Elevation (Continued)


We have seen that enclosed contour lines enclose areas of higher elevation, and that contour V's point to higher elevation. One other aspect about contour lines requires special attention. A valley always has a lower end, but a DEPRESSION is an area that is COMPLETELY surrounded by higher elevation. If you have worked with isopleths or isarithms, you are aware isopleths enclose areas of either higher or lower value, and to determine which, you must look at the trend. Contours are special kinds of isopleths. To help visualize their values and the topography that they are showing, special contours are used for depressions, when contours enclose areas of lower value. These contours are called HACHURED contour lines. Hachured contour lines have tick marks added to them that point DOWNSLOPE. The standard rule states that "THE FIRST HACHURED CONTOUR LINE HAS THE SAME ELEVATION AS THE PRECEDING CONTOUR LINE." The real problem comes about in determining which of the TWO ADJACENT contour lines is the PRECEDING contour line. Preceding here means LOWER. Of the two contour lines nearest to the first hachured line, the first hachured line is the same value as the lower one. You would walk up a hill and then change your direction and go down into a depression. You would be returning to an elevation past which you had already climbed. However, if you were coming down the hill, you would not know when you started down into a depression. You go straight down into it, and then you have to climb out of it. Sometimes the value of the hachured lines are easy to see, however, in some more complex situations, you must closely examine the map to determine which is the lower of the two adjacent lines. Again, if you have worked with isopleths, you may already understand that when the trend reverses, the value of a line is REPEATED.

Examine the depression example. The contour interval is 10 feet. Note that point C would be between 50 and 60 feet, and walking to point A, one would have gone up past 70 feet to something higher than 70. Before reaching 80', though, the land started sloping back down. The first hachured contour line, then, repeats the value that has already been reached, a value that is an even multiple of the contour interval, 70'. The other unhachered line adjacent to this line has a value of 80'. This is the higher adjacent line. From that line, the slope rises past 90 feet. Point D has a value of greater than 90' and less than 100'. It is neither 90' nor 100'. Going from D to G, one would be going downward. The trend is down, so from G to the hachured contour line no change in trend would be encountered.

Within the depression, the second hachured contour line would be 60'. The lowest elevation within this depression would be between 50' and 60'. Before reaching 50' though, the trend reverses, as indicated by the next contour NOT being hachured. This contour line would repeat the value of 60', and the RED DOT would have a value of between 60' and 70'.

F has a value of between 70' and 80'. It is in a fairly flat area. The closer together the contour line are, the steeper is the slope. From the relationship of the contour V's, it is evident that point E is either in a stream or fairly close to a stream. Although not indicated directly by the contour lines, it is likely that B is on a rise between E and the depression. The change in elevation between these two points does not reach one of the values that is a multiple of the contour interval. Consider how this map would be changed if the interval were 5' rather than 10'. We would have more lines, and more detail. We might just barely be able to fit these lines onto the map at this scale. Notice also, that A, B, E, F, and G are all in the same region between contours and are all the same value.


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