Maps Part 2Aug 25th, 2012 | By Garrison | Category: BFTC Featured
In Maps Part 1 we talked about the different maps and where to order them, here in Maps Part 2 lets talk about how to use your map. If you don’t know how to use your map when hiking, or camping in the backcountry then the only thing it will be good for is killing mosquito. Lets begin by identifying the important map features.
Map name: You will find the name in the upper and lower margins of American maps, and in the lower margin of Canadian maps. American maps also have the name in parentheses that indicate adjacent quadrangles.
Scale: You will find the representative fraction (1:24,000, etc) and bar scale in the lower map margin.
Dates: Most maps are very old; some were last field checked in the 1950s. A lot has changed since then, especially the location of roads, dams, and other man made features, but I think you already knew that. So when buying a map the key word here is revised and field checked this will tell you when the map was last updated. Certainly, an old map is better the no map at all, but if wont do if your planning a remote hunting, camping, hiking, or fishing trip. If you need more up to date information then that supplied by your map, contact your government agency ( U.S. Forest service, Department of Natural Resources, etc.) nearest your area of concern. There professionals require current maps for their daily work. If they can’t tell you where to get the maps you need, they will help you update yours.
Latitude and Longitude: Your map is a birds eye view of a tiny portion of the earths surface. Where dos that fit onto the big sphere? By using Latitude and Longitude we can reference any location to a precise spot on the globe.
Lines of Latitude (or Parallels): Run parallel to the equator, which is zero degrees. The longitudinal (north/south) lines that intersect at the poles are meridians, or lines of latitude. Longitude is measured in degrees east and west of the zero degree prime meridian, which originates in Greenwich, England. By splitting latitude and longitude degree readings into smaller units, called minutes and seconds, specific points on the earth may be accurately located via a coordinate system. the rules are simple;
1 degree = 60′ (minutes)
1′ = 60″ (seconds)
All this is needlessly academic, unless you plan to order a specific large scale maps and aerial photographs but is still good to know.
Aerial Photos: Lets say you are planning to canoe a remote river in Tennessee, your topo map says you’ll have to portage your gear around a major falls along the route. The map does not tell you where the portage begins, however. In fact, the contour lines ( we’ll talk more about that later) suggest there’s an impassable canyon wall on both sides of the river.
If you wanna know more then order the aerial photographs from which your map was made. Photos will bring into a clear perspective the most obscure features. Now, you’ll find a route around the falls…if one exists.
Aerial photos are expensive, and they are very large scale somewhere between 1:24,000 and 1:60,000. Since there are millions of there on file, getting exactly what you want requires precise identification of the specific land feature to the nearest 5 minute of arc.
Which Way Is North?
North as everyone north is always at the top of the map. well, sort of. For conventional route finding in the American Midwest, you can get by with that knowledge alone. However if you plan to travel in the East or West you’ll have to take into account the differences between the three Norths. Geographical North, that’s the direction in which the lines of longitude run. Since the vertical lines that describe the right and left map margins are true meridians, any line on your map drawn parallel to them run true North and South…Almost.
Meridians converge towards the poles, so the ones that run along your maps edges are not truly parallel to one another (and closer the each pole the less parallel they become). That’s what the neat lines are for.
To locate the true North – South meridian simply connect the neat lines of equal value at the top and bottom of the map. Similarly, you can plot a true East – West line (parallel) by connecting the latitude neat lines. Then, you can use these coordinates to accurately plot the latitude and longitude of any point on your map.
The Other Norths
All topographic maps have grid lines imprinted on their faces. grid lines began life as true meridian and parallels but became distorted when the spherical earth was flattened onto paper. Cartographers use a number of map projections to minimize this distortion, but all are subject to some small error. Grid lines do not point true North/South or true East/West. The error is reported in a declination diagram in the bottom map margin, and is usually small enough that it can be ignored by everyone but surveyors. Grid North is the North you’ll wanna use when using civilian and military topographic maps. Use true North with maps that don’t have grid lines imprinted on their faces.
Everyone knows that the compass needle does not point true North. It points towards the North magnetic pole that is located several hundred miles south of the real thing. The angular difference between the three Norths is reported in declination diagrams or legend information on all topographic maps.
Contour Lines: The thin brown lines on topographic maps are called contour lines. They give depth and elevation on the land. I’ll go over that in part 3
Map Symbols: There are dozens of map symbols most of which are obvious. But lest you forget, Canadian maps emblazon them all in the margin or on the back. American USGS maps are more subtle. They tease you with a few road classifications, then assume you know the rest or have a hand on there free pamphlet topographic map symbols, which list all of them.
You might also realize that for clarity map symbols are not drawn to scale, they always appear much larger than they really are. However, the geometric center of these symbols is accurately plotted. So shoot a compass bearing htere if you need to target a particular point.
Again stay tunes for maps part 3