Latitude/longitude

Latitude and longitude is the oldest map-coordinate system for plotting locations on the earth. The Roman scholar Ptolemy devised it almost 2,000 years ago. Ptolemy wrote about the difficulties of accurately representing the earth on a flat piece of paper and created latitude and longitude as a way of solving the problem. That’s pretty impressive for a time way before computers and satellites.

Latitude and longitude are based on a little math, but they’re not really complicated. Angles are measured in degrees, and they’re used for measuring circles and spheres. Spheres can be divided into 360 degrees; because the earth is basically a sphere, it can also be measured in degrees. This is the basis of latitude and longitude, which use imaginary degree lines to divide the surface of the earth.

The equator is an imaginary circle around the earth; the circles are an equal distance from the north and south poles and perpendicular to the earth’s axis of rotation. The equator divides the earth into the Northern Hemisphere (everything north of the equator) and the Southern Hemisphere (everything south of the equator).

Working with Map Coordinate Systems

A coordinate system is a way to locate places on a map, usually some type of grid laid over the map. Grid systems are a whole lot easier to use and more accurate than “take the old dirt road by the oak tree for two miles, then turn left at the rusted tractor, and you’ll be there when the road stops getting bumpy.”

A simple coordinate system can consist of a vertical row of letters (A, B, C) on the left side of the map and a horizontal row of numbers (1, 2, 3) at the bottom of the map. If you want to tell someone where the town of Biggs Junction is (for example), you put your finger on the city and then move it in a straight line to the left until you hit the row of letters. Then put your finger on the city again, but this time move down until you reach the row of numbers. You now can say confidently that Biggs Junction is located at A12. I call this the Battleship Grid System because it reminds me of the game where you call out coordinates to find your opponent’s hidden aircraft carriers, submarines, and destroyers. “B-3. You sank my battleship!”

A grid may be printed on the map or provide tick marks (representing the grid boundaries) at the map’s margins. Often maps have multiple coordinate systems so you can pick one that meets your needs or that you’re comfortable using. For example, USGS topographic maps have latitude and longitude, Universal Transverse Mercator (UTM), and township and range marks.

Most coordinate systems are based on x and y; where x is a horizontal value, and y is a vertical value. A location’s coordinates are expressed by drawing a straight line down to x and across to y. Mathematician René Descartes devised this system in the 1600s.
Letter-and-number coordinate systems are fine for highway maps, road atlases, and other simple maps where precise locations aren’t needed. However, if you want to focus on a precise location on a map, you need a more sophisticated grid system. That’s where coordinate systems such as latitude and longitude and UTM come in.

When you’re figuring out a location’s coordinates on a paper map, you have a fair amount of work to do, aligning the location with primary tick marks and then adding and subtracting to get the exact coordinate. With digital maps on a computer, that’s usually just a matter of moving the cursor over a location and watching with relief as the coordinates automatically appear. If you’re using a paper map, you can make life easier with free overlay grids and rulers from www.maptools.com. With these, you can print grids and rulers for different coordinate systems on clear transparency sheets.

Map Datums

A map datum is a mathematical model that describes the shape of an ellipsoid — in this case, the earth. Because the shape of the earth isn’t uniform, over 100 datums for different parts of the earth are based on different measurements.
Some serious math is involved here for getting into the nuts and bolts of map datums. If you’re the scholarly type, these Web sites provide lots of details on projections and datums:

• Datums and Projections: A Brief Guide http://biology.usgs.gov/geotech/documents/datum.html
• Peter Dana’s excellent Geographer’s Craft site www.colorado.edu/geography/gcraft/notes/notes.html

Datums all have names, but they aren’t stuffy sounding. Datums often have exotic, Indiana Jones-style names such as the Kerguelen Island, Djakarta, Hu-Tzu-Shan, or Qornoq datums. (The United States uses such boring datums as NAD 27 and WGS 84.)
You only need to be concerned with datums under a few circumstances, such as these:

• A location is plotted on two different maps.
• A map and a Global Positioning System (GPS) receiver are being used.
• Two different GPS receivers are being used.

In these instances, all the maps and GPS receivers must use the same datum. If the datums are different, the location ends up in two different physical places even though the map coordinates are exactly the same. This is a common mistake: GPS receivers use the WGS 84 datum by default, and USGS topographic maps use the NAD 27 datum. If you mix the datums, your location can be off by up to 200 meters (roughly 200 yards, if you’re metrically challenged).
Utilities can convert coordinates from one datum to another but it’s easier just to get all the datums on the same map.

Figuring Out Map Projections

Making a map is quite a bit more challenging than you may think. A cartographer’s first challenge is taking something that’s round like the earth (technically it’s an ellipsoid that bulges in the middle and is flat at the top and bottom) and transforming it into something that’s flat, like a map. Cartographers use a projection to reproduce all or part of a round body on a flat sheet. This is impossible without some distortion, so a cartographer decides which characteristic (area, direction, distance, scale, or shape) is shown accurately and which will be distorted.

Although my high-school geography teacher may smack me on the head with a globe for saying this, the average map user doesn’t need to know what kind of projection was used to make a map. There are some exceptions if you’re a cartographer or surveyor, but usually you won’t get in trouble if you don’t know the projection. So don’t panic if you can’t immediately tell a Lambert conformal from a Mercator or Miller projection. Just keep in mind what a projection is and that there are different types of map projections.

Aeronautical Charts

Maps designed for aviation use are charts (a term that can also refer to their marine counterparts). These maps provide pilots with navigation information including topographic features, major roads, railroads, cities, airports, visual and radio aids to navigation, and other flight-related data. You can find such aeronautical chart types as

• VFR (Visual Flight Rules)
• IFR (Instrument Flight Rules) Enroute
• Terminal Area Charts

You can find more about aeronautical charts by visiting the Federal Aviation Administration (FAA) National Aeronautical Charting Office (NACO) at www. naco.faa.gov.

FAA aviation charts aren’t freely available for download. The FAA offers a monthly service that provides all charts and updates on DVDs for a year, but the cost is over $300. A number of companies such as Jeppesen (www.jeppesen.com) and Maptech (www.maptech.com) make commercial flight-planning software packages that include digital charts, or you can try the www.aeroplanner.com, a Web service that provides digital charts and other services to pilots. Another noncommercial source of FAA sectional charts is http://aviationtoolbox.org/raw_data/FAA_sectionals.

Marine Charts

Marine charts are maps for inland, coastal, and deep-water navigation. Charts from the National Oceanic and Atmospheric Administration (NOAA) are commonly used for boating. They provide such important information as water depth, buoy locations, channel markers, and shipping lanes. See www.noaa.gov/charts.html for more on NOAA charts.
Marine charts aren’t available for all bodies of water. If you’re boating on a lake or a river, you’ll probably use a topographic map for navigation.

If you’re more of a sailor than a landlubber, check out Marine Navigator at www.maptech.com. This commercial marine-navigation program displays NOAA charts, aerial photographs, 3-D ocean-bottom contours, and tide and current tables.

Planimetric maps

Planimetric maps don’t provide much information about the terrain. Lakes, rivers, and mountain pass elevations may be shown, but there isn’t any detailed land information. A classic example of a planimetric map is a state highway map or a road atlas. Planimetric maps are perfect in cities or on highways, but they’re not suited for backcountry use.
When using planimetric maps, you’ll often encounter these terms:

• Atlas: An atlas is a collection of maps, usually in a book.
• Gazetteer: A gazetteer is a geographical dictionary or a book that gives the names and descriptions of places.