Courtesy of Wikimedia Commons, Latitude and Longitude of the Earth, Creative Commons license CC0
When gazetteers give a point location for a place, they often define that location with geographic coordinates derived from the grid abstractly superimposed on the earth by geographers long ago. Like the x- and y- coordinates you might have learned about in geometry class, these coordinates tell how far east or west (longitude) and how far north or south (latitude) to go from a point of origin. The point of origin is the point where an internationally agreed-upon "Prime Meridian" (longitude of 0, pole-to-pole meridian) crosses the Equator (latitude of 0, parallel circling the spherical earth, equidistant from the poles). This turns out to be a pretty effective system that has served geographers, navigators, the military, meteorologists, genealogists, and all sorts of lay people well, over many centuries, shared among nations, right into the digital age. There are other grid systems, but this is the one most universally used.
As part of this system, the sphere is divided into 360 degrees, and each degree into 60 minutes, each minute into 60 seconds -- a sexagesimal (base of 60) system. So latitude and longitude coordinates may be expressed in degrees, minutes, and seconds. For ease of computer applications, you may also see them expressed as decimal degrees, that is, degrees with fractions thereof to the right of a decimal point, to the base of 10, rather than 60, and with hemispheres designated as positive (east or north) and negative (west or south). For example, the location of New York City can be indicated by the point coordinates of City Hall at:
North latitude 40 degrees 42 minutes 45 seconds (N 40°42'45") or 40.71250000
West longitude 74 degrees 0 minutes 23 seconds (W 74°0'23") or -74.00638889
Courtesy of USGS.gov, Staten Island, 1898, 1:62,500, via topoView, with annotations added in red
Suppose you are looking for a place for which you've found the spelling, Zaszkow. Perhaps that was on a ship manifest or a naturalization paper for an immigrant ancestor. You know that it's in Eastern Europe, and perhaps you know that it was in Galicia, Austria-Hungary. Even more information is better, especially if you find out that there was more than one Zaszkow in Galicia. So if you know from family lore that it was near the city of Lemberg (now L'viv, Ukraine), so much the better. As of 11/14/2019, you would come up with zero results searching that name in the U.S. military's (National Geospatial-Intelligence Agency's) GEOnet Names Server, specifying Ukraine as the current country. Similarly, in the mid-20th-century gazetteers of the U.S. Board on Geographic Names (BGN), in the volumes for the U.S.S.R. which included Ukraine, with excerpt shown here, you would come up empty-handed.
With some creative thinking about possible spelling variants, or with the helpful cross-references in Where Once We Walked (not illustrated here), you would find the modern romanized spelling of Zashkov, with latitude and longitude as shown below, in the BGN gazetteer:
The "32001" in the right column above represents Ukraine; the third column numbers represent the latitude, and the fourth column numbers, the longitude. With 2 same-spelling possibilities in Ukraine listed, plotting the latitude and longitude of each place on a map would help you determine the one closest to Lemberg/L'viv.
Below is the index or key map to a contiguous-area map series covering Eastern Europe in the 1950s, including Ukraine and what was Galicia, Austria-Hungary prior to World War I. The rectangles on this map show the area covered by each map in the series. The red lines on the index map below show the plotting of the latitude and longitude of the Zashkov underlined in red above. The red lines on the index map intersect at the location of Zashkov. If you were looking at the full-sized index map, you would be able to read that the map covering this location is numbered NM 34-9.
Courtesy of Perry-Castañeda Library Map Collection, Topographic maps, Eastern Europe 1:250,000 (AMS), index map, with annotations added in red
Here is the corner of map NM 34-9 (of U.S. Army Map Service Series N501 for Eastern Europe at the scale of 1:250,000) that covers the latitude and longitude of Zashkov. You can see Zashkov circled in red on the map, with longitude almost 24 degrees east of the Greenwich Prime Meridian and latitude almost 50 degrees north of the Equator.
Courtesy of Perry-Castañeda Library Map Collection Topographic maps, Eastern Europe 1:250,000 (AMS), map NM 34-9, with annotations added in red
If you wanted to see this place on a pre-World War I map, perhaps to see for yourself that it was originally known as Zaszkow, or perhaps to get a sense of the area contemporaneously with your subject of research, the librarians in the Lionel Pincus and Princess Firyal Map Division could show you the index map to a contiguous area map series of Austria-Hungary that dates from 1877-1914, at the scale of 1:75,000, a scale large enough (zoomed in enough) to show small villages. However, these maps use as their frame of reference a Prime Meridian different from the current standard 0 degrees of Greenwich, England; their Prime Meridian was one used on many maps published in Europe in the 19th century and earlier. This meridian passes through Ferro (El Hierro, Ile de Fer), an island in the Canary Islands. As you can see below, in the upper left corner of the map that covers Zaszkow, next to the longitude indicator in the margin is the designation (in German) "oestlich v. Ferro," that is, 41 degrees and 30 minutes east of Ferro. Since the Ferro Prime Meridian is approximately 17 degrees and 40 minutes west of Greenwich, you would need to add 17 degrees and 40 minutes to the longitude east of Greenwich given in a modern gazetteer for Zashkov (23°59') in order to derive the longitude to plot on the earlier-era index map (shown below and to the right), and then on the detailed series map (shown below), to find Zaszkow.
