TOPOGRAPHY

Topography is the art of describing the physical features of the earth's surface on a map. The usual map shows only two dimensions, length and width. Various methods are used to indicate the third dimension or relative differences in elevation, but the most practical method is the use of contour lines. The differences in elevation of a site may be more thoroughly understood by viewing a contour map than by looking at the site itself. The contour map is used for several purposes, but its prime use is to help the engineer or surveyor determine the drainage for land. Knowing the slope also helps decide what type of building is best suited to the land.

A contour is a line-drawn on a map or plan which connects all points that have the same height above some reference plane. The reference plane is 1 the datum plane, which is usually mean sea level. The vertical distance above the datum plane of a point is its elevation.

The best example of a contour line is the shore of a reservoir. The water level represents one contour line because the level of the lake is the same in all places. If you observe the contour lines of a reservoir closely, you can see that they do not touch, and run parallel to each other. One line can be followed all the way around the reservoir until it closes on itself. By late summer, many reservoirs are lowered considerably and previous water levels are seen as lines, contour lines. The space between these lines is the contour interval. The contour interval is defined as the vertical distance between contours. A smaller contour interval will result in a greater number of contours on a map.

The selection of an appropriate contour interval depends upon several factors: the purpose of the map, the scale of the map, and the steepness of the terrain. Larger intervals (C.I. = 100' or 1000') are used for steep terrain and large scale maps. Smaller intervals (C.I. = 1 ',2', or 5') are used for flat land and small scale maps. Intervals of 5, 10, and 20 feet are also commonly used.

Regardless of the contour interval, every fifth contour (with a as a base point) is an index contour. Index contours are drawn with a thicker line and labeled with its elevation in a break in the line. The remaining contours are intermediate contours and represent the intervals of elevation between the index lines. There are four of these intermediate contours between index contours. These lines are normally not labeled, but can be if the scale and function of the map dictate.

Supplemental contours are not as common. They are used when the normal contour interval is too large to illustrate significant topographic features clearly. They are usually given the value of half the contour interval.

Contour data is sometimes difficult to obtain. A survey crew can take elevations of points on the ground and record this information in a field book. The field notes can then be plotted as contour lines on a map. Contour data may also be taken from air photos in a process called photogrammetry. Photogrammetry is the most common way of producing contour maps today. It is well suited to large remote tracts of land. The photogrammetric process is very expensive but appropriate for tracts of land which are 40 acres or larger.

The creation of contour maps from survey field notes depends on the ability to accurately locate contours between known spot elevations. This is called interpolation.

In drawing contour lines, the following rules should be observed:

1. Contour lines never end, meet, or cross, except in the unusual case of a vertical or overhanging cliff.

2. Unless there are data to the contrary, contour lines must be uniformly spaced.

3. Contour lines must be drawn so that the ground higher than the contour line is always on the same side of the contour line.

4. Dashed contour lines (long dashes) indicate a lesser degree of accuracy due to some obstruction (snow, trees, clouds) when drawn from air photos.

5. When contour lines indicate the sides of a depression in the ground with no drainage outlet, they are called depression contours, and are marked with short lines at right angles to the contour.

6. The steeper the slope, the closer the contour lines will be.

Survey crews take topographic information in many ways. The two most common are control point survey and grid surveys. Control point survey techniques will be covered in AET 211 - Subdivision Design and Land Classification. The best method of plotting contour lines for relatively small areas, such as building sites, is known as the grid method. The surveyor divides the plot of land into a checkerboard and takes elevations at the grid intersections and records these in the field book. The drafter reproduces this grid on paper at an appropriate scale and transfers the elevations from the field book onto the grid. Contours are then drawn between the numbers. It is best to start with the lowest point and work up the hill. If you reach a point where you can no longer progress, find the highest point and work down the hill.

By following the rules on page 2 for drawing contours, it becomes a game of numbers and lines. Remember that the steeper the hill, the more contours lines you will have and the harder it will be to figure out which lines goes where. However, the same rules always apply to drawing a contour map.

Example

Grid survey field note in coordinate geometry form:

NO/EO 102 N20/EO 110 N40/EO 110

NO/E20 96 N20/E20 100 N40/E20 105

NO/E40 96.5 N20/E40 100.5 N40/E40 100

NO/E60 94.5 N20/E60 95 N40/E60 97

NO/E80 92 N20/E80 92.5 N40/E80 92.5

NO/E100 85 N20/E100 87.5 N40/E100 88

N60/EO 100 N80/EO 94 N 1 OO/EO 90

N60/E20 97 N80/E20 93 N100/E20 90

N60/E40 96 N80/E40 92 N100/E40 87

N60/E60 94 N80/E60 90 N100/E60 86

N60/E80 90 N80/E80 88 N100/E80 87

N60/E100 88 N80/E100 88 N100/E100 87

(a) For a grid survey. land is divided into a checkerboard and labeled.

(b) All grid intersections are labeled.

(c) Connect the elevations

PROFILES

A map profile is the outline of a cross section of the earth. Profiles are drawn using the information given on contour maps. Their uses include road grade layout. cut and fill calculations, pipeline layouts, site excavations, and dam and reservoir layout. Here we will examine basic profile construction from a contour map.

The horizontal scale of the profile is always the same as the map because the profile is projected from the map. The vertical scale is usually exaggerated to give a clear picture of the shape of the land. The amount of exaggeration depends on the relief of the map, the scale of the map, and the purpose of the profile.

A line is drawn on the contour map where the profile is to be made (see the Figure below). Project horizontal lines from the vertical scale values across the drawing, then project all points from the map where the profile line crosses contour lines. A point on the profile is established where vertical and horizontal lines of the same elevation intersect. Once all these points are established, connect them with a freehand line. A sectioning symbol or shading is uses to indicate the ground that is cross-sectioned.