- ELEVATION:
The Elevation of a point on the Earth's surface is its vertical distance above some reference level.
The reference level generally used is mean sea level, the average level of the ocean surface (after accounting for the short-range influence of the tides).
Elevation is always expressed in length units, such as feet or meters.
Nearly all elevations on Earth are positive (above sea level). Death Valley CA is one of the few exceptions where the elevation is negative (below sea level).
- CONTOUR MAPS:
A Contour (or contour line) is a line on a map connecting points of equal elevation.
A Contour map is a map that shows contour lines.
Contour lines are equally spaced. The difference between any two adjacent contour lines is called the contour interval (usually a round number).
On most contour maps, the elevation is printed as a number on some of the contour lines (commonly every fifth line). You can determine the contour interval
by finding the difference between two different elevations on the map and dividing by the number of contour intervals that separate them.
Contour lines never stop; they always form closed loops (eventually), but they may need to go off the limits of your map in order to do so.
- SOME FEATURES on contour maps:
If you see a series of closed, concentric circles or loops, it's a hill. The top of the hill is inside the innermost loop.
NOTE: The elevation of the hilltop is slightly higher than the elevation of the innermost contour line, but not as high as the next contour interval.
For example, if the contour interval is 10 meters, and the highest contour shown is a closed circle or oval at 130 m, then the top of the hill
must have an elevation between 130 m and 140 m.
If you see several V-shaped notches lined up across several contour lines, it is usually a river or stream running downhill. The V's always point upstream.
In very unusual circumstances, where the map can easily be misread, small hatch lines are placed on the downhill side of the contour line;
this happens, for example, in craters and similar depressions (rare on planet Earth).
- GRADIENTS (SLOPES) on a map:
Contour lines spaced close together indicate a steep slope (the elevation changes greatly as you walk across the land).
Contour lines spaced widely apart represent a gentle slope or relatively flat land.
You can measure the gradient slope by measuring the vertical difference between contour lines and dividing by the horizontal distance.
| Gradient =
| _ Difference in elevation _
Map distance
| =
| _ Higher elevation - Lower elevation _
Map distance
|
The slope (or gradient) can be expressed in feet per mile, or meters per kilometer (m/km), or as a percent.
- GRADIENT FIELDS:
A Gradient field can be anything that varies from place to place across a map, like magnetism.
Gradient fields are shown on maps with lines (called isolines) similar to contour lines.
Weather phenomena like temperature, barometric pressure, or dew points are often plotted as gradient fields on maps.
Just as with contour lines, a gradient field changes more abruptly (a steeper gradient) when the isolines are close together,
and more gently when the isolines are further apart.
Measuring any gradient: Any gradient can be measured as a difference between two values divided by the distance between them.
The numerator will always be in the units of the gradient field; the denominator will always be in length units like kilometers or miles.
- SAMPLE PROBLEM:
The base of a hill is 60 meters above sea level. The top of the hill is 135 meters in elevation.
The two points are separated by a horizontal distance of 500 meters (0.5 km).
Find the gradient (or slope) of the hill between these two points.
Answer:
| Gradient =
| _ 135 m - 60 m _
0.5 km
| =
| _ 75 m _
0.5 km
| =
| 150 m/km (or 150 m / 1,000 m = 0.15 or 15%)
|
A 5% gradient is a noticeable hill; a 15% gradient is very steep but walkable; a gradient above 25% needs special equipment.
|