In the middle of this image,just above the tufts of grass,can be seen a short length of Hadrian's Wall.Adjoining this was an extra turret,placed equidistant between turrets 39a and 39b,called turret 39ab. But it was placed at the bottom of a dip in the land, and not at any advantageous position.So rigid was the 1/3 mile placement of the turrets.