For any non-ham people who land here, "QTH" refers to the location of a radio station. Here is the location of my station, expressed in a variety of ways:
Traditional latitude-longitude format uses degrees, minutes, and seconds.
The format often used by GPS receivers is degrees, minutes, and decimal minutes.
The format that is the easiest to compute with is simply decimal degrees. And, it provides the highest resolution for a given number of digits.
And, finally, we have UTM (Universal Transverse Mercator) which is similar to, but NOT the same as latitude and longitude. Most GPS receivers will give you UTM if requested.
Lest we forget, Maidenhead Grid Square is used as a simplified, low resolution, world-wide coordinate system. For my house it is:
If you want to convert to or from Grid Square coordinates, check out the calculator at Amsat.org.
And, now, for the first question! What are "NAD27" and "NAD83"?. For the answer, scroll down the page.
Your second question: How far would you miss my house if you used the wrong datum? Scroll further down the page for the answer.
In 1927, a standard location grid was established for North America. It was called NAD27, which stands for "North American Datum - 1927". Before this time each state in the United States had its own grid system. NAD27 was based on certain assumptions about the shape of the earth; its not really a perfect sphere after all. Points were given a certain latitude and longitude value, based on this grid.
After satellites had been up for a while, a more precise measurement of the shape of the earth was developed. And, with it, the latitude and longitude numbers had to be changed slightly. The new grid system for North America was called "North American Datum - 1983". Points on the surface of the earth didn't move once NAD83 was adopted, of course. But, what did change was the recognized latitude and longitude of a specific location. Check the next section to see how big this change was.
To compute the shift between NAD27 and NAD83, we need to know what the approximate circumferance of the earth is. To get circumferance, we start with the diameter of the earth. An internet search shows that the mean equatorial diameter of the earth is 12,756 km. Yes, thats not EXACTLY the same as the diameter going pole-pole, but the difference is not big enough to effect what we are going to compute here. If you remember your high-school math, circumferance is diameter multiplied by pi (3.14). So, the approximate circumferance of the earth is about 40,050km.
Next, if we went all the way around the earth, pole to pole, starting at the equator, we would pass 90 degrees of latitude going to the North Pole, another 90 getting back to the equator, another 90 getting to the South Pole, and yet another 90 back to the starting point. This gives us 360 degrees that we would have passed in one earth circumferance. Thus, in the north-south direction, one degree is worth 1/360 of the circumferance, or 111.3km.
The east-west direction is a bit tricky. If you head west from any point on the surface of the earth, you are travelling on line parallel to the equator. When you get back to your starting point, you have not traveled the whole circumferance of the earth (unless you started right on the equator). In fact, if you started just a few feet from, lets say, the North Pole, you would have traveled in a circle only a few feet in diameter! So, how far "around" the earth is it when you travel this way?
In fact, the circumferance of one of these tracks is C * COS(A), where C is the average circumferance of the earth and A is the angle in degrees of longitude where you are located. At my house, A is almost 45 degrees and COS(A) = 0.707. That small distance (27 minutes, 20 seconds, or 0.445 degrees) I am away from 45 degrees will effect this only slightly. After all, we are only trying to estimate all this. This makes the circumferance of the 45th Parallel to be close to 28,050km.
This distance is again divided into 360 degrees, so that 1 degree east-west, along the 45th Parallel, is about 77.8km.
Finally, getting down to the nitty-gritty, the difference between NAD27 and NAD83, at my house, is 0.0002°N, 0.0012°W. Multiplying this out, we get about 23 meters north, 93 meters west. Thus, if you use the wrong datum in your GPS, at my QTH, you could miss the house by (about) 75 feet north and 302 feet west for a total distance of close to 310'. If you were looking for a geocache at these coordinates (or setting a cruise missile target), that would be in a swamp (literally) or have spent a gazillion cruise missile dollars on nothing!
But, that's not the only reason to pay attention to datum. There are lots of U.S.G.S. topographic maps that were originally published before 1983. Newer editions can have lat/lon marks for BOTH datum systems. Older ones will have only NAD27 and the printing that tells you this fact may be quite obscure. When you use a GPS with an older map, you could be in for a real surprise unless you tell your GPS to display the correct datum.
Updated October, 2007