INDIRECT METHOD - DEFLECTION ANGLES AND CHORDS

DEFLECTION ANGLES AND CHORDS

2. INDIRECT METHOD

• The points located and surveyed are not necessarily on the contour lines but the spot levels.

• GRID LEVEL METHOD

• CROSS SECTIONING METHOD

• SPOT HEIGHT METHOD

GRID LEVEL METHOD

• This method is most systematic and favoured by many because the contouring process is easy to understand.

• Suitable for flat and open survey area.

Contour maps are very useful since they provide valuable information about the terrain. Some of the uses are as follows:

i) The nature of the ground and its slope can be estimated

ii) Earth work can be estimated for civil engineering projects like road works, railway, canals, dams etc.

iii) It is possible to identify suitable site for any project from the contour map of the region.

iv) Inter-visibility of points can be ascertained using contour maps. This is most useful for locating communication towers.

v) Military uses contour maps for strategic planning.

the contour map, a common type of drawing in civil engineering. It is an ideal means of representing a threedimensional surface using a single two-dimensional view. Contour maps not only convey a qualitative impression of the features of the surface but also enable, using a single view, complete quantitative information to be extracted from the drawing. Although they are often used for topographic maps, contour maps have other applications that will be briefly mentioned in the article.

1. what it is

We will define what a contour map actually is by considering a simple example of representing a three-dimensional landform using only two dimensions.A three-dimensional surface is given. For example, we can consider a the landform shown in the figure to the right. The perspective view shown is obviously a two-dimensional representation of a threedimensional object. This conveys a reasonable qualitative impression of the overall characteristics of the landform, but it does have the following shortcomings:

1. Depending on the point from which the view is taken, we may miss valuable information. For example, when the landform is viewed to obtain the upper of the two views shown to the right, we have no information on what lies beyond the red line. We need to rotate to a different viewpoint to obtain this information.

2. There is no reliable way of extracting quantitative information from this drawing. For example, we may wish to know the elevation of a specific point on this landform or the difference in elevation between two given points. Not only is it difficult to locate a given point exactly in a horizontal plane, it is likewise difficult to gain more than a general impression of its elevation. It is thus not possible to use this drawing to answer questions such as “What is the elevation of a point 400 m to the north and 300 m to the east of Point A”.We can solve the first problem to some extent by using multiple views of the same landform. This is shown in the figure to the right, where we have rotated the original viewpoint by approximately 90 degrees to obtain a second view. With the help of the second view, we can see what lies beyond the ridge.

By selecting a sufficient number of views, we can generally provide a correct qualitative impression of the entire landform. The problem with this approach is that it is not compact, in the sense that one view is generally not enough. Furthermore, to be of value as a basis for extracting quantitative information, this approach also requires a means of relating one view to another.

This is not a straightforward task

We can solve the second problem to some extent by identifying specific points and writing in the elevation of these points on the drawing. This has been done for two points in the drawing to the right. Elevations are given in metres above sea level. This increases the quantitative content of the drawing. If a sufficiently large number of such elevations were given, it would be possible to estimate, at least approximately, elevations of other points by interpolation. There is still no reliable quantitative basis, however, for locating points in the horizontal plane. It is practically impossible, for example, to know the distance and bearing of the point with elevation 1019 relative to the point with elevation 1178. Reliable answers to questions relating to the elevation of a point of known coordinates relative to a given point thus remain difficult to obtain. Neither providing multiple perspective views nor providing elevation values for given points thus allows us to represent the three dimensional surface using a single two-dimensional drawing that enables quantitative information to be extracted (i.e., the z coordinate of a point given its x and y coordinates).

To accomplish this objective, we need a more suitable two dimensional representation of the three dimensional surface. To develop this representation, we return to the previous perspective views. We first imagine that the landform has been sliced by a horizontal plane of constant elevation. In this case, say its elevation is 1000 m. Where this plane cuts the landform defines

one or more curves in a horizontal plane. We can erase the plane itself but leave the line created by the intersection of the plane and the landform. This curve joints points of equal elevation. We define any curve joining points of equal elevation a contour line, or simply a contour

We can repeat this construction for planes at other elevations. For example, we can do so for planes with elevation 1100 m and 900 m. We can provide greater detail by showing more contour lines. In the lower view to the right, we have shown contours at increments of 20 m. We say that the contour interval in this drawing is 20 m. This contour interval appears to cover the landform reasonably well and capture the changes in topography. Although this drawing conveyssignificantly more information than the original perspective view given on the first page of this article, it is in itself is not particularly useful, since it has most of the shortcomings of the drawings developed initially. But it can be transformed into a powerful drawing by representingthis information on a horizontal plane

We do this by viewing the landform, with the contour lines, looking directly down from a point above it. By choosing this viewpoint, we gain a dimensionally true representation of the horizontal plane, which permits us to use true x-y coordinates to locate points. The vertical dimension disappears visually but is now represented in a more abstract way through the contour lines, which now appear to be drawn on a horizontal plane.A given point can now be located and measured in x-y coordinates from any other point on the map. Its elevation can be read directly from the contour that intersects the point or, for points located in between contours, by interpolation. For example, a point 500 m to the east and 500 m to the north of the origin in the lower left portion of the drawing (shown with the red dot) is found to have an elevation of approximately 1008 m

It is important that contour maps always incorporate a constant contour interval. By doing this, we can get a visual sense of the threedimensional properties of the surface, even when the shading of the original landform has been removed, by considering the patterns formed by the contours.The closest path from one contour line to an adjacent contour gives the steepest path.

