or annotations. As an amplification of the map data, more  complete  descriptions  of  outcrops  are  entered  in notebooks  with  the  entries  keyed  to  the  field  map. Surveyors  support  the  geologist  by  preparing  basic topographic maps on which they plot the results of geological  investigations  and  then  make  such  tie measurements to geological features as the geologist may  require. The  geologist  uses  simple  survey  methods  in plotting geological features on a field map. Where an outcrop can be located with reference to a cultural or relief feature, it is generally plotted on a map by spot recognition.  In  other  cases,  the  relationship  of  a geological  feature  to  a  recognizable  topographic  feature is established by using a magnetic compass to determine direction and by pacing or taping to measure distance. Slope  or  small  differences  in  elevation  are  measured  by using a clinometer or hand level, while an altimeter is used where there are large differences in elevation. When the geological survey is keyed to a large-scale plan, the geologist generally uses a plane table and plots data with accuracy commensurate with the accuracy of the base plan. Base  Map  Surveys The survey for the base map should normally take place  before  the  geological  survey,  because  the geologist uses the map in the field to plot his data and to  determine  his  position  by  identification  of topographic  details.  If  aerial  photographs  are  available, the base map need not be made before the geological survey since the geologist can use the aerial photograph as a plotting base and later transfer the data to a base map. However, if possible, the base map should be prepared in advance, even in this case, as the number of aerial photographs needed to cover an area is generally too large to be handled in the field. Plane table topography is the method best suited to relatively  open  country.  In  the  absence  of  detailed instructions,  the  following  specifications  are  generally satisfactory: 1.  BASE  DIRECTION.  To  determine  a  base direction,  take  from  a  known  base  a  side  in  a triangulation net or a course of a basic control traverse. 2.  LOCAL  HORIZONTAL  CONTROL.  Use plane table traverses run in closed circuits or between known control stations of a higher order of accuracy or locate plane table stations by graphical triangulation. 3.  LOCAL  VERTICAL  CONTROL.  Where  the terrain is relatively level, carry elevation along traverses by  vertical  angle  or  stadia-arc  measurements,  adjusting elevations on closure at a basic control station. For rugged terrain mapped at one of the larger contour intervals,  barometric  or  trigonometric  leveling  is suitable. 4. SIGHTS. Use telescopic alidade. 5.  DISTANCE  MEASUREMENTS.  Use,  in general, stadid or graphical triangulation to locate points and stations. Certain measurements can be made most conveniently by pacing or rough taping. 6.  CONTOURING.  Locate  and  determine  the elevations of controlling points on summits, in valleys and saddles, and at points of marked change of slope. Interpolate and sketch contours in the field, using these elevations  for  control. 7. ACCURACY. Distance measurements by stadia should be accurate to 1 part in 500. Side-shot points located by pacing or other rough measurements should be accurate to within 25 feet. Take sights for traverse lines or graphical triangulation with care to obtain the maximum accuracy inherent in the telescopic alidade. The error in the elevation of any point, as read from the finished map, should not exceed one half of the contour interval. Topography  may  be  located  more  conveniently  in heavily  timbered  country  by  stadia  measurements  from transit-stadia traverse than by the use of the plane table, although the time required for plotting will be increased. The specifications listed above are generally applicable. Read horizontal angles on traverses to 1 minute and horizontal angles for side shots that will be plotted by protractor to the nearest quarter of a degree. Read vertical angles for elevation determination to 1 minute or use the stadia arc. Keep complete and carefully prepared stadia notes and sketches to assure correct plotting. When the geologist indicates that a map of a lower order of accuracy will fulfill his needs, plane table or compass  traverses  are  suitable. Use of Aerial Photographs If  aerial  photographs  are  available,  the  geologist generally  uses  them  instead  of  a  map.  The  most satisfactory  results  are  obtained  from  large-scale photographs,  1:15,000  or  larger.  Some  topographic features,  such  as  some  ravines,  rocky  knobs,  or sinkholes, are too small to be shown on maps. These 16-2