Copyright © 2002, Photographic Resource Center, Inc.
A Short Dictionary Digression: What are all these letters?
Many of the location-based numbers, signs, symbols, and devices seen in the Land/Mark exhibition function much like hyperlinks or overlays onto our world, allowing us to key into larger “networks” and make sense (or nonsense) of what is before us and where we are. Additionally, how we determine, create, and organize systems and units of measurements has a history and can reflect political, cultural, and societal trends. Consider this digression a map for some of the terminology, technology, and tools used by the artists in this exhibition.
Of all of these, GIS is perhaps the most umbrella of the terms, a geographic information system, which at its most general level is any computer system or software designed to manage location data. With this new technology, users can map and manage a whole host of information as well as return to “old” data and mine it for new information.
GPS consists of a constellation of at least 24 earth-orbiting satellites (with 3 extras in case of failure) armed with atomic clocks. GPS satellites orbit at an altitude of about 12,000 miles at 7,000 miles an hour, circling the earth twice a day, all the while transmitting low frequency radio signals to earth. The cost of maintaining the system is $400 million a year. There exist other international GPS systems, such as in Europe and Russia, but they are not as extensive or as useful. Today, a pocketsized GPS unit can be purchased for about $100.
Using a fairly simple mathematical calculation know as trilateration, the unit can determine your location, give or take 6-20 feet. Interestingly, the handheld GPS device actually uses its own inaccuracy to determine where you are. A receiver needs at least 3 satellites locked in to find its 2D location (latitude and longitude) and follow movement. With four or more satellites, a receiver can calculate the user's 3D location (latitude, longitude and altitude).
On May 1, 2000, former President Bill Clinton announced that "Selective Availability" (a sort of scrambling of signals to consumers) would be turned off, allowing GPS to be even more accurate. In the case of a national emergency, it could be “re-scrambled” for civilians while still allowing for miltitary use.
~ If you would like to experience what it is like to use a GPS receiver, there is one on display in the front foyer. You can check it out, and watch it as it locates where you are.
Latitude and Longitude
Each degree of latitude is approximately 69 miles apart with lines running from 0 degrees from the poles to 90 degrees at the equator. One minute of latitude is approximately one nautical mile.
Longitude lines, also known as meridians, converge at the poles and are widest at the equator. 0° longitude is located at Greenwich, England. Lines continue 179° east and 179° west until they meet at 180° (the International Date Line in the Pacific Ocean roughly follows 180°, but deviates to avoid island chains). Greenwich was established as the site of the Prime Meridian by an international conference in 1884. Before this, however, navigating at sea was treacherous and a terrible shipwreck led to a race to determine accurate longitudinal readings at sea. In the mid-18 th century, Britain established a Longitude Prize for anyone who could determine longitude to within one degree. An English clock designer, John Harrison realized that longitude had to do with the measurement of time (for every 15 degrees one travels, you lose or gain an hour). He invented a precise clock for use at sea, which he used to determine his position through triangulation in a trial in 1761. Although not initially convinced he had achieved this goal, the Board of Longitude finally gave him the prize many years later.
Latitude and longitude are expressed in the form of a number: Degrees, minutes (represented by '), and seconds (represented by "). There are 60 minutes in each degree. Each minute is then divided into 60 seconds. GPS receivers can convert this location-based code to a variety of permutations. You will see at least three variations, for example, on how to record locations numerically in this exhibition. Seconds, for example, can be further divided into tenths, hundredths, or even thousandths and expressed either as its own place or as a decimal. Geocaching uses the format HDDD MM.MM, which is a standard for GPS receivers (HDD means Hemisphere and degrees. MM.MM are minutes in decimal format). Additionally, in place of directions, N (North) and W (West) for example, + and – can be used: + in front of the first set of numbers indicates latitude North of the equator and – in front of the second set indicates longitude West of the prime meridian (it would be the converse for the Southern hemisphere and the East of prime meridian). The chosen numerical code of latitude and longitude is largely a function of how it will be used, accuracy desired, and preference.
To complicate the picture further, most US maps are drawn using another system, NAD27, or North American Datum of 1927, which calculates a coordinates from a single point in Meades Ranch, Kansas—approximately, the geographical center of the United States. NAD was updated in 1983 and no longer is based on a base point.
In the 1950s, the international community realized the need for a standard for geographical data and compatibility. The World Geodetic System (WGS) was established and over the course of years many datum sets were proposed—WGS60, WGS66, WSG72—each garnered by newer and more accurate methods and technologies. The most current set, WGS84, was produced in 1984 and is the current reference system used by GPS.
SMS , or short messaging service, was originally designed for use with GSM ( Global System for Mobile Communications). Akin to Wifi (wireless fidelity), cell phones are basically radios. The cellular system links the city through towers by creating “cells,” usually about 10 miles wide. When thought of in this way, such technology is territorial (roaming in and out of range) and functions by constantly locating where you are.
The Landsat program is the oldest system for acquiring earth imagery from space. The current satellite Landsat 7 has been in orbit since 1999 and can collect and transmit over 500 images per day. Although managed by NASA, its data is collected and distributed by the US Geological Survey.
What does this all mean?
With all of this changing technology, different datum sets, complex conversions, and even trees blocking satellite reception or buildings obstructing a cell tower—we still don’t seem to be able to explain exactly where we are, but we’re getting closer!Peter Van Demark, Director of GIS Products and Training, Caliper Corporation in Newton, MA (www.caliper.com) offered his expertise in the editing of this dictionary digression.