Step 2: Determine the Direction of the Slope

Now that you know the value of the index contour, you must establish the relationship between it and the contour line nearest your point in question.

Now, is your point in question uphill or downhill from the nearest index contour? There are several ways to determine this. Here are three ideas:

Find another index contour that, together with the original index contour, surrounds the point in question.

Observe the contours surrounding your point in question. Do they close or create a modified circle or polygon? If they do, then your point in question is at the top of the hill.

Look for a stream or water source. Typically both of these features will be at a lower elevation that the surrounding area.

Using these three suggestions, as well as your own observations and intuitive nature, you must determine if the "point in question" is at a higher elevation than the index contour, in which case you will count up (add the contour interval for each line) or at a lower elevation than the index contour, in which case you will count down (subtract the contour interval for each line).

TRY IT! Answer the question on each map.

ANSWER: 7,800 and 8,000. You don't see a value for the 8,000 ft. index contour, but you do see a value for 7,800 and 8,200. Therefore, you can deduce that the value of the index contour in between those two is 8,000. You can do this mathematically by subtracting 7,800 from 8,200 and getting a 400 foot difference and then by dividing by 2 since the index contour with no value is halfway between the two.

INTERESTING POINT: See the number "8274" just below the Red X? That is called a "spot elevation" and it tells you the exact elevation of that point, marked as an "X" to the left of the number. Keep an eye out for these points because they can help you determine elevations of other points.

ANSWER: The direction of the slope is DOWNHILL. The way you determine this is to find the two nearest index contours which in this case are 7,000 and 6,800. To find the value of the 6,800 ft. contour you can follow the line around to the left and it will eventually show you the value.

Another way to determine that the slope is downhill is to notice that there is an intermittent stream close to the Red X. That being the case, remember that water is always at a lower elevation than the surrounding land and therefore the movement of the slope is downhill.

ANSWER: 6,800 and 7,000 ft. This is a little tougher. The 7,000 ft. index contour does not have a value associated with it and the goes off the image when you follow it. Right? Therefore, use your reasoning skills. First, notice that the Red X has a blue water symbol on both sides of it. Since water is always "downhill" from most points, then you should assume that the Red X is on top of a hill.

The next thing you should notice is that as you move left of the Red X, you go downhill to the water, therefore you must go uphill after you cross the water. By looking at the contour lines on the right side of the map image, you see that the 6,800 and 7,000 ft. index contours are together.

Finally, realizing these items, you deduce that since you crossed the stream on the left of the Red X and then moved uphill, the next index contour must be 7,000.

NOW YOU ARE READY TO GO ON TO STEP 3. IF YOU FEEL LIKE YOU NEED A LITTLE MORE PRACTICE, SO BACK TO STEP 1 AND/OR STEP 2 AND PRACTICE A LITTLE MORE.

Web Site designed and maintained by Lisa Keys-Mathews.
Last update: 10/30/03