A Green Trails Topographic Map is distinguished by contour lines. Contours are placed on the map to represent lines of equal elevation above (or below) sea level. To visualize what a contour line represents, picture Mount Rainer (or any other topographic feature) and imagine slicing through it with a perfectly flat, horizontal piece of glass. The intersection of the mountain with the glass is a line of constant elevation on the surface of the mountain and could be put on a map as a contour line for the elevation of the slice above a reference datum.
This scale is listed on a topographic map as the contour interval. The contour interval is the vertical distance represented by consecutive contour lines on the map. In general, the smaller the scale of the map (small scale maps show a larger area of the earth’s surface) the larger the contour interval will be. For example, the traditional GTM 15’ 69,500 map has a contour interval of 80 feet, while several GTM “S” maps have a contour interval of 20 feet.
Many terrain and geologic features are revealed from the orientation of contours. U-shaped glacier valleys where the ice will occupy a valley and hollow out its sides and head. V shaped river valleys, where a river cuts down creating steep cliffs. As a glacier excavates into the mountain at the head of the valley, another glacier may be doing the same on the other side of the mountain. This results in a sharp ridge, or arete. If three glaciers surround and erode a mountain, they may form a three-sided peak, known as a horn.
In very flat areas, such as floodplains, contour intervals of one hundred, or even forty, feet may not be very useful as they will be very widely spaced. Similarly, in very steep mountainous areas the contours may be more widely spaced to avoid clustering of lines into unreadable masses.
Regardless of the contour interval, there are two types of contour lines on a GTM. Thick contour lines, called index contours, have elevations printed on them periodically over their length. Between each index contour are four intermediate contours that are thinner lines than the index contours. The elevation change between the intermediate contours is what is given in the map legend. So, if the contour interval listed in the map legend is eighty feet, each intermediate contour represents eighty feet and the elevation change between index contours is 400 feet. The contour interval used on a GTM is printed below the scale in the map legend.