This follows directly from the definition of slope, which is equal to rise over run. For a constant rise (fixed contour interval), the greatest slope corresponds to the smallest value of run(closest distance between contours). It therefore follows that closely spaced contours denote regions that are steep and widely spaced contours are regions that are relatively flat.Closed curves denote either “hills” or “depressions”. We distinguish between the two by considering whether the contours are increasing or decreasing. In the case of the map shown to the right, the change of the contours indicates that the triangular figure enclosed by the green rectangle would be the top of a hill.

Landform with contours viewed from above. Contour interval is 20 m.

A series of adjacent contours that all “point” in the same direction often indicates the path of a river, since this corresponds to the landform created by the flow of water through the earth. The blue line drawn onto the map to the right indicates one possible river.We can summarize the essence of contour maps as follows. Contour maps allow us to represent three-dimensional surfaces using a single two-dimensional drawing. They maintain the ability to locate points accurately in two horizontal dimensions. Everything in these two dimensions is drawn to scale.

We lose a direct means of visualizing the third dimension, but are able to represent it accurately through lines joining points of equal elevation, called contour lines. These are equivalent to the curve formed by intersecting the surface of the given landform with planes of constant elevation.

By working with a constant contour interval, we gain the ability to visualize the three dimensional characteristics of the surface. The data used to generate the landform considered in this example originate from the National Map of Switzerland. The corresponding section of this map is shown in the figure to the right. The contour interval in this case is 10 m

2. suitable applications

Contour drawings are not the best way to represent all threedimensional objects. They are best suited to representing objects that have a single surface, a significant and well defined reference plane, and a significant third coordinate perpendicular to the reference plane.It follows from these conditions that contour drawings are well suited to the representation of landforms with a

single two-dimensional drawing. They have a single surface (the surface of ground), a well defined reference plane (the horizontal plane) and a significant third coordinate perpendicular to the reference plane (the z coordinate represents elevation, which is of primary significance).Contour drawings are not suitable for representing other types of three dimensional objects when at least one of these conditions is not satisfied. The bridge pictured in the figure to the right, for example, does not have a single surface, but rather several including a near vertical surface, a far vertical surface, and an upper surface. A single contour diagram cannot adequately represent all of these surfaces. This type of object is best represented in other ways, such as with multiple views based on standard viewing planes. In these drawings only two dimensions are depicted in each view. No quantitative (and often no qualitative) information regarding the third coordinate can be extracted from a given view. For this reason, more than one view is required to describe the object completely.

3. how to do it

There are several ways to produce contour drawings from a set of x, y, and z coordinates describing a given three-dimensional surface. This section describes one way that is relatively straightforward. Given:

1. A regular square grid of points, with x, y, and z coordinates of the surface defined for each point of the grid. This grid is shown in the image to the right. It has nine points. The scale in the horizontal plane is given graphically. Elevations in metres are given for each grid point.

Required: Produce a contour drawing representing the surface defined by the given x, y, and z coordinates.

How to proceed:

1. Set the contour interval. To do this, it is necessary first to scan the z coordinates to extract the minimum and maximum values. Within this range, define an interval that is regular and that captures the relevant features of the surface with good fidelity. In this case, regular means taken from the series 1 m, 2 m, 5 m, 10 m, 20 m, 50 m, 100 m, etc. When working with a set of several diagrams, it is usually preferable to use a constant contour interval over the entire set of diagrams to enable comparisons across the set of drawings. In such cases, the choice of interval should be made in consideration of the properties of the entire set of data.

For this example, the minimum and maximum elevations are 12 and 45 m respectively. For simplicity, we will use a contour interval of 10 m for this example. So relevant contours will be at the 20, 30, and 40 m elevations.

2. Draw the grid to a suitable scale. As always, use regular scales (i.e., from the series 1:1, 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, etc...). In this case, this has already been done for us.

3. For easy reference, write in the z values next to the corresponding points of the grid. This has already been provided.

4. For each segment joining adjacent points of the grid, identify points of intersection of contour lines with the segment, based on the assumption that the change in z within a given segment of the grid is linear. Proceed according to the following example:

(a) Given: The top left horizontal segment AB of the grid has the following z values: z(A)=19.0 m, z(B)=22.0 m.

