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.
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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:
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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:
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The start of data problems.
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:
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Now let’s see what happens when we add the PLRs to this drainage network:
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Look carefully at the PLRs.
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:
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Duplicate PLRs.
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.
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In short, the really easy way to fix the commonly seen data problems like that describe above is to use numbers in the gully (or whatever upstream node exists on site) references, like this:
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Good data with no duplicate PLRs.
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The most common time that we see data problems like this is when people inspect upstream from an inspection chamber on site (let’s call it MH1) and they push the camera up 2 or 3 branch lines coming into chamber MH1. They label the upstream node as LatA, LatB and LatC, which then means that the PLRs are LatAX, LatBX and LatCX. So far, so good. But now they move on down the main line to MH2 and do the same thing again starting at A, so they inspect from MH2 upstream to LatA and LatB, making the PLRs LatAX and LatBX. Now we have 2 LatAX and 2 LatBX in the project and already the data is starting to fall apart. And of course they will swear blind that the pipes are different pipe because they are connected to MH1 and MH2, but this is just not the case in the data. The data sees them as the same pipe because they have the same PLR. Some pointers to help you demystify the nomenclature:
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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.
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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.
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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.
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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.
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To understand the STC25 reference, we must split it up into its 5 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 ip the km grid square starting at 00.
01 = numeric counter as described previously.
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OS grid tiles.
From this, we can deduce that the NZ 100km grid tile is in the North West 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:
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