coordinates

 Angular Coordinates Angular coordinates can specify any location on Earth, usually in Degrees of Latitude and Longitude. A degree (1/360th of a full circle), is divided into 60 "minutes" (1/60th of a degree), and a "minute" can be further divided into 60 "seconds". Latitude is measured North or South of the Equator, which is equidistant from the poles. Longitude is measured East or West of the Prime Meridian, which passes through the North and South poles, and the Greenwich observatory near London. For example, the middle of the Golden Gate Bridge is about 37 degrees, 49 minutes, 11.2 seconds North, by 122 degrees, 28 minutes, 39.3 seconds West. Calculating degrees, minutes, and seconds can be time-consuming, so some people prefer to work in decimal degrees. There are many other systems of Earth Coordinates, including rectangular grids like UTM.

 Universal Transverse Mercator (UTM) UTM is a metric system with many advantages over angular coordinates. Almost all large-scale USGS maps have UTM markings for quick and accurate position readings. UTM divides the earth into 60 zones of 6 degrees longitude each, and then into a rectangular grid within each zone. The middle of the Golden Gate bridge translates to UTM zone 10 S, 0545980 meters "Easting" by 4185742 meters "Northing", or "UTM 10S 0545980E 4185742N". With UTM you can measure distance, shape, and area, with 0.04% or less distortion. To achieve this, the grid allows a slight tilt from True North, but this "UTM grid declination" is only a few degrees, at most. The map here has UTM grid lines every kilometer. You may also notice a slight "Grid Declination" of 29 minutes - about 1/2 degree. As usual, the North Arrows exaggerate the "GN" angle. Angular coordinates are preferable to UTM on small-scale maps of very large areas.

 Map Preparation If your map has light blue tick marks along the edges, rather than a full UTM grid, you'll have to connect them to make a grid. Folding a map makes it more portable. Maps we sell can include a UTM grid and/or folding for no extra charge, or you can do these things yourself. All you need are a flat surface, a sharp pencil, and at least a two foot ruler. A mechanical pencil and drafting ruler are best: Locate two of the light blue tick marks on opposite edges with the same number, like "4206000m", or "4206" (without the last 3 zeroes). Carefully place the ruler near the two marks, and test the pencil to make sure it will land on the tick mark at both ends. Move your non-pencil hand to the middle of the ruler and press down firmly to keep everything in place. Slowly draw in the grid line. You may want to draw lightly at first to get the hang of it. Draw in the remaining parallel lines. Some ticks may not be numbered, so be sure to start with a numbered tick mark. Now turn the map 90 degrees, connect the tick marks along the other two edges, and you have a complete UTM Grid. When you're done gridding your map, you may also want to fold it: On the same flat surface, match up two opposite edges and fold. Now, fold each edge back to the middle fold, making a 4-section "fan". Unfold the map, turn it 90 degrees, and make the same folds as before. This prepares the map to fold in any direction. For better accuracy, you can use a calibrated scale or plastic "Grid Roamer" to read between the grid lines. Roamers for 1:24,000 scale maps are available from many of the same stores which sell USGS maps. Most maps and gpsMaps ordered from Wild Rose can include a Grid Roamer for \$10.

 Datum Mapping is never an exact process. The Earth is not quite a sphere, or even a flattened ball. This makes measuring coordinates and altitude far more difficult. Modern cartographers refer to a lumpy, flattened ball called a "geoid" to measure Latitude, Longitude, and Altitude. The "Earth Model" and reference points used to prepare a map are collectively called a "datum". Prior to the space age, all mapping was relative to known "benchmarks": steel poles, bronze marker plates, even trees. These were tied to a network of precisely measured "Geodetic Control Points", which in-turn related to a master marker or origin. The origin for the "North American Datum of 1927" is a marker at "Meades Ranch" in Kansas. Most USGS maps still use this datum, which your GPS receiver may call "NAD27-CONUS". There are hundreds of other datums in use around the world. A newer datum, the "World Geodetic System of 1984" (WGS84), covers the globe in one coordinate system, with the center of the Earth as the origin. Using the wrong US datum with a US map, coordinates can be off by up to 300 meters (1/5 mile). Other datum mismatches can cause far greater position errors.

 coordinates