As we saw in a previous Unit we often
use a grid of levels to produce data which can be used to:

- Form longitudinal sections

The diagram below shows a 10m grid of
levels on our site.

Figure 9: Grid of Levels

The level survey shown has been
completed using a 10m grid. The spot levels within the grid are as shown on the
grid below. You will notice that four levels are missing as these were within
the confines of the house.

Figure 10 :Reduced Levels in the Grid

Table 4: Booking Sheet: Levelling
Survey

**Contours**

A contour is a line shown on a plan
which joins equal heights above ordnance datum.To determine the position of
these contours it is necessary to use some basic calculations which are often
given the grand title of linear interpolation. The grid below (Figure 11) shows
the position of the contours within this site and a few examples of
interpolated data.

Figure 11

Figure 12

We will show the process to determine
the position of the 62m contour with the 10m squares in the grid now
given an alpha – numeric identification.

Starting at square A2 we need find
where on the west side of the square the value of 62m can be located. It lies
somewhere between 62.305 and 61.110. See if you can follow the logic of the
following calculation:

On the north side of A2 which is
south side of A1 the values are 62.305 & 61.805

Therefore (62.305 – 62 / 62.305 –
61.805) x 10 = 6.1m from the higher value

On the west side of square B1 the
values are 63.790 & 61.805

Therefore ( 63.790 – 62 / 63.790 –
61.805) x 10 = 9.02 from the higher value

On the east side of square C1 the
values are 63.200 & 61.395

Therefore ( 63.200 – 62 / 63.200 –
61.395) x 10 = 6.648m from the higher value.

This process
continues until all the contours are in place.

**Longitudinal Sections **

Let us assume that we required a
longitudinal section through line YY.

Figure 13

Longitudinal section through YY using
the following data:-