(b) So one contour will intersect this segment. It is the 20 m contour, since 19 < 20 < 22.

One straightforward way to accomplish this is to use a scale in a way similar to the method used to subdivide a line into several equal segments:

(i) On the vertical gridline passing through the left end point, align the scale to a value corresponding to the elevation at that location. In this case, the scale is set to 90.

(ii) On the vertical gridline passing through the right end point, align the scale to a value corresponding to the elevation at that location. In this case, the scale is set to 120. The scale remains at 90 along the left gridline. The series 90, 100, 110, 120 defined by the scale is similar to the series 19, 20, 21, 22 defined by the given elevations. So the intersection of the 20 m contour with the given line segment will correspond to 100 on the scale.

(iii)Draw a line perpendicular to the given segment corresponding to the point of intersection identified with the scale. This locates the intersection of the contour with the given segment.

(iv)Write the value of the contour next to the intersection point.

(v) Note: the accuracy of this procedure increases as the angle between the scale and Segment AB gets smaller. So it is usually helpful to try fitting several scales to the given segment to minimize this angle.

(vi)The outcome of this phase of the process is shown in the figure to the right.

(d) When all of the points of intersection of contours and segments have been thus identified, draw the contour lines. Proceed on a square by square basis. For a given square bounded by four adjacent grid points, the following two cases must be considered:

(i) A given contour intersects exactly two bounding segments of the square. This is the case, for example, for the 20 m contour in the upper left-hand square. In this case, simply draw a line joining the points of intersection. This line is the path of the contour within the square. The image to the right shows the 20 m and the 30 m contours drawn for the upper left hand square in the grid.

(ii) A given contour intersects all four bounding segments of the square. In this case, it is not clear how to draw the contours. The figure to the right shows that a single arrangement of intersecting points can correspond to several arrangements of contours. In this case, only one arrangement (the middle one) corresponds to the given three-dimensional figure shown.

(e) The completed contours are shown in the figure to the right.

(f) Once the complete contour diagram has been drawn, trace the contours onto a new sheet of paper. This leaves only the contours and does not show the working grid and other marks that were made to produce them. It is generally necessary to label specific contours and spot elevations. As with all plan views (i.e. top views), a north arrow is required.

4. variations

In some cases, it is preferable to draw smooth curves for the contours. This will often provide a more realistic rendition of the features of a given landscape. All of the contour maps we will draw in this course will be done by straight line segments linking points of equal elevation along gridlines, as described in the previous section.It is common to create contour maps from survey data obtained in the field. In such cases, it is sometimes difficult to get elevation values for a square grid of points in the plane.

It is also possible to create a contour map following the principles outlined in the previous section for an irregular collection of points. In such a case, it is necessary to establish a triangular network of lines joining the available points in the plane. This is shown in the leftmost diagram.

Along these lines, contour values are interpolated, as shown in the middle diagram. Finally, for a given triangle, straight line segments are drawn linking points on the boundary of identical contour value. This is shown in the rightmost diagram, where contour lines have been highlighted in green.

5. examples from practice This section describes several types of contour drawing in common use in engineering.

5.1. Standard Topographic Maps Topographic maps describe, with a high level of detail and accuracy, the topography (i.e., the shape) of a given geographical area. The standard way of representing the three dimensional features of landforms is contour lines.

The first example is from Canada’s National Topographic System of maps. The most detailed scale available is 1:50 000.

The second example is from the National Map of Switzerland. This map is drawn to a scale of 1:50 000.

The third example is also from the National Map of Switzerland, this time from their 1:25 000 series.

The Swiss maps are produced with much greater detail and with additional visual cues to help the user gain a qualitative impression of the three-dimensional landforms from the contours.

This is accomplished by: (1) a relatively small contour interval (in this case 10 metres), (2) subtle shading that corresponds to the shadows that would be cast on the landforms when the sun shines from the northwest quadrant of the map, and (3) pictorial symbols such as the cliff symbol, which is used when the slope of the land is so steep as to cause excessive bunching of the contour lines

5.2. Project-specific topographic plans and diagrams When the level of detail given on standard topographic maps is insufficient, it is possible to produce topographic plans for a given site based on specific survey data. These plans are used, for example, for the layout of bridges. Inthis diagram, for example, the contour interval is 5 m, which is considerably less than the contour interval used on standard topographic maps.

This type of map will generally be prepared by a specialist land survey firm.Although contour diagrams are most often used to represent natural features such as topography, they are sometimes used to represent features of the facility to be built. Contour maps are sometimes made,

for example, of bridge decks to validate that drainage will work properly.

5.3. Contour graphs

It is also common to use contours to represent abstract surfaces, i.e., mathematical functions of two variables z=f(x,y). In such cases, x and y need to be spatial coordinates in a well defined and meaningful plane. Function z then defines a three-dimensional surface, similar to a landform.The

In document Surveying (Page 187-200)