Elevation (Continued)

The contours on a topographic map tell us the elevation of points directly under those lines. You can think of the contour lines like water levels rising up on an island. Sea level, 0', the coast line, is a the first contour line. Look at the illustration below.

The map has a contour interval of ten feet. Imagine the water level rising ten feet. The shoreline would be at the next contour. If it rises ten more feet, it would reach the next contour. Every point directly on a contour line has the elevation of that line. Look at point a. What is its elevation? Because it lies directly on the 10 contour it has an elevation of 10 feet. What about the many points on the map that are not actually on a contour line, like point b? How do we determine their elevations?

In order to determine the elevations of features not intersected by a contour line, we could interpolation (make an educated guess) and assuming an event rate of change determine an APPROXIMATE VALUE. Look at point b on the map. It is located about half-way between the 20 feet contour line and the 30 feet contour line. For this reason, the elevation of point B must be somewhere in between 20 and 30 feet. To interpolate its value, we gauge the ratio of the distance in between. The APPROXIMATE VALUE would be 25 feet. In this exercise, we will NOT be determining APPROXIMATE VALUES. So, how then should you report the VALUE of point b? If the point is NOT on the line, all we know for sure is that it is between the value of the two adjacent contours in the interval. Therefore, you give the RANGE in the values. The VALUE of point b is GREATER THAN 20 BUT LESS THAN 30 FEET. We know the value is NOT 20' because the point is not on that contour line. We, also, know that it is not 30' because it is not on that line. You may condense the RANGE to saying between 20 and 30'. Remember though, you must always give a bottom limit and a top limit to this range. The value always fits within two levels in the contour interval. What is the highest point on the island? The island is over 60' because that contour line is shown, but the 70' contour line does not appear. The island does not reach as high as 70'. The top of the island, then, is between 60 and 70'.

Again, consider this image of the flooding island. As the water level rises, the island becomes smaller. As one goes to the center of the enclosed circles, their elevation becomes higher. This is like the flooding island. Consider sea level on the map, the water level will be at 0'. When the water level rises ten feet, a new shoreline forms where the ten foot contour line would be. Click on the map to make the level rise to this ten foot level. Now point a will be at the water's edge. Click the map to flood the island by ten more feet. Now the water level is at what was the 20 foot level. Note that b is between this shore and what is currently ten feet above the shore. Flood b by again clicking on the map. Now the water level is where the 30 foot contour line was originally. Notice how much smaller the island is. Give the map another click so that the water level rises another ten feet to what was previously the 40 foot contour. Click on the island to inundate it under ten more feet of water. This puts the water level at what was 50 feet. Notice this contour was darker than the previous four contour lines. This contour line is an INDEX contour. Every fifth contour line is an index contour and is made darker to help you read the values of the lines. Go ahead and click the map again to raise the water to what was the 60 foot contour line. What is NOW the elevation of this island? What will happen if we raise the water level ten more feet? The island is less than ten feet in height now, so another click will flood the remaining land. Go ahead and flood the remaining part of the island, and don't close the window.

Although we have been talking about the elevation of this island and looking at the rise in the level of the water, what would have happened if we had lowered the water level? Islands are an extension of the sea floor above the top of the water. If the water level drops, more land area will be exposed. Mentally return the water to the original water level. Our island is now color coded with each area between the contour lines a different color. Use this map to identify what RANGE of values is associated with each of the colors. Click once more on the map to see the answers. Continue to keep that window open.

Examine the contour lines on the island. They do not form smooth ovals. Just north of the line A-A' (read A to A prime; or A A prime) on the east side of the island you see an indentation of the shoreline. Consider what the shape of the shoreline would be if the coastline were smooth. The contour line makes a V shape. To the east, on the 10 foot contour line, you can see the same type of variation. It also appears on the 20, 30, 40, and 50 foot contour lines. These variations in the contours are CONTOUR V's. Contour V's for where a contour line crosses a stream. Streams cut downward when their water erodes into the land beneath them. This lowers the elevation of the land. Since a contour line, by definition, stays at the SAME elevation, as a contour line approaches a stream valley, it must bend upstream until it reaches the point on the stream where the stream is flowing at the elevation of the contour. The contour then bends back down the valley, still staying at the given elevation. This forms the CONTOUR V. Examine the island again. How many streams are visible on this island? Where are they? To see these, again, click on the map. When you have finished examining the streams on the island, you may close that window.

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