# If gauge charts are bad, why do cars have gauges?

It seems like data visualization experts generally disapprove of gauge charts (see here: What do you call a chart that looks like a half pie chart with a needle indicating a percentage?). The primary reason is that a gauge chart has a low data-to-ink ratio.

Ever since I was exposed to these concepts (a few Tufte books), I generally agreed with them, but today it made me wonder: if gauges are so inefficient at communicating information, then why do cars/boats/planes have lots of gauges on their dashboards? And does the answer to that question have some kind of relevance for creating software dashboards for large enterprises?

Edited to include some additional information I found:

I found a term, "glass cockpit", that refers to an airplance cockpit that has its mechanical gauges replaced with LCD screens. This gives credibility to the "convention" argument put forth by Wayne.

http://en.wikipedia.org/wiki/Glass_cockpit

Here's an iPad app that gives a dashboard-like readout of your car's telemetry, with no gauges to be seen.

http://itunes.apple.com/us/app/dashcommand-obd-ii-gauge-dashboards/id321293183?mt=8

I also found a gross example of digital gauges for cars (viewer discretion is advised).

http://www.chetcodigital.com/index-Automotive.htm

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If pie charts are bad, why do we have pies? ... Oh. ;-) –  cardinal Jun 14 '12 at 20:42
Tufte's data-to-ink ratio does not appear to be relevant in this problem, but Cleveland's studies of the speed and accuracy with which people interpret statistical graphics have a strong bearing on it. These studies suggested that people do not compare angles to each other as quickly or accurately as they compare lengths or parallel positions relative to a common baseline. One big problem with angles is that the comparison may depend on how the angles are oriented; that problem doesn't appear to be an issue with a dynamic gauge. So perhaps circular gauges are close to optimal at what they do. –  whuber Jun 14 '12 at 20:50

A (real) dashboard gauge needs to be: 1) physical, and 2) read quickly under circumstances that disturb concentration. In that sense, you want a low data-to-area ratio. Not to mention that when physical gauges were invented, digital (numeric) displays didn't exist so there was no real choice.

A software dashboard is not physical, and is not generally looked at in a pitching, moving vehicle with other vehicles whirring around it. So the effect of imitating a physical device doesn't buy you much.

EDIT: I'd also add that a physical dashboard only has a couple of key attributes to get across to you at (literally) a glance. A corporate dashboard needs to make a lot more detail visible, though of course things should be drawn/coded/organized in a way to also give a quick status.

That's part of the Tufte philosophy of dense detail in presentations that allow a broad view but also allow you to drill down. You car's dashboard doesn't let you drill down, basically because there's no need to.

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"when physical gauges were invented...there was no real choice" But I own a car built in 1999 that has more gauges than it does digital displays. Are you saying we still use gauges just out of convention? I do agree with your point on reading quickly, but would that not sometimes be a desirable quality in a software dashboard? (I'm playing devil's advocate here.) –  mehaase Jun 14 '12 at 20:29
(+1) Re: your last paragraph: Except of course for software systems meant to be used in-vehicle during operation, as found on many police vehicles and other industrial vehicles like in construction and the military. Interestingly enough, those hardware/software solutions often do look like dashboards. Touch screens, big buttons and judicious use of (high contrast) color play a very important role in usability and safety. These interfaces tend to look like anything but your typical GUI. –  cardinal Jun 14 '12 at 20:39
@mehaase: "Reading quickly" is quite different when you literally have a couple of seconds to glance down from the road versus when you're sitting at a desk in front of a monitor. I guess some financial trader will claim that they only have a couple of seconds to make a buy/sell decision or something, but realistically most corporate "dashboards" aren't used in such conditions. –  Wayne Jun 14 '12 at 21:44
My previous comment relates to the paragraph starting "A software dashboard is not physical...". I should have been mindful of future edits and not made references to paragraph placement. :) –  cardinal Jun 15 '12 at 13:51

In supplement to Wayne's fine answer, Robert Kosara has a recent post on his Eager Eye's blog about the very topic, Data Display vs. Data Visualization. In addition to as Wayne mentioned the goals of real-time visualization vs. more static displays might call for differences, he also mentions that gauges aren't very good for displaying multiple values. This is summed up nicely in his comment,

What you want to know is, how fast am I going right now? How much gas do I have left? What your speed was five minutes ago, or how much gas you had in your tank three hours ago, matters little.

So here is any obvious contrast between the goals of data visualization versus car-gauges, we pretty much always want to see multiple data values! And circular car-gauges are certainly a poor tool to do that. Sometimes we don't want to see multiple values though (a few circumstances are given in this question on the GIS site, What is the point of standard symbology?). And so we might expect other rules to which we apply the data visualization techniques in such circumstances. The GIS post I mention uses very flashy symbols/icons for point patterns that attempt to incapsulate the nature of the event (and sometimes visualization techniques like blinking dots to focus attention).

What I find interesting is that the work of Cleveland on comparing angles is still pertinent to car gauges though, and hence we still might expect a linear scale for a car gauge to work better than the circular display. So I suspect there might be more historical context as to why circular gauges were chosen (they are compact?), and it certainly may be this historical inertia as to why they are popular.

This much be a popular topic in the thralls lately, as the Visual.ly blog just came out with a post on the topic as well, Speedometer Design: Why It Works. In there they give credence to some of the things gung mentions in his post that I am somewhat critical about in the comments, in particular how we develop a gestalt for identifying locations around the circular display.

I think I'm partially coming around to this notion. A circular display provides more visual distinction between general areas than does a linear one. For a general example, it is easier to quickly tell the difference between a needle pointing to 3 o'clock and a needle pointing to 12 o'oclock than it is to tell the difference between 15 and 12 on a linear scale.

I'm still not totally convinced though, and I say rubbish to the notion that acceleration is easier to distinguish on a circular scale (or even if it is information we need the dashboard to inform us about anyway) that the visual.ly blog post mentions. Just my opinion though, I'm not sure any of us have been citing directly pertinent experimental results on human perception. Cleveland's is a start, but not likely to give an entirely satisfactory answer to these particular circumstances.

That being said the multiple data values are still the main crux of the argument, circular displays aren't good for multiple data values.

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+1 This seems to be on the right track. I think Cleveland's principles can be pushed further. Recall that he asked subjects to compare quantities being displayed on charts. Comparison with angular displays was neither as quick nor accurate as comparison of position along a marked axis. In effect, a dial gauge gives a very clear position along a curved axis. As such it shares some of the good properties of graphics that use position to represent quantities. It also suffers a bit, too: we may have a harder time distinguishing speeds near the top of a gauge compared to speeds at the sides. –  whuber Jul 16 '12 at 15:21
Historically, the reason for a dial gauge is clear: Maxwell's equations suggest the most basic way to translate an electric current into a physical motion is by placing a small coil of wire inside a static magnetic field (e.g., wrap the wire around a magnet). Not only simple, but cheap, reliable, stable, nearly linear over a long range, and easily calibrated. –  whuber Jul 16 '12 at 15:26