Conversions in Measurement too accurate for you? Here take this…

ApproximateConversion.swift

import Foundation

extension Measurement where UnitType: UnitLength {
  func approximatelyConverted(to otherUnit: UnitType) -> Measurement<UnitType> {
    switch (unit, otherUnit) {
    case (UnitLength.meters, UnitLength.feet):
      return Measurement(value: value * 3, unit: otherUnit)
    case (UnitLength.feet, UnitLength.meters):
      return Measurement(value: value * 3 / 10, unit: otherUnit)
    default:
      return converted(to: otherUnit)
    }
  }
}

extension Measurement where UnitType: UnitSpeed {
  func approximatelyConverted(to otherUnit: UnitType) -> Measurement<UnitType> {
    switch (unit, otherUnit) {
    case (UnitSpeed.knots, UnitSpeed.metersPerSecond):
      return Measurement(value: value * 2.0, unit: otherUnit)
    case (UnitSpeed.metersPerSecond, UnitSpeed.knots):
      return Measurement(value: value / 2.0, unit: otherUnit)
    default:
      return converted(to: otherUnit)
    }
  }
}