Why bother learning how to do a measured survey?

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Why not commission someone to a carry out a 3D laser scan, so you don’t have to do anything?

In an office like ours, projects involving existing buildings come up all the time. Inevitably this means that we need reliable drawings of those buildings and therefore need to carry out measured surveys. 

But there are plenty of specialist companies that could do a measured survey of a building for us. And there are plenty of technologies that could automate things or make things easier for us.

So why do we prefer to do measured surveys ourselves and why do we take the trouble to train architects how to do it?

I could give some simple, practical reasons, such as keeping costs down or not being reliant on other people’s schedules etc, but the real reason is to do with how we think about things. We want people to start to develop an understanding of the relationship between themselves, the physical world and the drawings that we produce to describe it.

If that’s starting to sound way too metaphysical, let me illustrate things with a simple case study. One where I make a mistake, show my inexperience but thankfully get away with it.


Case Study

During my first year of professional experience, I was asked to design a new information centre in the iconic Preston Bus Station. Although this was primarily an alteration and shop-fitting exercise, it had most of the characteristics of a full building project.

I carried out a measured survey of the relevant part of the building and attempted to draw the results. I found that the geometry that I’d measured didn’t quite work out on the drawing. A problem that was compounded by the fact that I’d forgotten to take a couple of dimensions.

So I went back to the building to check what I’d done and to take the missing dimensions. I updated the drawings and made the geometry work as best as possible. Subsequently, I designed the project to fit the information I had and the works were tendered and a contract awarded.

One crucial aspect of the design was that the information centre was to be pre-fabricated and had to be installed to fit between two existing walls. I duly included the dimension between the two existing walls on my drawing.

But after the contract was let, I started worrying about this dimension. What if my measurement was a bit inaccurate and the builder had relied on it? What if my measurement was a lot inaccurate? Too small and the information centre might not fit. Too big and there could be significant gaps all round.

I couldn’t get it off my mind, so I went back to the building to recheck my measurement again. In fact, I went back on three separate occasions with different measuring devices to make sure I hadn’t made a mistake.

In the end, the project turned out fine, and my worries proved unfounded. But that was mostly down to the actions of the builder who later explained two crucial things:

  1. He didn’t expect to be told the exact dimension between the walls. He expected the drawings to record the approximate dimension with an instruction for him to take his own site measurement. So he took his own measurement anyway.

  2. Because the information centre was to be pre-fabricated, transported and manually moved into position, he wanted to ensure that there was some flexibility in its length (regardless of how my construction drawings told him to make it). This was partly to make it easier for him to get a large, heavy object into place and partly to allow for his own survey inaccuracy. He achieved the flexibility in length by considering the shadow gaps at each end to be make-up pieces. He thought of them as being of variable width and a separate installation. I had thought of them as being of fixed width and integral to the construction.


So what did I learn from this experience?

Quite a lot actually. It made me conscious of the following things:

  • All surveys include inaccuracies.
  • They may also include mistakes.
  • I was wrong to think of my drawings as a fixed and accurate picture.
  • Some measurements become much more critical than others.
  • Survey drawings are a record of an imperfect process.
  • I didn’t fully understood how my design was going to be built.

Let me explain these points in a bit more detail:

1. Accuracy

When you carry out a survey yourself, you become aware of a host of problematic issues that come up when measuring things. For instance:

  • Metal tapes have a sliding end stop which may not be sliding correctly.

  • Measuring into corners is tricky with a metal tape.

  • Fabric or plastic tapes sag and need to be pulled tight.

  • Knowing for sure that a laser measurer has measured to the right place.

  • Obstructions which mean that you’re forced to 'eyeball' the measurement.

If you’ve ever done a survey, you’ll recognise these issues. There are plenty more. But when you were doing the survey, you probably glossed over them because there were no alternatives and you needed to get on with things.

Then, when you were drawing things up on the computer, you encountered problems in fitting the measured geometry together. It wouldn’t quite work out. So, when nobody was looking, you fudged it!

All of this is normal.

2. Mistakes 

Mistakes are different to things not being accurate and can cause more significant problems. 

They come about for all sorts of reasons, such as writing a number down incorrectly, misreading a measuring device, or forgetting to take a critical dimension.

Sometimes mistakes become apparent because they cause an anomaly in relation to other parts of the survey. Sometimes, though, they remain hidden and can be the cause of much bigger problems further down the line.

3. How you think

This is the real nub of this post.

Becoming conscious that the survey is not accurate and may contain mistakes, means that how you think about it should change. If you now regard the survey as only an approximate representation of the existing building (and possibly wrong), then you are more likely to treat it with caution because it has become less reliable in your mind.

This is a good thing. It keeps you alert.

Of course, we want the survey and the drawings that we subsequently produce to be sufficiently reliable for our purposes. So knowing why we’re doing the survey becomes an important consideration too.

The question then arises, how reliable is the survey? Which leads me to the next point.

4. Critical Measurements

Referring back to my earlier case study, when I carried out the survey, I hadn’t made any design decisions. As the design developed, certain dimensions became more critical than others. In this case, the dimension between the two walls.

This means that depending on the relationship between the existing building and the proposal, parts of the survey may become more or less sensitive as the design process unfolds.

Remaining conscious of this, and having the flexibility of mind to act as necessary as things develop, is all important. 

5. Drawings

As you become more conscious of the issues involved, it’s likely that your approach to preparing the drawings will also change.

For instance, it is often worthwhile having a set of ‘survey drawings’ that are separate from the design drawings. That way you can record things that are important at the time of the survey, such as who did it, what the weather was like, restrictions to access or obstructions that impeded your work.

You can record on the drawings why the survey is being carried out and for whom, thereby implicitly (or explicitly) putting a limitation on its use and deterring third parties from using it for other purposes.

When using information from the survey in design drawings, you can record in the notes of those drawings, where the survey information came from.

And when you’re receiving survey information from other people for use in your projects, you will treat their information with the respectful scepticism that the circumstance demands.

6. Buildability

So far I have concentrated on the inaccuracy of the survey process. The other thing that carrying out a dimensional survey reveals is that the buildings themselves aren’t accurate either.

Rooms that appear to have right-angled corners are not square, surfaces that are meant to be horizontal and flat are neither, verticals that are supposed to be plumb are inclined and things that are intended to line up, don’t.

All of which will be of great interest when it comes time to get something built yourself.

So, returning to the original question, why bother learning to do a measured survey?

Because it gives you some experience of all the things I’ve mentioned so far. But more importantly, because it helps you build a healthy mental model of your drawings and the building in question. The process makes you conscious of the fact that the information is approximate and it encourages you to continually assess whether it is accurate enough.

It helps you develop the right frame of mind so that you are confident in your abilities, yet cautious about what you’re doing because you know that the process is not perfect.


If you’re just starting out, or wanting to improve your survey skills, watch this space as I’ll be returning to the subject in more detail in future posts.

 
Andy FosterComment