Compass declination

[cavemanjonny]

Something else to consider when you are buying a compass off of the internet: is it properly balanced? In general, compass needles (or discs) are balanced for either the northern hemisphere or the southern hemisphere. Balancing is designed to offset the effects of magnetic inclanation (dip). The north-seeking end of an unbalanced needle will tend to point towards the ground in the northern hemisphere and away from the ground in the southern hemisphere.

The needle on an unbalanced compass will not rotate smoothly since it tends to bump into the compass housing. The needle will also generally not line up with the numbers, instead hovering below or above them, making it even more difficult to use. This might be a bit of a moot point in the case of compasses with the numbers printed directly onto a north-seeking disc, as is the case with most instruments used for surveying caves.

Many compasses come in a northern hemisphere version and a southern hemisphere version. Some are even geared towards more narrow latitudinal bands. You might want to double check that the compass you buy will work where you intend to use it.

[NZcaver]

I thought NZcaver's posts were very good, and I was particularly impressed by his first post. I was going to comment on the original thread, but I thought, "What is the point? NZcaver has covered all the details, and he has even included the appropriate map."

Thanks, George! I try to pay it forward by passing on what I've learned, just as other cavers have patiently done with me over the years. Occasionally I get a few details wrong here and there, but that's life.

Also Bob Thrun has a good point. "Why order a compass from the US, when you can get it from Britain?"

That is a good point. Buying local makes sense, especially for good customer service and support. However I'm guessing different markets may mean different retail prices in each country. So with the huge US consumer market, new surveying instruments may still be cheaper to purchase in the US than in the UK - even when shipped over the Atlantic twice (I don't know this for sure, but I've seen it happen with plenty of other items). The used market also seems significantly larger in the US, especially with satellite dish installers selling their old instruments on eBay all the time.

Suunto makes a compass that has a declination adjustment. It does this by sliding a thin sheet of transparent plastic with a ssecond hairline in front of the compass capsule. The space between the sheet and the capsule provides a place for water and mud to collect. The problem with having two hairlines visible should be obvious. As I recall, the adjustment range is plus or minus 7 degrees. This is not enough for my caving area.

Thanks - I did not know this. (Hence my earlier comment about Suuntos not having declination adjustments - wrong again!)

Something else to consider when you are buying a compass off of the internet: is it properly balanced?

See my first post in this topic - but be sure to ignore the misinformation in some of my later posts.

[cavemanjonny]

See my first post in this topic - but be sure to ignore the misinformation in some of my later posts.

Sure enough!! You beat me to it .