Logarithmic scale

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Various scales: lin-lin, lin-log, log-lin and log-log. Plotted graphs are: y=x (green), y=10x(red), y=log(x) (blue).

A logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself. Presentation of data on a logarithmic scale can be helpful when the data covers a large range of values – the logarithm reduces this to a more manageable range. Some of our senses operate in a logarithmic fashion (Weber–Fechner law), which makes logarithmic scales for these input quantities especially appropriate. In particular our sense of hearing perceives equal ratios of frequencies as equal differences in pitch. Logarithmic scales are either defined for ratios of the underlying quantity, or one has to agree to measure the quantity in fixed units. Deviating from these units means that the logarithmic measure will change by an additive constant. The base of the logarithm also has to be specified, unless the scale's value is considered to be a dimensional quantity expressed in generic (indefinite-base) logarithmic units. On most logarithmic scales, small values (or ratios) of the underlying quantity correspond to small (possibly negative) values of the logarithmic measure. Well-known examples of such scales are:

• Richter magnitude scale and Moment magnitude scale (MMS) for strength of earthquakes and movement in the earth.
• ban and deciban, for information or weight of evidence;
• bel and decibel and neper for acoustic power (loudness) and electric power;
• cent, minor second, major second, and octave for the relative pitch of notes in music;
• logit for odds in statistics;
• Palermo Technical Impact Hazard Scale;
• Logarithmic timeline;
• counting f-stops for ratios of photographic exposure;
• rating low probabilities by the number of 'nines' in the decimal expansion of the probability of their not happening: for example, a system which will fail with a probability of 10^-5 is 99.999% reliable: "five nines".
• Entropy in thermodynamics.
• Information in information theory.
• Particle Size Distribution curves of soil

Some logarithmic scales were designed such that large values (or ratios) of the underlying quantity correspond to small values of the logarithmic measure. Examples of such scales are:

• pH for acidity;
• stellar magnitude scale for brightness of stars;
• Krumbein scale for particle size in geology.
• Kardashev scale for technological advance in physics.
• Absorbance of light by transparent samples.

Graphic representation

A log scale makes it easy to compare values which cover a large range, such as in this map

A logarithmic scale is also a graphical scale on one or both sides of a graph where a number x is printed at a distance c·log(x) from the point marked with the number 1. A slide rule has logarithmic scales, and nomograms often employ logarithmic scales. On a logarithmic scale an equal difference in order of magnitude is represented by an equal distance. The geometric mean of two numbers is midway between the numbers. Logarithmic graph paper, before the advent of computer graphics, was a basic scientific tool. Plots on paper with one log scale can show up exponential laws, and on log-log paper power laws, as straight lines (see semilog graph, log-log graph).

Logarithmic and semi-logarithmic plots and equations of lines

When values that span large ranges need to be plotted, a logarithmic scale can provide a means of viewing the data that allows the values to be determined from the graph. The logarithmic scale is marked off in distances proportional to the logarithms of the values being represented. For example, in the figure below, for both plots, y has the values of: 1, 2, 3, 4, 5, 6, 7, 8, 9 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100. For the plot on the left, the log₁₀ of the values of y are plotted on a linear scale. Thus the first value is log₁₀(1) = 0; the second value is log₁₀(2) = 0.301; the 3rd value is log₁₀(3) = 0.4771; the 4th value is log₁₀(4) = 0.602, and so on. The plot on the right uses logarithmic (or log, as it is also referred to) scaling on the vertical axis. Note that values where the exponent term is close to an integral fraction of 10 (0.1, 0.2, 0.3, etc) are shown as 10 raised to the power that yields the original value of y. These are shown for y = 2, 4, 8, 10, 20, 40, 80 and 100.

Plots of the log (base 10) of values of y (see text) on a linear scale (left plot) and of values of y on a log scale (right plot).

Note that for y = 2 and 20, y = 10^0.301 and 10^1.301; for y = 4 and 40, y = 10^0.602 and 10^1.602. This is due to the law that

<math> \log(AB) = \log(A) + \log(B).\, </math>

So, knowing log₁₀(2) = 0.301, the rest can be derived:

log₁₀(4) = log₁₀(2 × 2) = log₁₀(2) + log₁₀(2) = 0.602

log₁₀(20) = log₁₀(2 × 10) = log₁₀(2) + log₁₀(10) = 1.301

Note that the values of y are easily picked off the above figure. By comparison, values of y less than 10 are difficult to determine from the figure below, where they are plotted on a linear scale, thus confirming the earlier assertion that values spanning large ranges are more easily read from a logarithmically scaled graph.

Plot of the values of y (see text) on a linear scale.

Log-log plots

If both the vertical and horizontal axis of a plot is scaled logarithmically, the plot is referred to as a log-log plot. The equation for a line on a log-log scale would be:

log(F(x)) = m log(x) + b,

F(x) = (x^m)(10^b),

where m is the slope and b is the intercept point on the log plot. The example plot shown below is for the equation log(F(x)) = m log(x) + b, for m = −10, b = 20.
Plot on log-log scale of equation F(x) = (x⁻¹⁰ )(10²⁰).

Semi logarithmic plots

If only the ordinate or abscissa is scaled logarithmically, the plot is referred to as a semi logarithmic plot. The equation for a line with an ordinate axis logarithmically scaled would be:

log(F(x)) = mx + b

F(x) = 10^{(mx + b)} = (10^mx)(10^b)

The equation of a line on a plot where the ordinate axis is scaled logarithmically would be

F(x) = m log₁₀(x) + b.

Estimating values in a diagram with logarithmic scale

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One method for accurate determination of values on a logarithmic axis is as follows:

1. Measure the distance from the point on the scale to the closest decade line with lower value with a ruler.
2. Divide this distance by the length of a decade (the length between two decade lines).
3. The value of your chosen point is now the value of the nearest decade line with lower value times 10^a where a is the value found in step 2.

Example: What is the value that lies halfway between the 10 and 100 decades on a logarithmic axis? Since it is the halfway point that is of interest, the quotient of steps 1 and 2 is 0.5. The nearest decade line with lower value is 10, so the halfway point's value is (10^0.5) × 10. To estimate where a value lies within a decade on a logarithmic axis, use the following method:

1. Measure the distance between consecutive decades with a ruler. You can use any units provided that you are consistent.
2. Take the log (value of interest/nearest lower value decade) multiplied by the number determined in step one.
3. Using the same units as in step 1, count as many units as resulted from step 2, starting at the lower decade.

Example: To determine where 17 is located on a logarithmic axis, first use a ruler to measure the distance between 10 and 100. If the measurement is 30mm on a ruler (it can vary — ensure that the same scale is used throughout the rest of the process).

[log (17/10)] × 30 = 6.9

x = 17 is then 6.9mm after x = 10 (along the x-axis).

See also