survey, for example, is a route survey required in the design and construction of a particular road or highway. The initial activities included in a route survey are as follows: reconnaissance survey, preliminary-location survey, and final-location survey. On  the  other  hand,  a  long  established  Navy  base might already have well-marked horizontal and vertical control  networks  and  up-to-date  topographic  maps available. Then perhaps neither a reconnaissance nor a preliminary survey would be required. The road could probably be designed by using the existing design data, and  the  fieldwork  would  begin  with  making  the  final location survey. In summary, the extent to which data is  already  available  is  an  important  factor  in determining   what   field   operations   have   to   be performed. RECONNAISSANCE   SURVEY.—   A  recon- naissance survey provides data that enables design engineers to study the advantages and disadvantages of a variety of routes and then to determine which routes are feasible. You begin by finding all existing maps that show  the  area  to  be  reconnoitered.  In  reconnaissance, studying existing maps is as important as the actual fieldwork. Studying  these  maps  and  aerial photographs,  if  any  exist,  will  often  eliminate  an unfavorable  route  from  further  consideration,  thus saving your reconnaissance field party much time and effort. Contour  maps  give  essential  information  about  the relief of an area. Aerial photographs provide a quick means for preparing valuable sketches and overlays for your field party. Direct aerial observation gives you an overview  of  an  area  that  speeds  up  later  ground reconnaissance  if  the  region  has  already  been  mapped. Begin the study of a map by marking the limits of the area to be reconnoitered and the specified terminals to  be  connected  by  the  highway.  Note  whether  or  not there are any existing routes. Note ridgelines, water courses,  mountain  gaps,  and  similar  control  features. Look  for  terrain  that  will  permit  moderate  grades without   too   much   excavating.   Use   simplicity   in alignment and have a good balance of cuts and fills; or use a profile arrangement that makes it possible to fill depressions with the cut taken from nearby high places. Mark the routes that seem to fit the needs and that should be reconnoitered in the field. From the map study,  determine  grades,  estimate  the  amount  of clearing  required,  and  locate  routes  that  will  keep excavation to a minimum by taking advantage of terrain conditions. Mark stream crossings and marshy areas as possible  locations  for  fords,  bridges,  or  culverts. Have  the  reconnaissance  field  party  follow  the route or routes marked earlier during the map study. Field  reconnaissance  provides  you  with  an  opportunity for checking the actual conditions on the ground and for noting  any  discrepancies  in  the  maps  or  aerial photographs. Make  notes  of  soil  conditions, availability of construction materials, such as sand or gravel,  unusual  grade  or  alignment  problems,  and requirements   for   clearing   and   grubbing.   Take photographs  or  make  sketches  of  reference  points, control   points,   structure   sites,   terrain   obstacles, landslides,   washouts,   or   any   other   unusual circumstances. Your reconnaissance survey party will usually carry lightweight instruments that are not precise. Determine by compass the direction and angles. Determine the approximate elevations by an aneroid barometer or altimeter. Use an Abney hand level (clinometer) to estimate elevations and to project level lines. Other useful  items  to  carry  are  pocket  tapes,  binoculars, pedometer  and  pace  tallies,  cameras,  watches,  maps, and  field  notebooks. Keep design considerations in mind while running a  reconnaissance  survey.  Remember  that  future operations may require further expansion of the route system presently being designed. Locate portions of the new route, whenever possible, along roads or trails that already exist. Locate  them  on  stable,  easily drained, high-bearing-strength soils. Avoid swamps, marshes, low-bearing-strength soils, sharp curves, and routes  requiring  large  amounts  of  earthmoving. Keep the need for bridges and drainage structures to a minimum. When the tactical situation permits, locate  roads  in  forward  combat  zones  where  they  can be concealed and protected from enemy fire. The report you turn in for the reconnaissance field party must be as complete as possible; it provides the major data that makes the selection of the most feasible route  or  routes  possible. PRELIMINARY   SURVEY.—   A   preliminary survey is a more detailed study of one or more routes tentatively selected on the basis of a reconnaissance survey report. It consists essentially of surveying and mapping  a  strip  of  land  along  the  center  line  of tentatively  selected  route. Some   of   the   activities   associated   with preliminary   survey   are   as   follows:   running a a a 14-26