The Square Kilometer is a unit of surface measure, which corresponds to the area of a square with a kilometer of side. This measure is equivalent to one million square meters, or even to one hundred hectares. In many of the medieval cities of Europe, the area that was protected by the walls that surrounded the city, was about one square kilometer. With an area of 59.1 km², Manhattan has a population of 1,640,000. This gives an average of 27,750 people/km², which makes it one of the most densely populated areas on the planet!
One Hectare equals to one square hectometer or one hundred ares. This in square meters results in 10,000 m². To get the idea of what represents one hectare, just imagine a square terrain, with one hundred meters on each side. If you are a football fan, you can use the measure of the area of the playing field as equivalent to about one hectare. This is essentially an agrarian measure, widely used in calculating the size of agricultural areas, forests and natural areas. For example, at the time of fires, the burning area is always expressed in hectares.