Depth locators, carefully attached to a fly, help accurately set indicator depth.
Set your indicator for an approximate depth. Attach the depth locator to your fly. The depth locator pulls the indicator beneath the surface. Guage the distance between your indicator and the surface above to accurately set your fly depth once the depth locator has been removed.
Depth locators can also be used to help set fly depth when suspending flies vertically above the bottom using fast sinking lines in deep water, a method often referred to as dangling.