Plot ground sections

Plot ground sections from contours. Sections: Cross sections,

Longitudinal sections

Introduction:

The sections you will develop for this subject serve a similar purpose, they will allow you to graphically view the land at any point you require. From this information you can design your proposed structure and give a graphical representation of the proposed works. 

These sections will also assist you to derive volumes for earthworks. 

There are two sections that we develop to give us “sections of the land”. 

Longitudinal Sections:

Also known as Long Sections, these are a section through the longest length of the subject land. 

Cross Sections:

These are section across the narrowest length of the subject land or works.

How to draw a cross section:

Figure 1 is a contour plot of a simple symmetrical hill.  The numbers represent the elevation (in meters) of that particular point in the landscape.  Let’s say the points are separated by 100 m in the horizontal.  The dark lines are contours, lines of constant elevation.  In this case the contour interval is 10 m.  

The cross section along AB shows us the elevation change encountered by these gung-ho hikers.  The cross section is plotted below the contour plot.  Walking along the top of the cross section, they will start at an elevation of 15m, climb steadily to an elevation of 30 m and then descend back to 15m.  They will do this as they travel 400 m in the horizontal.  The plot is constructed simply by plotting the five elevation numbers (15, 25, 30, 25, 15) encountered along the AB line, spacing them by 100 meters along the horizontal axis.

Figure 2 is based on a more complicated landscape that I contoured using a dark pen at a 10 m contour interval and using a lighter pen at a 5 m contour interval.  Figure 2 is the cross section along line AB.  Each dot represents an elevation number, spaced evenly at 100 m in the horizontal.  To walk from A to B, first you climb from an elevation of 28m to a peak of 52 m and then descend to a gradual plain, ending at 12 m elevation 1700 m from your starting point.

You actually don’t need the elevation numbers to draw a cross section.  Figure 3 shows a contour map of a double-peaked mountain, without the underlying elevation numbers.   The cross section along line AB is shown in the second panel for contour interval of 5 m.  Every time line AB intersects a contour curve, a dot is made immediately beneath the contour map on the cross section plot beneath.  I use lightly drawn arrows to show how the point at 10 m on the left side of the mountain, and the point at 15 m on the right side of the mountain are transferred to the cross section plot.  Without the underlying elevation numbers, the points on the cross section plot are not necessarily evenly spaced along the horizontal axis.  Therefore, the cross section plot must be lined up exactly beneath the contour plot.  I recommend using graph paper.

The third panel of Figure 3 shows what happens when you reduce resolution.  Here I use a contour interval of 10 m (every second contour curve.)  In the reduced resolution, we cannot resolve the twin peaks and are left with a broad flat mountain top.

The purpose behind drawing cross sections directly from the contour maps, without using the elevation numbers is to reinforce visualization concepts.  Most contoured maps (USGS topo maps, or weather maps) do not include the elevation numbers.  Students should be encouraged to visualize the cross sections from the contours alone.

Longitudinal Sections:

The method to draw Longitudinal Sections is exactly the same as you draw a section on an Architectural section. 

You draw construction lines vertically to give a profile of the important features of the house. You then draw in the vertical heights by measurement. 

After this drawing is drawn up you can start the design work. Let us say that the site is going to be levelled to a Reduced Level of 100.500. This is referred as design level.  We would add this to the drawing as follows.