If the width of the instep (AB) at its widest part is less than 1cm, the foot is considered as flat (right). If the width of the footprint at its narrowest part (BC) is less than 1cm, the arch is considered high (center). All other footprints are considered "normal" (left).
Adapted from "The Influence of Footwear on the Prevalence of Flat Foot," The Journal of Bone and Joint Surgery, 74B(4), 1992, pp. 525-527.
Flat feet of themselves are not necessarily a problem. But people with a flat foot (or the opposite condition, a highly arched foot) that are painful, are certainly in need of treatment. Flat feet and high arches that are associated with ankle, knee or low back pain are also in need of treatment.
The causes of flat feet are many, but in the older age group decreased exercise and increased weight add to the mechanical disturbances of the foot. Flat feet can also produce conditions known as heel spurs and/or plantar fascitis.
The best and first line of defense for a flat foot condition is a good supportive shoe (Nike, New Balance, K-Swiss, and Avia) but usually some type of arch support is beneficial.
The over-the-counter pads and insoles with arch support such as Spenco and Dr. Scholls are beneficial in treating this condition. Foot massagers can also aid in relief.
But the most effective means of controlling flat foot or a high arched foot are custom molded orthotics. Shoe inserts, such as the Arch-Mate Foot Support inserts or Tuli's Gaitors 3/4 Length arch supports may provide relief. But if pain continues, by far the best option is a custom fitted orthotic which are made to fit your feet.