Not all maps are created the same. Some maps have terrain profiles without showing declination. Some show declination without showing scale, some have scale but fail to show neat or grid lines. When first picking up a map you need to orient yourself to what data it does convey.
Scale: The first thing I always check is scale. Scale represents the ratio of actual distance on the ground to what the map compresses it to. A typical hiking scale is 1:62,500. This means for every 62,500 inches of actual ground distance, there will be one inch of map. The lower the scale, the more detail that can be in the map; the higher the scale, the less detail. To check for scale, typically it will be listed in the legend.
Declination: The next most important thing to know is the declination. Declination will be shown in the declination diagram or nay be written somewhere in the map data in the margins or in a guidebook. Declination is the difference between true north (where north really is), magnetic north (where the compass will point), and grid north (where north is drawn on the map). If this is confusing, I'll try to explain.
True north is normally represented by a star in the declination diagram. This represents the exact centerline of the planet along it's axis.
Magnetic north is the direction that a compass will point in. The earths magnetic field does not run exactly in line with its axis. The difference between the true north and magnetic north is slightly higher in some places than others, typically the further you get from the equator either north or south.
Grid north is where north is represented on the map. Because maps normally must link together, and they are trying to represent a curved 3 dimensional surface on a 2 dimensional medium, there exists a normal error in how a map aligns to either true or magnetic north.
Now that I have tried to explain these elements of north, I cover why it's important. If you want to navigate on a correct course, you must stay as close to the correct azimuth you want to travel as possible. As I mentioned in lesson 2, every method of compass hold has a margin of error already, there is little desire to increase this by any reason. If you use a standard compass and navigate using center hold method, then fail to compute for a declination of 4 degrees, you could end up a full 15 degrees off course! We will cover how to use this information more in lesson 5.
Legend: Legends are used to show what is represented by map symbols used on the map. Information such as shelters, campsites, highways, trails, etc. are standard stuff. Legends may or may not contain other useful information such as the scale, declination, contour interval, and coordinate scale.
Contour Interval: Contour interval shows how much elevation is represented by each intermediary contour line. There are three types of contour lines:
Index lines - these are the thick contour lines. They are usually every 5th line and have the elevation of that line written somewhere along the line.
Intermediary lines - these are the normal contour lines used on a map. The elevation is determined by locating the closest index line then counting up or down to the elevation of the intermediary line then adding or subtracting the contour interval.
Supplementary lines - this is a dashed contour line. It is used when the map makers need to show elevation to a point that does not meet the needs of a full intermediary line. Elevation to a supplementary contour line is 1/2 the contour interval.
Elevation Profile: Elevation profile is not used on all maps, but is used on trail maps like Appalachian trail maps to give the hiker an idea of how steep or long an uphill/ downhill ascent will be. I personally don't use them much except to figure out where low or high ground may be at a quick glance - helpful for finding water sometimes. My experience is that they are not very accurate for giving a true idea of steepness and sometimes miss hills and dips on ridgelines. It's sometimes fun for me to look at one after I'm done to figure out how wrong it was.
Coordinate Scale: A set of scales for measuring distance on the map. They are normally nest to the map scale in the marginal data or map legend. This will be covered more in lesson 5.
Adjoining Map Sheet Diagram: Maps sometimes have a diagram that shows which maps are to the sides of the map your using. This is helpful when ordering another map, or figuring out which map you need next out of the stack you are carrying. Trail maps typically only list the maps that adjoin along the trail, while military and quadrant maps will list which map is on every side and diagonally.