features,  as  well  as  the  larger  topographic  fares,  such as  stream  channels  and  swamps,  can  be  observed directly from aerial photographs. The photos also can be used to prepare a base map for portrayal of the field data by tracing planimetric details from an uncontrolled mosaic  with  spot  elevations  added  from  field  surveys. The geologist may satisfactorily use contact prints of aerial photographs in place of the base map except where  large-scale  plans  for  engineering  purposes  are  to be the base. In such a case the distortion within an aerial photograph  does  not  permit  plotting  of  geological  data commensurate with the accuracy of the final plan. Map  Base  for  Detailed  Geological  Surveys Detailed  geological  surveys  generally  cover  a specific map area geographic region, or specified site from scales of 1:62,500 to 1:600 or larger. In general, the very large scales are used for specific engineering or mineral  development  problems. SITE  PLANS  AND  PROPILES.—  Geological data  affecting  foundation  designs  at  construction  sites are plotted on plans drawn to scales of 1 inch = 50, 100, 200, or 400 feet. Contour intervals may range from 1 to 10  feet,  depending  upon  the  roughness  of  the  terrain Plane table mapping is suited to plotting the topographic features, ranges, and reference points used to locate drill holes, rock outcrops, and other geologic data. When plotting contours on a 1- or 2-foot interval, you should try to locate points that are actually on the contours or to determine elevations at the intersection of closely spaced grid lines staked out on the site. In addition to a plan, the geologist may require that profiles be drawn along selected lines or that the boring logs of test holes be  plotted  to  suitable  scales. USING A TOPOGRAPHIC MAP AS A BASE MAP.— The base map for a detailed geological survey is  a  complete  topographic  map  or  plan  with  relief expressed  by  contours.  Simple  colors  and  symbolization of basic details are used so that they will not conflict with the overlay of geological information that is shown by colors and symbols. Published topographic maps are used  where  suitable.  The  geological  survey  is  expedited if the map base is from a quarter to double the scale of the  map  on  which  the  information  is  to  be  presented. Enlargements of the base map, rather than other maps of a larger scale, are generally used to satisfy these requirements.  This  permits  the  direct  reduction  of geological data to the scale of the final map with a minimum amount of drafting. When no topographic map is available or if the existing maps are not suitable, a base map or plan must be  prepared  from  detailed  topographic  surveys.  Culture and relief (contours) should be shown in the greatest detail possible. The survey for the base should conform to third-order accuracy where large geographic areas are concerned.  Maps  made  from  aerial  photographs  by precise instrument methods can be used in place of field surveys. Altitude or elevation of the intersection of boreholes and the surface should be accurate to the nearest  one-half  foot. PEDOLOGICAL  SURVEYS Sometimes  there  is  a  requirement  for  pedological mapping for the purpose of locating the limits of sand or gravel deposits suitable for concrete aggregates, road materials, or for other construction operations. In such a case, the pedological survey conducted under the direction  of  the  soils  engineer  and  the  surveyor’s mission would be one of support to the soils engineer’s objective. The  engineer’s  objective  in  a  pedological  survey  is to prepare data in plan and profile symbolizing soils and outcropping  on  maps,  overlays,  and  sketches  for subsequent engineering uses. The following approaches may  be  used  in  conjunction  with  a  soils  survey operation: 1.  Aerial  photography  may  be  used  when  an extensive area is to be surveyed. Usually no survey measurements are required in this case. 2. Maps of an area that extend several square miles are   required   when   an   initial   study   or   technical reconnaissance is needed for an engineering project. Low-order  survey  measurements  usually  suffice  for  the preparation of a reconnaissance sketch upon which the soils engineer can plot the pertinent data. 3. A sketch of an airfield, for example, is frequently required  by  the  soils  analyst  before  construction planning can be initiated. In this case, the surveyor applies low-order measurements to prepare a sketch (1 inch = 100, 200, or 400 feet) upon which the soils engineer plots the results of soil tests and findings. Aerial  Photography Photo coverage of the area under consideration aids in  the  establishment  of  control  for  the  pedological survey. The use of vertical aerial photographs in the planning  phase  of  outlining  ground  control  will  speed the  survey  regardless  of  the  size  of  the  area  to  be 16-3