The first thing an expert does after being retained is to examine the property in question very carefully. He examines each and every building from cellar to roof - first as to its general condition of repair; then as to the number of apartments and stores, if any, and the number of rooms in each apartment. He draws a diagram of each floor, showing location of all the improvements and the staircases, examines the kind of roof, and also the cellar, as to whether concreted, flagged or what, and familiarizes himself with every detail, so that he can be asked any question regarding it. Then he ascertains whether the property in question is a key, or has a controlling influence over the adjoining property. For instance, it may be next to a corner and partake of the value of the corner, and might absolutely control the same. As the next step, he proves the value of the land by actual sales he has made in the neighborhood, or that have been made, so that he can substantiate his valuation as being the actual market value at the time title vested in the City.