Visit the EarthView web site to meet the team and learn about the project.

Tuesday, July 20, 2010

Latitude/Longitude Converters

Geographers and other scientists use latitude and longitude to identify places on earth reliably and precisely. Every location on earth has a unique combination of latitude (north or south of the equator) and longitude (east or west of the Prime Meridian). Geographer Matt Rosenberg explains these concepts in more detail in his article Latitude and Longitude: Discover the Secrets of Parallels and Meridians.

Latitude and longitude are each expressed as degrees or fractions of degrees. At the equator, a whole degree can be used to locate a place to within a mile or so -- a suitable level of precision for locating a city, for example. A degree comprises 60 minutes, so that a latitude or longitude expressed in degrees and minutes is precise to roughly 100 feet, which is suitable for a lot of purposes, such as finding a house. Fractions of an arc minute are known as seconds, and again there are 60 to each minute. At the equator, coordinates expressed as degrees, minutes and seconds would be precise within a couple of feet. Fractions of a second might be used for even more precise work, such as the location of property boundaries, the corners of a building, or the location of a well. (The caveat "at the equator" is used above because the surface length of a degree of latitude changes slightly from the equator to the poles because of the oblateness of the earth's shape, and the length of a degree of longitude changes dramatically, down to zero at the poles, because of the convergence of the lines there.)

Followers of this blog may already be familiar with the latitude/longitude lookup utility created by Steve Morse, which allows users to look up addresses and find latitudes and longitudes. What is especially useful about this site is that it does not draw on just one source. It uses various mapping web sites, showing that although each site is precise to the equivalent of inches or millimeters, they are only as accurate as the datasets upon which they are based. For any given address, users may find small or large differences, depending on how large a property is, and how each database treats its proximity to nearby roads.

For various reasons, it is often useful to express latitude and longitude as decimal fractions of degrees, rather than minutes and seconds. This is particularly true if distance or area calculations are being made on the basis of the coordinates. Geographers usually prefer the conventional terms, however. Conversion from one to the other is relatively simple, but it can be tedious. For this reason, the conversion utility from the FCC is particularly helpful. It allows users to enter coordinates in either format (d-m-s or decimal) and convert easily to the other. Another interesting utility allows the distance between two points to be calculated from their respective coordinates.

Have you ever heard that if you could dig a really deep hole, you could get to China? From locations in North America, this would not be the case, even if digging such a whole were possible. Geographers can use latitude and longitude to make a simple calculation of where such a journey would end, by finding the antipode. In fact, for most locations on land, the antipode is in a body of water.

"Earth Sandwich" enthusiasts have made a fun geographic project of marking antipodes by making 8,000-mile-thick sandwiches of the earth, with a piece of toast carefully located on opposite sides of the planet at the same time. A similar hobby is the marking of degree confluence points in the geodetic grid, one of which is located very close to EarthView's home in Bridgewater, Massachusetts.

No comments:

Post a Comment