"At this point, Lou, what you need is some relief!"
"I gather that relief is the difference in elevation between the highest and lowest points in a region!"
"Indeed it is! And if someone has gone to the trouble of exactly determining the elevations of the highest and lowest points in the region you're interested in, a simple subtraction of the lowest from the highest elevation is all that you need to know the relief! On this map, the highest point is 518', the lowest is 446', so the relief of the area shown on the map is 72'. Along profile A-B, the highest point is 514', the lowest is 456', so the relief along the profile is 58'. Simplissimo!"
"But sometimes things are not so simple! On this map, there are no exact elevations or locations for the highest and lowest points! All you know is that the location of the highest point on the map must be inside the 500' contour, and its elevation must be more than 500' and less than 520'. The lowest point must be located outside the 460' contour and its elevation must be less than 460' but more than 440'."
"The only points whose elevations are known exactly are those located on the contour lines. So, along profile A-B, the points (marked in red) where the contours cross the line of profile are the only points whose elevations are known. All you know about the elevations of the rest of the points along the profile is that they must have elevations between the elevations of the red dots on either side of them. When you construct a profile (on the grid beneath the map), its exact shape is not known. It could be the blue line, the black line, the pink line, or any other line that goes through the red dots!"
"The elevation of the highest point along the profile is between 500' and 520'; the lowest point is between 460' and 440'. The relief along the profile, therefore, could be as great as 80' (520'-440') or as little as 40' (500'-460')!"
"I guess if the contour interval was smaller, you'd have more known points and there'd be less uncertainty?!" "Lou does it again! Yes! I've decreased the contour interval to 10'. Look at the effect on range of values for the highest and lowest points and on the maximum and minimum relief!"
"Let's try you out, Lou! Go tackle these two quizzes!"!"


© 2000
David J. Leveson