Common improvements require special consideration. See Sections 10–55, 10–56, and 10–57 of the Manual of Surveying Instructions. A common improvement is one that tends to benefit several claims of a common group. It must have been constructed in its entirety subsequent to the location of each claim of the group, or at least $500 must have been spent on the improvement subsequent to the location of each claim of the common group. All claims within range of benefit must be included, even though they were previously surveyed, or if they are locations which are not included in the survey. The field notes must recite all the claims involved, whether previously surveyed or unsurveyed. [Illustration] [Illustration: OPEN PIT MINES ] [Illustration: AN ACCESS ROAD IN RUGGED AREA. SUCH ROADS DO NOT AS A RULE COUNT TOWARD PATENT EXPENDITURE. ] [Illustration: AN ADIT (TUNNEL) LEADING TO EXTENSIVE UNDERGROUND WORKINGS ] Improvements need not be surveyed with the same accuracy as the claim boundaries. Stadia measurements are acceptable, although they should be made with a Philadelphia Rod and distances kept under 500 feet. Small workings may be measured with a Brunton Compass and steel tape, but some point of the working, such as the mouth of a tunnel or cut, must be tied to a corner of the survey using a transit. Surveys of extensive underground workings may be taken from the claimant’s records if such surveys were executed under the supervision of a mining engineer. Extreme care should be exercised in entering old workings. In addition to unsafe timbering, they may be poorly ventilated resulting in bad air that could be fatal. Rather than risk an accident, the extent of such workings can be estimated from the size of the dump. Ties to discovery workings require special mention in view of the directive from the Assistant Director of Technical Services, Washington, D.C., dated March 15, 1978. Normally the discovery working is on the lode or intersects it and a tie is given along the lode line to the point of discovery, such as the face of the discovery cut, the mouth of the discovery tunnel, the center of the discovery shaft, etc. However, in the case of blanket veins that are essentially horizontal a presumed lode line or center line need not be shown, and the discovery working may be anywhere on the claim. In such cases, the tie may be given from the nearest corner of the survey and included in the description of that corner, or a right angle tie may be given from a point on the nearest end line.