(Remember that since there are 60 minutes to the degree, when you add 59 and 40 minutes, that equals 1 degree and 39 minutes; so you carry the 1 and add it to the 23 and 17 to make 41 degrees.)
This explains why you find Zaszkow on the older map below at longitude of 41 degrees and between 39 and 40 minutes east.
New York Public Library Digital Collections, image ID 1226354, with annotations added in red
Since an international conference in 1884, the meridian (pole to pole grid line) passing through Greenwich, England, has been an internationally agreed-upon "Prime Meridian." That means that it serves as the 0-degree meridian or origin from which degrees of longitude are measured to the east and west. But before (and even after) 1884, maps have used meridians passing through other locations as their origin for identifying and determining east-west locations on the earth. A common Prime Meridian for European maps even into the early 20th century was that passing through Ferro, in the Canary Islands, which is approximately 17 degrees and 40 minutes west of Greenwich. On maps in The New York Public Library, here are some of the places that give their names and locations to Prime Meridians, along with their degree and minute distances east or west of Greenwich:
Considering latitude and longitude as an international and precise language of location, you can be grateful if you find the name of the place you are looking for in a gazetteer that gives those geographic coordinates. Not all maps show latitude and longitude, but many world atlases found in public libraries do (for example the National Geographic Atlas of the World, the Times Comprehensive Atlas of the World, the Oxford Atlas of the World). In these, you can plot the location of the place by using the grid numbers on the east (usually right) or west (usually left) margin of the map to identify the degrees and minutes north or south of the Equator; and using the grid numbers on the north (usually top) and south (usually bottom) margin of the map to identify the degrees and minutes east or west of the Prime Meridian. With your eye or finger or handy straight-edge, figure out where the lines passing through those coordinates intersect. That intersection point is the approximate location of the place you are looking for.
If you are trying to locate a very small town or village, the chances are that the name won't appear in a world atlas, and you'll need a more detailed -- and larger-scaled -- map to find the name in the designated location. (By checking the alphabetical index of place names in the atlas first, you can determine if the place name should appear.) So what do you do next? You can try searching on a larger-scaled map of the appropriate country, region, or province. But in many cases, your best bet is to turn to what is variously referred to as a set map, a series map, or a sectional map presenting topographic information. This type of map is part of a so-called "contiguous-area map series," a series of many maps that together cover a large area at a consistent scale. These series come in various scales, and the larger the scale (ratio of distance on the map to actual distance on the earth, with larger scale represented by smaller number in the ratio -- for example, 1:250,000, 1:100,000, or 1:50,000, as opposed to 1:1,000,000 or 1:5,000,000), the greater the chance that small village names will appear on these maps.
Once you have access to a contiguous-area map series covering your area of interest, the next challenge is to figure out which map in the series covers the specific area and place you are looking for. An indispensable aide in this stage of the process is an "index map" or "key map" for the series as a whole. This is a small-scale map of the series' area of coverage (say, Eastern Europe or the whole of Ukraine, when you are looking for a village in Ukraine), with a superimposed grid. The rectangles of the grid represent the constituent maps of the series; the rectangles (for maps, also called quadrangles) outline each map's coverage area and show a name or number by which each map is identified. With latitude and longitude demarcated in the margins of the index map, you would follow the same process described above, plotting the coordinates north or south of the Equator and east or west of the Prime Meridian to determine where the lines of your parallel of latitude and meridian of longitude intersect. The grid rectangle containing your intersection point represents the map where your place should appear. In the case of a collection of paper maps, such as in the Lionel Pincus and Princess Firyal Map Division, the staff would then pull out that map for you, and once more, you would follow that process of plotting the lines of your latitude and longitude to find the intersection point on the map.
Here are a couple of caveats:
To address the extra steps of the latter caveat above, and to see an example in order to review the steps already given, see the illustrations and explanations given in the box at left.
Longitude numbers underlined in red in the upper border of this index map segment range from 38 to 44 degrees east of the Ferro Prime Meridian.