Latitudes essentially are the angular distances measured north and south of the equator, the parallel that circles the globe equidistant from the poles. They range from 0 degrees at the Equator to 90 degrees at the poles. Lines of constant latitude are referred to as parallels. Longitudes, on the other hand, are the angular distances east or west of a common meridian, and, on a globe, converge at the poles. From pole to pole the points along the same longitude are referred to as a meridian.
Latitude is a natural geometric reference point, in that its foundational metric is the earth's equator. Longitude, on the other hand, is measured from an internationally-established Prime Meridian fixed in the late nineteenth century at Britain's Royal Observatory at Greenwich, which we recognize as 0 degrees longitude. The points where latitudes and longitudes converge are referred to as geographic reference coordinates. In its shape the Earth more or less is a sphere. For geographic reference we divide this sphere into 360 degrees, and each degree into 60 minutes, and each minute into 60 seconds. Geographic reference coordinates thus can be expressed in degrees, minutes, and seconds. Because our base number is 60, we refer to it as a sexagesimal system. Virtually every place name and geographic feature on Earth has a pair of geographic reference coordinates identifying its latitude and longitude.
More and more these days, latitude and longitude are expressed in decimal degrees, or degrees with fractions to the right of a decimal point to a base of 10 rather than 60. In this case the hemispheres are either positive (east of Greenwich or north of the Equator) or negative (west of Greenwich and south of the Equator). Consider the location of the Library of Congress in Washington, D.C. Its geographic coordinates can be rendered in two ways, either in degrees, minutes, and seconds or as decimal degrees, as in the following:
38° 53′ 19″ N = North latitude 38 degrees 53 minutes 19 seconds = 38.888611
77° 0′ 17″ W = West longitude 77 degrees 0 minutes 17 seconds = -77.004722
Geographers employ other ways of expressing latitude and longitude, but the two shown here are the most common. Nevertheless, it is important to recall that we rely upon the sexagesimal system in reckoning latitude and longitude, which uses sixty rather than one hundred as its base.
Virtually all modern gazetteers and other geographic sources identify place and feature name coordinates, specifically longitude, in relation to the Greenwich prime meridian. Maps published in the nineteenth and the first half of the twentieth century, on the other hand, may employ other prime meridians, which leads to potential confusion.
In an effort to alleviate the problem, reference staff have compiled a table for converting Greenwich prime meridian to other meridians used on European topographic maps (PDF, 8.1 MB). Those meridians include the island of Ferro, Pulkovo, Amsterdam, Athens, Bern, Brussels, Bucharest, Helsinki, Copenhagen, Lisbon, Madrid, Oslo, Paris, Rome, Sofia, and Stockholm -- all actual prime meridians used in the preparation of topographic maps. This printed resource is also available for use in the Geography and Map reading room, and reference staff are willing to assist with interpreting it upon request.
Before 1884 (and, indeed, for many years thereafter) cartographers utilized a variety of prime meridians when preparing charts and maps, including large-scale topographic surveys at the national level. But the long-standing need to standardize the depiction of longitude on nautical charts to ensure the safety of mariners at sea led to the selection of the Greenwich meridian in the United Kingdom as the Prime Meridian of the world. Nevertheless, the older previous prime meridian is the one likely to appear on historical maps and charts, especially those produced prior to the Second World War.
A country's prime meridian was a great source of national pride, and map makers in Europe continued to rely on traditional meridians in creating maps and charts. A national prime meridian usually was fixed with great care to ensure accurate national surveys and precise astronomical observations, hence a national prime meridian usually being placed at the longitude of a national observatory. As an example, France's Observatoire de Paris was established along the Paris meridian in 1667, from which location the Cassini's undertook the first large-scale mapping of a nation based on trigonometrical surveys. The most famous meridian perhaps is Great Britain's prime meridian, fixed in 1851 at the Royal Observatory at Greenwich, and chosen in 1884 by an international commission as the world's Prime Meridian. An enduring exception, however, proved to be Ferro, an island in the Canaries, which was utilized primarily by Austria and Germany well into the twentieth century. Indeed, so legendary was the island group (then considered the Fortunate Islands) that Martin Waldseemüller used them as the prime meridian on what is arguably the most revolutionary map in cartographic history, his 1507 Universalis cosmographia secundum Ptholomaei traditionem et Americi Vespucii aliorumque lustrationes.
