Originally Posted by
Winemaker
Absolute value.
Speed is distance over time. Velocity is distance over time with a direction component, a vector. The ground speed of this slow flying Cessna is 10 knots; it may not be making any progress towards its intended target, but its speed over the ground is 10 knots. Its velocity vector is 10 knots in the direction of the wind. There's no 'negative' speed, there's speed in the direction opposite to the intended travel. If you want to plot this on an X -Y axis then you could certainly make the direction it ends up traveling as -X, the speed (magnitude) is still 10 knots, the direction is negative.
GS isn’t calculated as the absolute value of airspeed - headwind. If you think it is then you should edit that wiki page accordingly.