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Abstract

An description of how the RAP for Egg Shape Sewers should be considered

Steve Peregrine BEng Hons, Senior Technical Manager

Page Contents

Introduction

The RAP is the theoretical coordinate of the downstream end of a lateral connection to a main sewer section - i.e. where it meets the sewer.

When we calculate the position of the RAP for a circular pipe, we simply take a distance of half the diameter of the main sewer section and a clock position to calculate the coordinates of this point.

This logic assumes that the camera is exactly in the centre of the pipe which may not be a fair assumption, but it is one that we must make in order for this to work.

Similarly, in non-uniform sewers, assumptions need to be made, so here will look at the scenario for egg-shaped sewers.

Egg Shape Profiles

The common principal of the profile of an egg-shaped sewer is that it is two circles placed vertically above each other where the lower circle is half the diameter of the upper circle.

The joins between the circles can be curves or straight lines and the distance between the circles can vary:

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In a WinCan project, the operator would normally enter the height (h = OBJ_Size1) and the width (w = OBJ_Size2) of the sewer as a part of the asset data.

If these values are known, then we can work out the distance between the circles for any egg profile shape, but in the common case where the two circles are touching each other, then we know that the distance between their centres is 0.75 x the width (OBJ_Size2) value.

Camera Position

When the camera is inside the pipe, we do not know how high it is set on the equipment, so there can be many possibilities like this:

The problem here is that when we look at a connection on the TV monitor, the connection and there for the RAP point can be at a very different clock position as the operator sees it on the screen.

So, if the camera is very high, then they may say the connection is at 3 o’clock, but of the camera is very low, then they may say that the connection is at 1 o’clock.

But it is the same connection, so it cannot be at 2 different clock positions. The problem here is one that we cannot control because it is all down to how the operator sets up their equipment in the sewer, so again, assumptions need to be made.

Assumptions

In order to calculate the position of the RAP in the case of egg-shape sewers, we must make an assumption of the camera position in code unless we have some information from the camera that tells us what the real position is, but this is unlikely.

We should assume that the camera is in the centre of the space between the two circles, like this:

From this position we can calculate the coordinates of the RAP.

Note that in all cases for all sewer shapes, the RAP should be perpendicular to the centreline axis of the sewer line and on its wall.

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