3 min read

measurement units

2022-03-15

Data mtcars has a column named mpg. mpg means miles per gallon. ‘Mile’ and ‘gallon’ are units for length and volume. A mile is approximately 1.6 kilometers and a gallon is approximately 3.7 liters. Mile and gallon sound unfamiliar to people who live outside England or U.S.A. because international standard units for length and volume are meter and liter.

In this post, we will learn how to convert a unit to another unit, for instance, we will convert mpg to km/L, which is more comprehensible to people who use SI units.1

Units in R

Vectors(the most common data structure in R) do not contain information of measurement units. Units are implicit and units should be converted by users. But as history tells us, unit conversion should be treated carefully because it can cause serious damage to the whole project2.

Package units

Using the package units, we can easily convert units accurately. And the data is plotted, units will be included in the x- or y-label automatically.

First install package units.

install.packages('units')

And load all the necessary packages and data

library(units)
## udunits database from C:/Users/Seul/Documents/R/win-library/4.1/units/share/udunits/udunits2.xml
library(dplyr, warn.conflicts = FALSE)
data(mtcars)

To get the information about the data mtcars, we can do help(mtcars). It will show the measurement unit for each column. mpg is measured in unit of miles per gallon, disp is measured in unit of cubic inch, hp is measured in unit of gross horsepower, wt is measured in unit of 1000 lbs, and qsec is measured in unit of sec per 1/4 mile.

It is sad that mpg(miles per gallon) is not registered in the package units, but we can register it ourselves. The code below installs a new unit called mpg_US as interantional_mile/US_liquid_gallon.

install_unit(name='mpg_US', def='international_mile/US_liquid_gallon')

Now we can use mpg_US. mtcars$mpg is measured in unit of mpg(US) and mtcars$wt is measured in unit of kilogram.

units(mtcars$mpg) = 'mpg_US'
units(mtcars$wt) = 'kg'
mtcars$mpg %>% head
## Units: [mpg_US]
## [1] 21.0 21.0 22.8 21.4 18.7 18.1

If we want to convert the unit mpg(US) to SI unit km/L, we need to do the below.3

units(mtcars$mpg) = 'km/L'
mtcars$mpg %>% head
## Units: [km/L]
## [1] 8.928017 8.928017 9.693276 9.098075 7.950187 7.695101

We can easily plot the relation between mpg and wt using the package ggplot2. But do not forget to load ggforce beforehand.

library(ggplot2)
library(ggforce) # without this, the code below will raise error!
## Registered S3 method overwritten by 'ggforce':
##   method           from 
##   scale_type.units units
ggplot(data=mtcars,
       aes(x=mpg, y=wt)) + 
  geom_point()

Summary

  • Use units::units()<- for setting unit for measurements.
    • Use units::units()<- for converting unit.
    • Use units::units()<-NULL for deleting unit.
  • Use install_unit(name=, def=) for introducing new units.
  • Use valid_udunits() to show all the units available from the package units.
valid_udunits() %>% head 
## udunits database from C:/Users/Seul/Documents/R/win-library/4.1/units/share/udunits/udunits2.xml
## # A tibble: 6 x 11
##   symbol symbol_aliases name_singular name_singular_aliases name_plural
##   <chr>  <chr>          <chr>         <chr>                 <chr>      
## 1 m      ""             meter         "metre"               ""         
## 2 kg     ""             kilogram      ""                    ""         
## 3 s      ""             second        ""                    ""         
## 4 A      ""             ampere        ""                    ""         
## 5 K      ""             kelvin        ""                    ""         
## 6 mol    ""             mole          ""                    ""         
## # ... with 6 more variables: name_plural_aliases <chr>, def <chr>,
## #   definition <chr>, comment <chr>, dimensionless <lgl>, source_xml <chr>

  1. International System of Units↩︎

  2. It is well known that the failure of MCO(Mars Climate Orbiter) is due to inadequate unit coversion.↩︎

  3. If the objective is simply to reset the unit, do units(mtcars$mpg)=NULL; units(mtcars$mpg)='km/L'. This will not convert unit but just replace the unit with another unit.↩︎