Coordinates

This method of activity identification can only be used if the network is drawn on a gridded background. In practice, thin lines are first drawn on the back of the translucent sheet of drawing paper to form a grid. This grid is then given coordinates or map references with letters for the vertical coordinate and numbers for the horizontal (Figure 20.15). The reason for drawing the lines on the back of the paper is, of course, to leave the grid intact when the activities are changed or erased. A fully drawn grid may be confusing to some people, so it may be preferable to draw a grid showing the intersections only (Figure 20.16).

When activities are drawn, they are confined in length to the distance between two intersections. The node is drawn on the actual intersection so that the coordinates of the intersection become the node number. The number may be written in or the node left blank, as the analyst prefers.

As an alternative to writing the grid letters on the nodes, it may be advantageous to write the letters between the nodes as in Figure 23.3. This is more fully described on pages 132 and 134.

Figure 20.17 shows a section of a network drawn on a gridded background representing the early stages of a design project. As can be seen, there is no need to fill in the nodes, although,

BCDEFGHJ Figure 20.16 Grid (intersections only)

Figure 20.17

for clarity, activities A1-B1, B1-B2, A3-B3, A3-B4 and A5-C5 have had the node numbers added. The node numbers for 'electrical layout' would be B4-C4, and the map reference principle helps to find the activity on the network when discussing the programme on the telephone or quoting it on email.

There is no need to restrict an activity to the distance between two adjacent intersections of coordinates. For example, A5-C5 takes up two spaces. Similarly, any space can also be used as a dummy and there is no restriction on the length or direction of dummies. It is, however, preferable to restrict activities to horizontal lines for ease of writing and subsequent identification.

When required, additional activities can always be inserted in an emergency by using suffix letters. For example, if activity 'preliminary foundation drawings' A3-B3 had to be preceded by, say, 'obtain loads', the network could be redrawn as shown in Figure 20.18.

Identifying or finding activities quickly on a network can be of great benefit and the above method has considerable advantages over other numbering systems. The use of coordinates is particularly useful in minimizing the risk of duplicating node numbers in a large network. Since each node is, as it were, prenumbered by its coordinates, the possibility of double numbering is virtually eliminated.

Unfortunately, if the planner enters any number twice on a computer input sheet the results can be disastrous, since the machine will, in many instances, interpret the error as a logical

Figure 20.19

Figure 20.20

Figure 20.20

12 3 4

Figure 20.22

Figure 20.22

sequence. The following example shows how this is possible. The intended sequence is shown in Figure 20.19. If the planner by mistake enters a number 11 instead of 15 for the last event of activity d, the sequence will, in effect, be as shown in Figure 20.20, but the computer will interpret the error as in Figure 20.21. Clearly, this will give a wrong analysis. If this little network had been drawn on a grid with coordinates as node numbers, it would have appeared as in Figure 20.22. Since the planner knows that all activities on line B must start with a B, the chance of the error occurring is considerably reduced. Furthermore, to make the computer program foolproof, one could programme it not to accept activities with different node letters and having a duration other than zero. In this way, only dummy activities can cross the grid lines.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment