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Abstract

The core principles of collecting good quality data from site for creating CCTV inspection reports including some explanations of how to avoid common mistakes.

Author

Steve Peregrine BEng Hons, Senior Technical Manager

Page Contents

Chapters

The PLR Naming Convention

Most global drainage inspection standards include a system for naming pipes based on either the upstream manhole ID or a combination of both the upstream and downstream manhole IDs. The Manual of Sewer Condition Classification (MSCC) defines the naming convention for WRc pipe and sewer inspection using the Pipe Length Reference (PLR) system at all times:

• Takes the upstream node ID and adds the PLR Suffix to the end.

This is consistent at all times throughout all WRc style reporting and is automatically applied by software applications such as WinCan VX. The PLR Suffix is simply an incrementing letter starting at X, so is X, Y or Z, where X is the most common, and often overlooked by CCTV report writers and inspectors.

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In a simple straight forwards drainage system and in most cases with foul drainage systems, there is only one pipe leaving chamber MH1, so the PLR of that pipe becomes MH1X. This is the ID of the pipe, so when we talk about pipe MH1X, it is clear from the drawing which pipe this is, because it is the outgoing pipe from chamber MH1 and it doesn’t matter where it goes or what the downstream chamber’s ID is.

This logic happens all the time inside inside WinCan VX with WRc projects and a lot of people ignore the PLR field and the PLR Suffix field inside the section header because the PLR Suffix defaults to X for all new pipes, and the PLR itself is created automatically when the header is saved.

The big problems with CCTV inspection data begin when people use inappropriate naming conventions for the nodes in their project, and fail to follow the guidance warnings on screen that they are creating duplicate assets. Here is an example of common bad data:

So, the user creates a drawing or starts to create headers inside the software application to inspection this network of pipes. They are going to inspect:

• MH1 upstream to gully GY.

• MH1 downstream to MH2.

• MH2 upstream to gully GY.

Now let’s see what happens when we add the PLRs to this drainage network:

Look first at the PLR for the pipe from MH1 to MH2. It looks good. As explained, we take the upstream manhole ID and add the X to the end giving is MH1X.

But now look at the PLRs for the two gully legs. Notice that they are both GYX. Why? Because the upstream node ID of both pipe is GY, so the PLR of both pipes is GYX:

What does this mean in plain and simple terms? This means that there are 2 pipes with the same ID, so these are both the same pipe. But, I hear you say that, “They are not the same pipe because one goes to MH1 and the other goes to MH2.” Back to basics - the PLR is defined by the Upstream node ID and the PLR Suffix, and the downstream node ID has no affect on the PLR whatsoever.

WinCan VX will warn the user that they are creating a duplicate asset (remember, the PLR is the ID of the asset), but it will also allow them to ignore the warnings and carry on. The reason for this is there are a small number of drainage inspection standards around the world that do not define a naming convention for pipes, but not many.

How do we avoid problems like this? The simple solution is to use unique IDs for every node point on your site sketch regardless of whether it is a manhole, catchpit, gully, rainwater pipe or anything else. There is no specified way to do this, but a simple and robust technique is to use a few letters and a number which additionally describe the use of the asset (i.e. foul water, surface water or combined) and its type, like this:

This list can be extended or modified as required by the user because it is only a suggestion for a simple and robust naming convention that is easy to follow and is informative. It should be remembered that the upstream and downstream node ID fields in the WRc reporting in WinCan VX only allow for 10 characters maximum. The reasons for this are because the manhole naming convention described in Appendix A of the MSCC only require 10 characters to name any manhole in the country, and the xml data format exchange file also limits this field to 10 characters. This naming convention is described in detail in Understanding STC25 Manhole References.

In short, the really easy way to fix the commonly seen data problems like that described above is to use numbers in the gully (or whatever upstream node exists on site) references, like this:

Because the name of the pipe (the PLR) is defined by the upstream node ID and the PLR Suffix, this allows a manhole or other node to have up to 3 outgoing pipes. This may seem a strange concept to many CCTV inspectors, particularly at a domestic level because they have very likely never seen a manhole with 2 or more outgoing pipes. However, this is extremely common in highways drainage and is constructed this way by design because on highways (more than any other place) it is essential to get the water off the road at all costs and as quickly as possible during heavy rainfall events so as to avoid traffic accidents. Those inspecting highways drainage will commonly come across drainage designs like this as an example:

Now the manhole MH1 has 3 out going pipes, so this is where we start to adjust the PLR Suffix value. By default it is always X, and in cases like this, the X PLR should always be the ‘most significant’ outgoing pipe (i.e. most likely the deepest and the largest outgoing pipe).

Then, we use Y (maybe Z as well) to name the other outgoing pipes as MH1Y and MH1Z (remember. the PLR only considers the upstream node and the PLR Suffix). There are of course now 2 pipes going from MH1 to CP1 and a 3rd pipe going from MH1 to somewhere else. This is why we need to be able to accurately and uniquely define the ID of each pipe in the ground. One pipe, one ID.

There is no hard and fast prescribed way of deciding how to use which PLR Suffix for each pipe except that the X pipe should always be the primary outgoing pipe. After this, you can select to name the PLR Suffix for the other outgoing pipes by:

• Rising up through the manhole from the bottom (will be explained why this makes sense in the next paragraph) as shown in the diagram above, so the Y PLR is the next highest one from the X, and the Z PLR is the highest one.

• Going clockwise around the manhole from the X position (in the example above, the Y and Z PLRs would be swapped over).

• Random selection. This is not recommended, because all that matters is that every pipe in the project has a unique PLR, and contractors should be consistent with how they manage and deliver their data across all projects and customers.

What if there are more than 3 outgoing pipes?

The chances of this are extremely rare but not impossible. At the time of writing this, the author has only ever come across a manhole with 4 outgoing pipes once (after 30 years in the business), and yes it was on a highways project. There is known to be a surface water manhole in the North West of England which has 7 outlets. All 7 outlets go to the same next manhole through a syphon to the other side of a river, and the 7 pipes in the upstream manhole are all along one wall and at ever increasing heights from the bottom of the chamber so that the rain water flows first through pipe 1, then through pipe 2 etc as the level of rainfall increases or pipes have failed.

The MSCC simply advises the user to use whatever character they wish after Z. The issue we have here is that under MSCC rules, this can only be one character (because the xml data format has a limit of 11 characters on the PLR value - 10 for the upstream node plus the PLR Suffix).

Then additionally, we have the very old (but still regularly used) STC25 manhole reporting system which specifies incoming pipes as A, B, C, D etc, so using A as a PLR Suffix for an outgoing pipe may be misleading.

It is recommended to go back to the letter W and then start going backwards from there for outgoing pipes 4 and upwards (i.e. X, Y, Z, W, V, U etc).

Why is the highways drainage system shown above been constructed this way?

We never really know how hard it is going to rain when it rains and highways are impervious so they create an awful lot of runoff water which we need to catch and get away from the highway surface as quickly as possible to protect driver’s lives.

Under a normal light to medium rainfall event, the water will come into the system and run away down the X pipe MH1X without any drama.

But, what if it is raining extremely hard or pipe MH1X is blocked with silt (common on highways drains). Ok, no drama, the manhole MH1 will fill up a bit with water and then the additional rainwater will start to flow down pipe MH1Y.

Now what if it is either raining harder than ever before in a ‘once in a hundred years rainfall event’, or both the MH1X and MH1Y pipes are blocked or the CP1X pipe is blocked. The highways engineers still want to get the water off the road, so now the chamber MH1 will fill up until the water starts to flow down the MH1Z pipe, which as you can see goes off in a completely different direction to a a different part of the drainage network because the chances of 2 different parts of the network failing at the same time are very slim, but not impossible.

This is great idea which includes built in resiliency into the drainage network and usually works well, except that during normal operational conditions, we are only alerted to a problem when all three pipes are blocked or failed, so the remedial works will be much more significant than if there is only 1 outgoing pipe.

HADDMS Pipe Referencing

The HADDMS inspection standard follows the same PLR Suffix naming convention of Upstream Node plus PLR Suffix, except that it does not X, Y and Z for the PLR Suffix. Instead, it uses a dot and a numeric incremental counter like .1, .2, .3 etc so the MH1X pipe in the diagram above would be MH1.1, MH1Y is MH1.2 and MH1Z is MH1.3.

The HADDMS data format system does not consider the WRc xml file data and specifies its own limits for the number of characters allowed on nodes and pipe IDs, so the use of 2 characters here is of no consequence in this data system.

Other than that, the logic is exactly as described here.

Infonet Data

The Infonet data management system from Innovyze is commonly used across a number of water companies and users of this data often see a column in asset lists for CCTV and cleaning called ‘Link Suffix’ which usually has a numeric value. If you see a spreadsheet with 1,000 lines of pipes, then the must common link suffix (maybe ¾ of the lines) will be 1, them a smaller number of 2s and an even smaller number of 3s. It is even possible to see numbers going up as high as 8 or 9.

It is mistake to assume that the Link Suffix is the same thing as the PLR Suffix in WRc data where 1 = X, 2 = Y, 3 = Z etc. This is not the case.

Infonet contains a useful logic where inlets and outlets (i.e. not just outlets) at a manhole are numbered and when we do not know where they go or what the node is at the other end of the pipe, then their GIS system will create a nominal 1m long stub pipe at the clock position defined by the user, so at least there is an indication that there is a pipe here but we don’t know where it goes or where it comes from.

This works well in Infonet and helps with connectivity analysis, but there is no concept of this in the WRc MSCC data stream which is why using this value to create PLRs is not recommended.

Good Site Drawings

Everything described above regarding avoiding problems with data in software applications can be avoided by taking care and time to create good quality site drawings. The process described here considers a domestic CCTV inspection but the recommended logical approach can be extended to any type of CCTV inspection.

The golden rule of CCTV inspections is to always start with a site sketch before getting the camera system ready to go, and take time to make it useable. If you do your job correctly on site and deliver you drawing and inspection data aback to the office in a good shape, then nobody will ever have to call you up a few days later and start asking some questions while they are trying to write the report and make some rehab recommendations.

Step 1 on arrival on site - start with a drawing layout of the property site showing the outline of the buildings, significant points of interest and the property boundaries that are relevant, something like this:

The sketch already contains some useful text information and the general site layout. The significance of writing down the surface type is that once we do the inspection and find that there are some serious problems, we may need to recommend a dig-up and if this is the case then the people in the office doing the quotation will want to know what the surface condition is so that they can quote accurately.

Step 2 - safely pop up some manhole lids and have a look at surface visible node points (gullies, SVPs, RWPs etc), and also look a little more closely at the building for the signs of small windows with frosted glass and 20mm overflow pipes poking through walls for evidence of things like downstairs toilets inside the property. From this, you should quickly be able to come up with an approximation of the drainage layout for the site based on your own expertise, like this:

Notice in the drawing we have used red lines for foul drains and blue for surface water objects. There is not hard and fast recommendation for this and different water companies use different colours for this so you can be fluid, but it is advisable to at least have some coloured pens to hand for this reason, particularly where there are separate foul and surface water systems at the site.

Step 3 - add some annotations to the drawing to clearly identify each node point on the site and do not forget to make up node IDs for ‘invisible’ connector nodes. Add some information about the physical attributes of the pipes and nodes based on observation. All of this will make life a whole lot easier when it comes to actually carrying out the CCTV inspection:

Step 4 - crack on with the CCTV inspection. Life will be really easy now when it comes to entering the upstream and downstream nodes for each pipe that you camera. Just be a little careful of the connector nodes. The WRc inspection standard requires that pipe inspections are done from end to end, so in the example above, we could inspect IC2 - IC3 either upstream or downstream, and we would never inspect IC2 to CN1 or IC3 to CN2. We would simply inspect the whole pipe and add the appropriate junction or connection observation does at CN1 and CN2 as required and carry on until the end of the pipe at the next chamber or node point.

If we needed to inspect the pipe from WC1 to CN1 in a situation like this where there is no chamber at CN1, then we need to practice the ‘Indian Rope Trick' with the camera where we tie a piece of rope to the spring behind the camera head, push the camera up from IC3 to CN1 without any recording and with the rope attached and then push the camera cable while pulling the rope so that the camera flicks into the junction. Now, we pause, set the distance counter to zero on the camera and create a new clean inspection on the lateral (this is a true lateral - a pipe coming into a manhole is not a lateral) WC1 to CN1, upstream.

Step 4 - make some notes based on what you have seen and deduced during the inspection for the office:

That’s it, your job as a CCTV surveyor is complete provided that the video files and data that you have recorded on your CCTV camera match up with this drawing. Based on the information reported here, the office rehab manager can either go with your recommendations or create their own, but regardless of which way they go, they have enough information in the video files, pictures and (most importantly) the very good site sketch to make all the good and proper quotations needed for this job.

Understanding STC25 Manhole References

It is common when working with water company asset records to see STC25 manhole references like NZ24567401 (made up example) and although this seems like a mad set of numbers and letters, the way these are constructed is actually quite simple and is described in Appendix A of the MSCC.

The first 8 digits of this manhole reference represent a 100m square grid on the ground by GIS coordinates and the last 2 numbers are a numeric counter, so 01 is the first manhole inside this 100m grid, followed by 02, 03 etc and a maximum of 100 manholes 00-99 (extremely unlikely) in this 100m grid space.

Some water companies reserve 00-49 for foul water manholes and 50-99 for surface water manholes but this is not set in stone across the UK - you should check with your client on how they handle this.

To understand the STC25 reference, we must split it up into its 6 constituent parts:

• NZ 24 56 7 4 01

  • NZ = the 100km grid tile that you are currently in as described and specified by the Ordnance Survey. Remember 100km = 100,000 metres.

  • 24 = the 24th kilometre across the NZ box from the bottom left corner starting at 00.

  • 56 = the 58th kilometre up the NZ box from the bottom left corner starting at 00.

  • 7 = the 7th 100m segment across the km grid square starting at 00.

  • 4 = the 4th 100m segment up the km grid square starting at 00.

  • 01 = numeric counter as described previously.

     

From this, we can deduce that the NZ 100km grid tile is in the North East of England, and if we count 24Km East from the bottom-left corner of the tile and 56Km North from the bottom-left corner of the tile, then we hit the red cross:

The red cross is the location of the 1km grid square and then we split this 1Km square up into 100m square tiles to find the 7th square across and the 4th tile up from the bottom left corner.

Using this system we can define the geometric coordinates of every mahole in the country to within a single 100 x 100 metre square.

What this means for regular CCTV operators is;

• you can do what you want with the last 2 characters, they are unique identifiers only, and

• you cannot assume what the correct values for the first 8 characters are because you may be standing on a prescribed 100m square right now but if you take one step to the right or left then you could be in a different square and you would never know it. There are no golden lines on the ground where these squares start and end.

WinCan VX Map includes an option to click on a point on the screen against a given background location plan and the appropriate STC25 reference will be generated for you. How does it do this? Because we know the coordinates where the mouse was clicked on the screen and the first 8 digits are geo-spacial so they are not in any question or open to modification by the user.

Given that the Water and Sewerage Companies (WaSCs - pronounced Wazzucks ) usually use 2 numeric digits for the last 2 values when we are plotting uncharted assets, we cannot be absolutely sure in any 100m grid square that the last 2 digits ‘01’ have already been used by the WaSC so an easy solution when using WinCan Map VX is to let the software create the correct STC25 reference and then edit the last 2 character sonly to 2 letters rather than 2 numbers. The best suggestion for this to prevent duplicates on the client’s mapping system is to use your initials for the first uncharted asset, so the first manhole in the 100m grid might be JG (if that’s your initials), then JH, JI, JK etc.

HADDMS Version of STC25 References

HADDMS also uses the same type of node referencing as the MSCC, but they have 2 additional characters in the geometric part of the string, and they are not limited to only 10 characters.

This means that with HADDMS numbering, we are focusing down to a 10m grid rather than a 100m grid and a typical HADDMS node reference might typically look like NZ2456_7348a where:

• NZ 24 56 _ 73 48 a

  • NZ = the 100km grid tile that you are currently in as described and specified by the Ordnance Survey. Remember 100km = 100,000 metres.

  • 24 = the 24th kilometre across the NZ box from the bottom left corner starting at 00.

  • 56 = the 58th kilometre up the NZ box from the bottom left corner starting at 00.

  • _ = fixed text character.

  • 73 = the 73rd 10m segment across the km grid square starting at 00.

  • 48 = the 48th 10m segment up the km grid square starting at 00.

  • a = alpha node counter inside this 10m grid.

As a point of interest, and if the standard character lengths allowed it, we could specify a 1m grid on the ground using a manhole reference like NZ2456734854a, so this can be achieved with only 13 characters. The chances of there being more than 1 drainage asset plotted in a 1m grid square are extremely low, but possible. It is common to see double gullies side-by-side on the inner ring of roundabouts where the road surface level is angles downwards in towards the centre of the roundabout.

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