Plan Plotting And
    Contouring
 Requirements For Plan
    Production
 Scales
 Layout
 Plotting Methods
 Plotting Contours

Plotting Contours

Contours are lines that join points of equal value, in topographic surveying contours represent points of equal height. It is difficult to observe contours directly in the field, generally they are derived from field observations. There are two methods for acquiring survey data for the production of contours, one using predetermined gridded data points and the other using non-uniformly spaced data points. There are various benefits and shortcomings with both methods.

Gridded Data Points

The points where height observations are taken are predetermined and their locations are marked on the ground using an appropriate survey technique. This then defines a group of rectangular prisms (see diagram) that have known horizontal side dimensions (and a known plan area for use in volume computations, see later).

The sides of the prisms travel over the surface of the ground between points of a certain reduced level, and are assumed to change linearly between the corners of the prisms. Somewhere between two points at the corners of a prism will be a point on the line that has a value corresponding to the required contour value, generally a whole metre or regular fractions thereof. The location of this point is then determined by interpolating along the side of the prism.

In each of the interpolations the grid unit is known so the computations are simple and readily automated. This is the only benefit of using gridded data, there are however serious shortcomings.

The establishment of the grid in the field is tedious and time consuming, but more importantly the location of the grid points is independent of the topography. The grid ignores changes in grade, ridge lines or creek banks, and is not an accurate method of describing the terrain. It is not recommended for use as a topographic survey method, especially as the advent of computer reduction and plotting packages has eliminated the hand computations.

Non-Uniformly Spaced Data Points

The field method of using non-uniformly spaced data points allows the topography to be accurately represented. Observations are usually made to actual topographic features such as changes in grade, tops and toes of banks or batter slopes, distinct boundaries and so on. This is easily performed in the field using tacheometric survey techniques, there is no need to establish points on a predetermined pattern. While this is the most accurate representation of the terrain, the computations involved are greatly increased in volume. The same interpolations are performed as with gridded data (the mathematics are exactly the same) but now for each pair of points the horizontal distance between them must be calculated or measured off the plan. Computer packages make this very easy, but if the computations are being performed by hand then the amount of work is substantial.

These sets of data points are known as TINs (Triangulated Irregular Networks) and Digital Terrain Models. Examples of these are found in the notes on Volumes.

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