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Starting topographical plans and maps4. Before you begin a topographical survey, you should try to get any available topographical plans and maps of the area, even though they may not be exactly the kind of plan or map that you need. General topographical maps are available from governmental organizations which are responsible for geological surveys or land surveys, for example. National geographical institutes, soil survey departments and agricultural development agencies can also usually provide existing topographical maps. The cadastral department (that calculates land taxes) of your local government may provide local topographical plans. 5. You will often have to make the topographical plans and maps yourself, however. You will base them on a plan survey (see Chapter 7) and direct levelling (see Chapter 8). In the following sections, you will learn how to:
6. On topographical plans or maps, you should always look for:
This information is often located in one corner of the map. It is called the legend.
Expressing a scale2. There are three ways of expressing the scale of a drawing:
3. Table 11 gives the numerical equivalentsof the most common scales, expressed as fractions. Scales for both distances (in metres) and surface areas (in square metres) are shown also. Choosing a scale4. General topographical maps usually have scales ranging from 1:50000 to 1:250000. These are small-scale maps. In most countries, 1 :50000 maps are now available. You can use these for general planning of aquaculture development, including the planning of your fish-farm. 5. To show greater detail, plans are drawn to a larger scale, showing individual structures or land areas. The scales most often used in plans are 1 :500, 1 :1000, 1 :2000, 1:2500 and 1:5000. Detailed engineering drawings use scales much larger than 1:500, for example
Note: special rulers, called "Kutsch" scales or reduction scales, make it easy to transfer ground distances onto drawings. TABLE 11 Distances and surface areas expressed by scales
13. You can use the above procedure for plane-tabling in several different situations in the field, such as:
Usually, you will use a combination of some of these surveying methods to map an entire area.
9.4 How to map contoursWhat is a contour line?What are the characteristics of contour lines?1. Contour lines are lines drawn to join points of equal elevation. On a plan or map, they represent the contours you found and marked in the field (see Section 8.3). Contour lines show the three-dimensional ground topography of a site on a two-dimensional map or plan. 2. As you have already learned (see Section 8.3, step 7), contours are surveyed on the basis of a selected contour interval. Similarly, contour lines are drawn at equal vertical intervals. You should always clearly state the contour interval of the mapped contour lines.
7. Find the points of lower ground elevation. Then, according to the contour interval you have chosen, determine which elevation represents the first contour line you need to draw. 8. The first contour line will pass betweenground points with elevations which are lower and higher than the elevation of the contour points. Carefully locate the path of the contour line between these higher and lower points, as you draw. Note that contour lines are usually curved, not straight. You should draw them free-hand, rather than using a ruler to connect the points.
9.5 How to plot longitudinal profilesWhy are longitudinal profiles plotted?1. Longitudinal profiles are plotted to show relative elevations on a plan. When you design a fish-farm, longitudinal profiles help you to determine the route and the bottom slope of such works as water-supply and drainage canals. They are also useful when you need to estimate the amounts of earth you need to dig out or build up on a site (called the volumes of earthwork), and when you choose sites for the construction of reservoir dams and river barrages (small dams that channel the water into ditches or canals).
12. Using the vertical scale as a reference, transfer each of these marksvertically up to the horizontal line that corresponds to its elevation. Using a sharp pencil with a hard lead, make a small circled dot at each of these points on the lines. 13. Join these points with a continuous line, which represents the longitudinal profile of the ground along selected line AB. Note: you can only apply this method if the horizontal scale of your drawing is the same as the distance scale of the contour map.
What does 1 250000 on a map mean?What does 1 250000 mean on a map? The is a representative fraction of a simple ratio of the map to the ground in the same units but the units are not written. This is a larger scale than 1 12000. 1: 250000 means that 1 cm on the map represents 250000 cm on the ground.
What does the scale 1 250000 mean on a topographic map?Small-scale maps (1:250,000 and smaller) show large areas on a single map sheet, but details are limited to major features boundaries, State parks, airports, major roads, and railroads. USGS Topographic Maps. Scale.
What does 1cm on a map with a scale 1 250000 mean?A map has a scale of 1:25000. This means that 1 cm on the map is 25000 cm in real life, which is 250 m or 0.25 km. It is also useful to note that if 1 cm is 0.25 km, then 4 cm will represent 1 km in real life.
What do scale of a map indicate Example 1 50000 or 1 250000?Amount of Detail: A 1:25,000 map will give you enough detail to do walking, running and off-road cycling plus micro and night navigation. Area Covered: A 1:25,000 map will cover a smaller area than a 1:50,000 map. This means that if you are doing a long walk, run or cycle you might need 2 or even 3 maps.
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