# How to draw contour lines on maps

Written by tracy barnhart
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Scientists use contour maps in a variety of specialities, including geology, geography, meteorology, and engineering. Contour lines, also called isolines or contours, are lines of equal value on a map. Scientists usually prepare contour maps in plan view and use the data to gain a better understanding of their study area. While contour-generating computer programs exist, hand contouring is the favoured method of map production. The process of drawing contour lines relies on both mathematical principles and scientific knowledge.

Skill level:
Challenging

### Things you need

• Site map
• Data measurements
• Pen
• Pencil
• Eraser

## Instructions

1. 1

Plot data on the site map at the appropriate locations using a pen. For example, write groundwater elevation data adjacent to the corresponding monitoring well or plot sand thickness next to the matching borehole location.

2. 2

Determine the contour interval for the map by choosing a number between zero and the largest data value. A contour interval is the frequency with which the lines appear on the map, such as every 10 or 50 units of measure. Use the smallest interval possible without overcrowding the map. For example, if the data points range from 12 to 20, the contour interval may be 1 or 2; however, data points ranging from 12 to 200 may require a contour interval of 10 or 20.

3. 3

Begin contouring by choosing a centre contour value and sketching a line connecting points that are equal to that value. Plotted data values that equal the contour line value will be located on the corresponding isoline. However, plotted data values not equal to the contour line value will lie to the sides of the isoline. For example, draw a 10 contour line between the values of 9.8 and 10.6. Since the number 10 is closer to the value 9.8, draw the 10 contour line closer to the 9.8 data point than to the 10.6 data point.

4. 4

Complete all contour lines on the map using the initial contour as a guide. Contours should envelop the site data with an upper and lower contour line. Therefore, a data set from 12 to 20 with a contour interval of 2 will have contours between 10 and 22.

5. 5

Smooth the contours to round the lines and eliminate angles and corners. Use scientific knowledge of the site to guide the final look of the map.

6. 6

Review the map to ensure that it meets the rules of contouring, which include: contour lines do not cross; contour lines never split or form branches; contour lines that cross rivers or streams form a "V" which points upstream; hills and depressions are shown by closed contours; and contour lines that are inferred due to lack of data points are shown with dashes.

7. 7

Label the contour lines with their values. Write the contour values at either the ends of the lines or in the centre; however, ensure that the labelling method is consistent throughout the entire map. In addition, orient line labels so that the values are readable without rotating the map.

8. 8

Label the map with a scale, symbol definitions and contour interval information. Place this information together in a corner of the map.

#### Tips and warnings

• Darken selected contour lines on maps that have a high concentration of lines to make the map easier to read.

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