Method of acquiring horizontal distance between camera and target, and camera and surveillance system adopting the method

A surveillance system comprising:

• a camera connected to a host device, the host device acquiring a horizontal distance between the camera and a target which is a horizontal distance between a central point of an optical system of the camera and a ground landing point of the target,
• wherein the host device is configured to acquire a relative target angle which is an angle between ground and a camera-target connection line which connects the central point of the optical system of the camera and the ground landing point of the target,
• wherein the host device is further configured to acquire a height of the camera which is a height of the central point of the optical system of the camera on a basis of a vertical position of the ground landing point of the target, and
• wherein the host device is further configured to set a quotient of the height of the camera divided by a tangent value of the relative target angle as the horizontal distance between the camera and the target.