Seeing is Not Measurement

By The Metric Maven

Bulldog Edition

The old saying “seeing is believing” is true, but unfortunately, what you then believe about what you’ve seen, may not be real. We all make unconscious assumptions based on our experience. Our brains have to automatically dismiss a lot of information or we  probably would not be able to function. Our brain would be overloaded with too many ideas to process. The brain has to choose what is important, and what isn’t, very quickly. Only a small amount of information reaches our conscious mind. The problem is we just don’t realize that our brain does this. We see what little our sub-conscious want’s us to see and the rest is left behind.

For instance, here’s a little puzzle:

A father and son are driving in a car. The father approaches a railroad track upon which a train is about to cross. The father tries to beat the train, the car is hit and the father is killed instantly. The son is still alive when paramedics arrive. He is rushed to a hospital and prepped for immediate emergency surgery. The surgeon walks in to begin the operation, looks down and says “I can’t operate on him, he’s my son.”  How is this possible?

A lot of very intelligent people have puzzled over this tale. They mull it over without resolution, and when told: “The surgeon is his mother” are somewhat embarrassed, but should not be. Your mind makes assumptions like this on a regular basis. If you show a very young child a class glass of milk in a short stubby glass, and then pour the exact same amount into a tall thin one, they will often believe there is now more milk. This seems absurd to adults, but we have been exposed to weights and measures during our education.

The use of perception to separate a mark from their money in trade is an old and long established one. When every locality had its own unit of measure for the inch, foot, pound, perch and so on, people were cheated by localized ad hoc redefinition. Some merchants had weights they used for purchasing and another set for selling, with the difference going into the pocket of the trader. If done cleverly, the customer would have no idea they had been cheated. The arrival of the metric system changed everything. A single, simple system, which can be very easily verified, took one variable away from trade cheats. Nation after nation realized that a system that could be relied upon to be honest, and would promote international trade. The metric system is also more efficient in manufacturing, takes less time to teach in school,  and in everyday use is much simpler. The metric system swept the globe, with the notable and sad exception of the United States.

We need a measurement system we can rely upon because our perception is often flawed. If we all work in meters, and I’m going to measure an object, I can bring my meter stick, the seller can bring his, and we can compare. The two meter sticks better match, or there is a problem. Then even if we see something like the figure shown below, we realize that our perception is the problem and not the meter stick.

This is called the Muller-Lyer illusion. The line segments (a) and (b) are of identical length, but what you “see” is that (b) is longer than (a). Your mind continues to insist that this is so even when a ruler is placed on the line. I could create a false ruler, which shows what your mind insists is true, but this would be easily testable against a standard.

People are generally willing to estimate sizes and have considerable confidence in their values. I see this constantly in local news reports where the people involved in a story estimate distances as if they were measured to exactness with lasers. This certainty that “seeing is measurement,” can often lead to surprising errors of judgement. For instance, I asked a coffee house barrista and a young man chatting with her a prosaic question: “How long are the centerline stripes, and the gap between them on an Interstate Highway?”  After some discussion, their final answer was three feet for the stripes and five feet for the gap between. The stripes would therefore be about 1 meter in length, with a gap of 1.5 meters between them. Below is a diagram from a California Traffic Manual:

Centerline pattern for use on two-lane streets and highways (normally used on local streets and highways)

As you can see the length of each centerline strip is 3.66 meters, or obviously over 3.5 times as long as estimated. The gap is almost 11 meters or over 7 times as long as estimated. I must admit the first time I stood and looked at the centerlines on a highway, it came as a surprise how long they are, compared to how they look when driving a car.

It came as a pleasant surprise that the California DOT defines all their street and road markings in metric, as well as the speeds, which define the lengths used. Below 50 Km/h, the length of the stripes are 2.14 meter, and the gaps 5.18 meter. They are still considerably larger than estimated.

I went out one morning to a side street with a 56 Km/h speed limit in my neighborhood, and put a meterstick down next to a yellow centerline. Below is a photograph.

