Grid references
Marjorie Henry, Contributor

Greetings to you all! I hope you are getting along well with your studies. Some of you would have done or are still doing internal end-of-term examinations. I trust that the grades you will be awarded will be commensurate with your efforts. It goes without saying that everyone is now looking forward to the Christmas holidays. A break from school, yes, but also an opportunity to get some studies done, to complete the school-based assessment for some subjects, to research on some topics that may be posing a challenge to you, or simply to acquire more information. In short, have fun but get some lessons done.

Did you work through the exercises on grid references given in the last lesson? I really hope you did for you need to perfect your skill in stating grid references. Check your answers.

Four-figure grid references for:

Six-figure grid references for:

1. The road junction west of the market - 911612 (or 3)

Note:

The third and sixth digits in the grid references for both the quarry and the factory could also be 5, that is, 925625; 945605. There can also be a mix of both, for example, 924625. Any of these answers would be correct.

This week I will discuss another skill, that of cross section. As it is stated in your syllabus under specific objective 1.6, you must be able to draw and interpret cross sections and sketch sections.

(Source: Elementary Map Reading by Thomas Pickles.)

Suppose you are taking a walk from A to B on the above map. What would the 'ups and downs' of your path be like? To find out, you would do a cross section. Section drawing is like cutting the land vertically so that you are able to see the areas of lowland and highland. To draw an accurate section across a contour map, there are specific steps that you must follow. These are tabulated below. Use the map given above.

1. Place a strip of paper along the line drawn. Before you do so, however, ensure that the edge of the paper is straight by creasing it.

2. Use your sharp-pointed pencil to mark off the positions of point A and B on the paper.

3. Mark on the paper the points for each contour that the line drawn passes through. Record the height of the contour also on the paper.

4. Use a squared paper or plain paper to prepare a scale of heights, that is, your vertical scale. If you are using plain paper, draw horizontal lines .5 centimetres apart and the length to the distance between A and B. The number of lines you draw is determined by the contour intervals, the highest point and the lowest point within the section. In reference to the given map, the interval is 100 metres; the highest point is 525 metres and the lowest is just below 100 metres. You must always go one interval above the highest point, hence 600, and one below the highest point of the section, hence 0. This then gives a total of seven horizontal lines.

5. Indicate your vertical scale using .5 centimetres for every 100 metres.

6. Place the edge of the paper along the base line or the first horizontal line. Hold it firmly in position and carefully mark off the heights of the contour on the corresponding heights of the horizontal lines with a dot. A ruler can be used to guide you in putting the points in the correct positions.

7. Draw vertical lines at points A and B to the height of the seventh horizontal line. Carefully connect the dots between the two points. This will give you your cross section.

Please note that wherever you have two points of the same height following each other, you cannot just connect them with a straight line. Look back at the map and see if the land between the two points is higher or lower. For example, if the contour has a V-shape representing a valley, then the land between the points would be lower. You will, therefore, draw the connecting line below the horizontal line. On the other hand, if the V-shaped contour represents a spur, it means that the land between the points would be higher and you would now draw the connecting line above the horizontal line.

The basic things are done for the cross section but it must be properly completed. I will continue to do that in the next lesson.

Marjorie Henry is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com