For researchers who have no trouble in calculating prime meridians on historical maps on their own, it may be helpful to know that the most commonly used prime meridians on large scale topographic maps of East Central Europe, Eastern Europe, and European Russia are Ferro, Pulkovo, and Paris. In relation to the Greenwich prime meridian, which lies at longitude 0° 0′, the other three prime meridians are situated as follows: Ferro lies 17° 40′ west of Greenwich; Pulkovo lies 30° 20′ east of Greenwich; and Paris lies 2° 20′ east of Greenwich. This means that when using a prime meridian based on Greenwich coordinates, we need to adjust for the prime meridian in use on the maps.
Maps of Austria-Hungary, Germany, Poland and western Russia will likely employ the Ferro prime meridian, in which case we will need to add 17° 40′ to a given Greenwich longitude. For maps of Imperial Russia, which use both Pulkovo and Paris as their meridians, we will need to add, respectively, either 30° 20′ or 2° 20′ to our given Greenwich longitude. And, depending on a place name's location with regard to the prime meridian at Pulkovo Observatory (11 miles south of Saint Petersburg), it could lie east or west of Pulkovo.
Researchers should also bear in mind that the earth is divided into 360 degrees, which in turn is subdivided into 60 minutes, and thereunder by 60 seconds. As a result we calculate longitude using the sexagesimal system, which is a numeral system with 60 as its base.
As examples, let us consider the Ukrainian cities of Uzhorod and Poltava. Uzhorod lies in the historical region of Galicia at the crossroads of Central and Eastern Europe. Uzhorod is situated at longitude 22° 17′ east of Greenwich. But, it appears on Austro-Hungarian and Polish maps by its longitude calculated according to the Ferro prime meridian. Recalling that Ferro lies 17° 40′ west of Greenwich, we need to add 17° 40′ to our Greenwich longitude. We convert that to Ferro in the following manner:
|Conversion of Uzhorod to Ferro Meridian:||22° 17′ east of Greenwich||+||17° 40′||=||39° 57′ east of Ferro|
Poltava, on the other hand, is in eastern Ukraine, and appears on Imperial Russian maps, which use Pulkovo and Paris as their prime meridians. Both lie east of Greenwich, in which case we will have to subtract their respective differences in longitude. As noted, Pulkovo lies 30° 20′ east of Greenwich, whereas Paris lies 2° 20′ east of Greenwich. Poltava lies at 34° 33′ east of Greenwich. We convert that to the Pulkovo and Paris prime meridians as follows:
|Conversion of Poltava to Pulkovo Meridian:||34° 33′ east of Greenwich||-||30° 20′||=||04° 13′ east of Pulkovo|
|Conversion of Poltava to Paris Meridian:||34° 33′ east of Greenwich||-||02° 20′||=||32° 13′ east of Paris|
Geography and Map Division reference staff are very familiar with the complexity surrounding the conversion from one prime meridian to another. Several years ago foresighted staff members created a valuable resource for mitigating that difficulty.
The link below takes us to a table for converting Greenwich prime meridian to others used on European topographic maps. Those meridians include the island of Ferro, Pulkovo, Amsterdam, Athens, Bern, Brussels, Bucharest, Helsinki, Copenhagen, Lisbon, Madrid, Oslo, Paris, Rome, Sofia, and Stockholm -- all prime meridians at one time in use by their respective nationalities for the preparation of topographic maps. The prime meridians employed most frequently on maps of East Central Europe, Eastern Europe, and Russia are the island of Ferro, Pulkovo, and Paris.
From right to left there are seventeen columns, one for each prime meridian, beginning with Greenwich. If a modern gazetteer provides us with a specific longitude measured from Greenwich, we can easily find its companion figure from one of sixteen other meridians. Starting at 0° 0′ at the Greenwich Observatory, we can scroll down one degree in each row until we reach 59° 59′ east of Greenwich, and scroll down even further to 10° 59′ west of Greenwich, which is not necessary for our purposes. Nevertheless, a series of concomitant longitudes appears to the right of our basic figure.
If a patron continues to have difficulty converting converting longitudes, s/he can contact Geography and Map Division reference staff for assistance.