Meterstick with alternating orange and white 100 mm designations on a yellow centerline stripe (click to enlarge)

The meterstick is dwarfed by the centerline. Clearly the centerline length is about 3.5 meters. (Tip o’ the hat to Sven for pointing out this centerstripe illusion to me)

Ok, you might say, but what about a “real world” example? Ronnie Cohen, of the UK, wrote an interesting monograph  called Made in Britain: Not Made to Measure. The current situation in the UK is that they were half-way through metrication, when reactionary politics abolished the UK metric board in 1980. They now live with a mixture of units. In 1977 retailers were asked to voluntarily convert to metric units, here is what happened with carpet in the UK.

… That ended in a fiasco in 1977 when one major retailer of carpets discovered that it could gain a huge advantage over its rivals by going back to selling by the square yard. Subsequently, their rivals reverted to imperial units and sold carpets  by the square yard. According to Jim Humble, the last director of the UK Metrication Board, “The product which brought all voluntary retail initiatives to a full stop was the experience of the floor covering and carpet retailers. Their 1975 change to sales by the square metre started well, but in 1977 one of the major High Street retailers found enormous commercial advantage in reverting to sales by the square yard. Consumers could not be persuaded to believe that goods costing, for example, £10 per square yard or £12 per square metre were virtually priced the same. Consumers bought, in very significant volume, the apparently cheaper priced imperial version. Metrication of carpet sales entered into full scale reverse and the Chambers of Trade and retail associations pressed for firm Government leadership, i.e. compulsory cut-off.”

…. This example shows why the use of measurement units must be regulated and traders must not be allowed to choose any measurement units they like. This is essential for consumer protection. Freedom of choice in measurements would lead to chaos in the marketplace, make it hard, if not impossible, for consumers to compare prices and quantities and would be a charter for rogue traders to rip off their customers at every opportunity. If traders had such freedom, rogue traders would give you the smallest quantity that they could get away with.

Our measurement intuition fails us. That’s why humans began to create measurement methods in the first place. A proliferation of measurement units nullifies much of their advantage to humankind. Because we in the United States have not embraced the metric system, a pound of feathers still weights more than a pound of gold. Of course it’s obvious that an ounce of feathers weight less than an ounce of gold–right? (avoirdupois versus troy). We measure carbon 60 with one measurement system (grams), and carbon in a diamond lattice with another (carats). I believe it’s long past the time to have a mandatory metric changeover in the United States—unless of course—you’d like to buy this bridge in Brooklyn I have for sale.

Updated 2012-09-11

I’m a metric rebel who believes that prefixes which magnify should be capitalized, so I use Km/h deliberately. I often get contacted about this by those who have read available style manuals. It is quite welcome, as from it I know they read the blog—but I stubbornly will not conform. Thanks.

Updated 2012-09-18 California’s Metric to English Transition:

Longtime reader Todd wrote to the California Department of Transportation to inquire about their apparent return to Imperial units. Below is the body of his email and the DOT’s response.

I recently read an article that mentioned roads and road design. I was
surprised and happy to learn that those specifications in the CA DOT were
So, I went to your site and found there was a more recent publication in
“English.” Why is that? Was there a lot resistance to the metric
measurements? Lawsuits? Seems like a sad and counterproductive move to go
back to the imperial measure when we as a country are the last to go metric
and are in bad need of it.

Just curious.

Be well.


Their response:

Mr Todd,

Thank you for your question on why the California Department of
Transportation (Caltrans) adopted the use of English units in October 2006
as its preferred system of units and measures after having used metric
units of measure for ten plus years. Caltrans predominately switched back
to the use of the English system of units and measures to align our
practices, procedures, products, and services with those of our primary
customers and partners; the contracting industry, other State and local
agencies, and the citizens we serve.  I would not say that there was a lot
of resistance to the use of metric units.  However, the contracting
industry reported to us that using metric units added costs to Caltrans
projects because they would need to translate the metric units on the plans
into US Customary (English) units as part of their processes.

Kevin Herritt, Project Manager
Metric to English Transition
Division of Design

Yes, they apparently have a Project Manager for Metric to English Transition.

The Metric Maven

One thought on “Seeing is Not Measurement

  1. I am surprised that the California DOT spec’d everything in metric, but when I went to the web site, the have recently published an “English” one as well. ( Perhaps they gave into the crying complaining metric hating local contractors. My answer would be “learn metric or lose the contracts.” But, California is the state of litigation, so it was probably easier and cheaper to publish the other book of specs. (sigh) You’re right about how the lines look shorter than they really are. It must be the perspective we have from our vehicle makes them look that way.

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