A system for inventory management, comprising: a two-dimensional grid of fiducial marks within a workspace; a plurality of inventory holders , wherein each of the plurality of inventory holders can be located at a position associated with one of the fiducial marks ; a plurality of mobile drive units , each mobile drive unit operable to: detect, as the mobile drive unit moves within the workspace, multiple ones of the fiducial marks to determine the location of the mobile drive unit and/or measure the movement of the mobile drive unit based on detection of fiducial marks; move beneath the inventory holder at a first one of the fiducial marks, wherein the first one of the fiducial marks corresponds to a current location of the inventory holder ; lift the inventory holder at the current location; move, while the inventory holder is lifted, from the current location to a destination, using the fiducial marks to navigate the workspace by detecting multiple fiducial marks along a path of the mobile drive unit to the destination and updating the location of the mobile drive unit based on the detection of the fiducial marks; lower the inventory holder at the destination; and move away from the destination. a two-dimensional grid of fiducial marks within a workspace; a plurality of inventory holders , wherein each of the plurality of inventory holders can be located at a position associated with one of the fiducial marks ; a plurality of mobile drive units , each mobile drive unit operable to: detect, as the mobile drive unit moves within the workspace, multiple ones of the fiducial marks to determine the location of the mobile drive unit and/or measure the movement of the mobile drive unit based on detection of fiducial marks; move beneath the inventory holder at a first one of the fiducial marks, wherein the first one of the fiducial marks corresponds to a current location of the inventory holder ; lift the inventory holder at the current location; move, while the inventory holder is lifted, from the current location to a destination, using the fiducial marks to navigate the workspace by detecting multiple fiducial marks along a path of the mobile drive unit to the destination and updating the location of the mobile drive unit based on the detection of the fiducial marks; lower the inventory holder at the destination; and move away from the destination. detect, as the mobile drive unit moves within the workspace, multiple ones of the fiducial marks to determine the location of the mobile drive unit and/or measure the movement of the mobile drive unit based on detection of fiducial marks; move beneath the inventory holder at a first one of the fiducial marks, wherein the first one of the fiducial marks corresponds to a current location of the inventory holder ; lift the inventory holder at the current location; move, while the inventory holder is lifted, from the current location to a destination, using the fiducial marks to navigate the workspace by detecting multiple fiducial marks along a path of the mobile drive unit to the destination and updating the location of the mobile drive unit based on the detection of the fiducial marks; lower the inventory holder at the destination; and move away from the destination. <EOS>