Unit System documentation

Question

Is there any documentation on what consists of the unit system "SIBase","Imperial", "SI" and "Metric" in the UnitConvert function, and the rule of what unit is returned in the results? Cause I have notices in doing UnitConvert[Quantity[x, LengthUnit], "Metric"] sometimes returns quantity with units in m, and other times in mm.

Answer 1

i dont know about documentation, but "Metric" and "SI" appear to be the same and give a unit based on the magnitude, while "SIBase" consistently gives meters.

  UnitConvert[Quantity[#, "feet"], "Metric"] & /@ {.00001, 0.001, 
       0.01, .1, 1., 10., 10000.}

{Quantity[3.048, "Micrometers"], Quantity[304.8, "Micrometers"], Quantity[3.048, "Millimeters"], Quantity[3.048, "Centimeters"], Quantity[30.48, "Centimeters"], Quantity[3.048, "Meters"], Quantity[3.048, "Kilometers"]}

  UnitConvert[Quantity[#, "feet"], "SI"] & /@ {.00001, 0.001, 0.01, .1, 
      1., 10., 10000.}

{Quantity[3.048, "Micrometers"], Quantity[304.8, "Micrometers"], Quantity[3.048, "Millimeters"], Quantity[3.048, "Centimeters"], Quantity[30.48, "Centimeters"], Quantity[3.048, "Meters"], Quantity[3.048, "Kilometers"]}

  UnitConvert[Quantity[#, "feet"], "SIBase"] & /@ {.00001, 0.001, 
       0.01, .1, 1., 10., 10000.}

{Quantity[3.048*10^-6, "Meters"], Quantity[0.0003048, "Meters"], Quantity[0.003048, "Meters"], Quantity[0.03048, "Meters"], Quantity[0.3048, "Meters"], Quantity[3.048, "Meters"], Quantity[3048., "Meters"]}

The base unit itself plays into the heuristic as well, example:

UnitConvert[Quantity[.158125 10.^-4, "light year"], "Metric"] -> 1. au
UnitConvert[Quantity[93. 10^6, "Miles"], "Metric"] -> 1.49669 10^8 km