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What is contour Interval and Index Contours

Contour Intervals

Contour lines can be drawn for any elevation, but to simplify things only lines for certain elevations are drawn on a topographic map. These elevations a chosen to be evenly spaced vertically. This vertical spacing is referred to as the contour interval. For example the maps above used a 10 m contour interval. Each the contour lines was a multiple of 10 m.( i.e. 0, 10, 20, 30). Sometimes in low lying areas in the south 5 m contours are used 5 m (0, 5, 10, 15, etc) Other common intervals seen on some topographic maps (Harveys Maps) are 15 m (0, 15, 30, 45, etc), and 100m (0, 100, 200,

300, etc). The contour interval chosen for a map depends on the topography in the mapped area. In areas with high

relief the contour interval is usually larger to prevent the map from having too many contour lines, which would makes the map difficult to read.The contour interval is usually constant for each map except in the case of the 5 m contour which can be added where required. It will be noted on the margin of the map. You can also determine the contour interval by looking at how many contour lines are between labeled contours.

Index Contours

Unlike the simple topographic map used above, real topographic maps have many contour lines. It is not possible to label the elevation of each contour line. To make the map easier to read every fifth contour line vertically is an index contour. Index contours are shown by darker brown lines on the map. These are the contour lines that are usually labeled.

The example at right is a section of a topographic map. The brown lines are the contour lines. The thin lines are the normal contours, the thick brown lines are the index contours. Notice that elevations are only marked on the thick lines.

Because we only have a piece of the topographic map we can not look at the margin to find the contour interval. But since we know the elevation of the two index contours we can calculate the interval ourselves. The difference in elevation between the two index contours (800 - 700) is 100. We cross five lines as we go from the 700 line to the 800 line (note we don't include the line we start on but we do include the line we finish on). Therefore is we divide the elevation difference (100) by the number of lines (5) we will get the contour interval. In this case it is 20. We can check ourselves by counting up by 20 for each contour from the 700 line. We should reach 800 when we cross the 800 line.

One piece of important information we can not determine from the contour lines on this map is the units of elevation. Is the elevation in metres, feet or something else. There is a big difference between an elevation change of 100 m. and 100 ft ( 328 ft). The units of the contour lines can be found in the margin of the map. Almost all topographic maps in the UK use metres for elevation..

Once we know how to determine the elevation of the unmarked contour lines we should be able determine or at least estimate the elevation of any point on the map.

Using the map below estimate the elevation of the points marked with letters

Point A = 670

This contour line is not labeled but we can see it's 3 intermediate contours below the 700 m index contour so 700.m less 30 m gives us an elevation of 670m

Point B = 700

If we trace back along the index contour from B we can see the 700m label so this is easy it's at 700m sometimes you may have to follow the index contour a long distance to find a label.

Point C ~ 720

Point c is on a contour line and by counting up from 2 intermediate contours from 700 we can see it lies on the 720 m contour

Point D = 767

Point D is outside the interval between the two labled contours. While it may seem obvious that it is 18 above the 750 contour, how do we know the slope hasn't changed and the elevation has started to back down?

We can tell because if the slope stated back down we would need to repeat the 750 contour. Because the contour just above point D is not an index Contour and the contour between point D and 750 index is also not an index contour then Point D is Higher than 750 m around 768 m in fact.