How many tsp of ground ginger is a 1/2-inch piece of fresh ginger?
Well, ground ginger is never a good substitute for fresh ginger, especially outside of baking. But if you're in a pinch, use about 1 tsp ground ginger to a 1/2-inch piece of fresh. If this is a savory dish, cook it for a few seconds in your oil first before you mix it into everything else. This will help release the flavors and keep that powdery sort of taste out of your dish.
Fresh ginger is definitely preferred, but most packages of ground ginger have some sort of comparison on the label, as with vanilla extract.