Warning - the Watanic Bowmen club has archery events on adjoining land, next to the parking area. Be careful.
Geocaching has taught me more about different coordinate systems and different ways of representing the numbers for the coordinates. The cache page on geocaching.com gives the coordinates in degrees, minutes, and thousandths of minutes. UTM is also listed. And there is a link to click to see conversions to other formats and other datums. The most common method of writing the coordinates for a geocache is latitude and longitude, in the form of degrees, minutes, and thousandths of minutes. And this is in the WGS84 datum.
I never knew about different datums before geocaching, but now I know. It's a good thing we generally don't have to worry about which datum to use for geocaching, since for example around here the difference between using NAD27 vs. WGS84 is almost 150 feet.
| Format |
Example |
First coord precision |
Second coord precision |
| DD MM.MMM |
N 42 51.000 W 71 32.123 |
6.1 feet |
4.5 feet (changes with latitude) |
| DD.DDDDD |
42.85000, -71.53538 |
3.6 feet |
2.7 feet (changes with latitude) |
| DD MM SS.SS |
N 42 51 59.22, W 71 32 07.38 |
1 foot |
9 inches (changes with latitude) |
| UTM |
19T E 292893 N 4749103 |
1 meter |
1 meter |
Decimal degrees is another common format. For example, N 42 51.000 W 71 32.123 is the same as 42.85000, -71.53538. To convert, just take the minutes (including the thousandths) and divide by 60 (because one minute is a sixtieth of a degree). The result becomes the fractional part of a degree (the numbers after the decimal point). In this example, 32.123/60 is equal to 0.53538.
UTM is totally different, using a set of rectangular grids instead of latitude and longitude. But the nice thing about UTM is the units are always one meter, the same in both direction (north-south and east-west), and the distance between units doesn't change with location like longitude does.
I have solved mystery cache puzzles that used coordinates in all these different formats. Sometimes part of the puzzle is figuring out what format the answer is supposed to be in.
When writing coordinates, or any quantity really, the precision of the number is important. How many of the digits are really significant information? For geocaching, it turns out that thousandths of a minute is a very appropriate precision. For example, the difference between N 42 51.000 W 71 32.000 and N 42 51.001 W 71 32.000 is a thousandth of a minute to the north, which is about 6 feet. And the difference between N 42 51.000 W 71 32.000 and N 42 51.000 W 71 32.001 is a thousandth of a minute to the west, which is about 4.5 feet. It wouldn't make any sense to try to get more precise coordinates for a geocache, because our GPS receivers aren't even that accurate. Also, wouldn't it take some fun out of it if you could actually locate a cache within inches by using a GPSr? Well, now that I think about it, sometimes I would like that extra precision for LandRocket and Smoking Monkeys micro caches!
What about the precision of other formats? If you convert to decimal degrees, how many digits after the decimal point do we need, to keep approximately the same precision as a thousandth of a minute? A thousandth of a minute is 0.001/60 which is 0.000016666 degrees. So since a thousandth of a minute is a little more than a hundred thousandth of a degree, we need to represent the coordinate to the hundred thousandth of a degree (five digits after the decimal point) and no further. To clarify with an example, the difference between 42.85000, -71.53333 and 42.85001, -71.53333 is about 3.6 feet. So it doesn't make any sense to use more than five decimal places to give geocache coordinates in decimal degree format.
Degrees, minutes, and seconds are also sometimes used. A second is a sixtieth of a minute. So one second of latitude is about 100 feet. A tenth of a second is 10 feet, a hundredth of a second is just 1 foot. Less for longitude. So we need only two significant digits after the decimal point in seconds to represent a 1 foot precision. An example is N 42 51.987 W 71 32.123 is equal to N 42 51 59.22, W 71 32 07.38. An example calculation is .123*60 = 07.38, in other words 0.123 minutes equals 7.38 seconds. Yes, it is the same with minutes and seconds of time, too. This format is the default used in Google Earth. When I first installed Google Earth, I was dissapointed it didn't support DD MM.MMM format. But I just upgraded to version 4.3.7204 and it now has that option.
Sometimes it does make sense to use more precise numbers, if you are doing a lot of mathematical manipulation or conversions. Keeping track of more significant digits in the calculations can reduce rounding errors and give better results.
Notice that the distance between degrees of latitude is basically the same anywhere in the world, while the distance between degrees of longitude varies greatly, depending on how far from the equator you are. Earlier I said .001 minutes of longitude is about 4.5 feet. That is true for southern New Hampshire. In New York City it is closer to 4.6 feet. In Florida, it is about 5.4 feet.
The cache is located within 3,000 feet of the given coordinates, which can also be used for parking. Fizzycalc is an excellent tool for converting coordinates, calculating distances, and projecting from a waypoint. The cache isn't really a large ammo can. After you make a good guess at the puzzle solution, you may verify your answer.