from somewhere else, but I don't understand why there is that much difference between NOAA's data and your measured data. Nor do you have to try to explain.

Also: an 1.5 to -1.5 is within the "error range" of an accurate Suunto compass.

I tried, when I ran OTR compass course, to loan out only one set of instruments. Granted these people probably weren't the best instrument readers (those people brought their own instruments), but the azimuths (with the same compass) were all over the place, from 4 to -4 degrees. It told me that the compass reader is more of a factor than the compass.

I have to agree that compass courses do find gross errors and gross problems.

Does the fact that you are relatively close to zero declination (in Kentucky) have any effect on the numbers?

By the way, this was one of the big problems the Wright brothers had. Once they realized that the previously calculated numbers for the shape of a wing were wrong, and they invented the wind tunnel and came up with their own numbers--well, then they could fly.

I have had that problem with vanishing long posts too.

[Roppelcaver]

The +/- 1.5 degrees was after compass deviations were applied. FS/BS on compass helps reduce the human error component (improving technique with awareness is one aspect).

Reason for differences are due to local anomalies (e.g., systemic such as buried iron meteorites, or random like sunspots).

Controlling readers, normalizing compasses for deviations, and correcting for a declination (real time, or constant) has proven positive effect based upon empirical cave survey data (4x better than without).

[George Dasher]

"Empirical cave survey data."

I've got to remember that term. Does it impress the women?

[Squirrel Girl]

"Empirical cave survey data."

I've got to remember that term. Does it impress the women?

Hey, baby, it impresses me!

[George Dasher]

How about this one:

"Quiescence carbonate deposition."

[Squirrel Girl]

How about this one:

"Quiescence carbonate deposition."

Well, that *would* turn me on, except it seems to have a grammatical error. Shouldn't it be "quienscent carbonate deposition"?

[Spike]

Could you post the declinations given by the various calculators? I'm curious to what the spread actually is.

[George Dasher]

I too would like to see a posting of the declinations...

[icave]

I'll try to get the number out again this weekend. After speaking with one of my government contacts, I have decided to trust the NOAA calculations. I also found an error on my part, being that some of the declination adjustments were given as decimal degrees, while others were in degrees and seconds. The differences were much less when I corrected the seconds to decimal degrees.

I also agree that compass reader variation may play a bigger part than small errors in declination, but it's always a good choice to use the best data available.

- Mike