indicates the ground surface level outside the building is marked “Grade” or “Grade Line.” The elevation of a surface at a particular point is a grade   elevation.    A grade elevation may refer to an existing, natural earth surface or to a hub or stake used as a reference point, in which case the elevation is that of  existing  grade  or  existing  ground.  It  may  also  refer to  a  proposed  surface  to  be  created  artificially,  in which case the elevation is that of prescribed grade, plan grade, or finished grade. Grade  elevations  of  the  surface  area  around  a structure are indicated on the plot plan. Because a natural earth surface is usually irregular in contour, existing   grade   elevations   on   such   a   surface   are indicated by contour lines on the plot plan; that is, by lines  that  indicate  points  of  equal  elevation  on  the ground.  Contour  lines  that  indicate  existing  grade  are usually made dotted; however, existing contour lines on maps are sometimes represented by solid lines. If the prescribed surface to be created artificially will be other  than  a  horizontal-plane  surface,  prescribed grade  elevations  will  be  indicated  on  the  plot  plan  by solid  contour  lines. On  a  level,  horizontal-plane  surface,  the  elevation is the same at all points. Grade elevation of a surface of  this  kind  cannot  be  indicated  by  contour  lines because  each  contour  line  indicates  an  elevation different  from  that  of  each  other  contour  line. Therefore,  a  prescribed  level  surface  area,  to  be artificially created, is indicated on the plot plan by outlining the area and inscribing inside the outline the prescribed  elevation,  such  as  “First  floor  elevation 127.50  feet.” BUILDING LAYOUT LEARNING  OBJECTIVE:  Upon  completing this section, you should be able to determine boundaries  of  building  layout. Before  foundation  and  footing  excavation  for  a building can begin, the building lines must be laid out to  determine  the  boundaries  of  the  excavations. Points  shown  on  the  plot  plan,  such  as  building corners,  are  located  at  the  site  from  a  system  of horizontal  control  points  established  by  the  battalion engineering aids. This system consists of a framework of stakes, driven pipes, or other markers located at points of known horizontal location. A point in the structure,  such  as  a  building  corner,  is  located  on  the ground  by  reference  to  one  or  more  nearby  horizontal control  points. We cannot describe here all the methods of locating a point with reference to a horizontal control point of a known  horizontal  location.  We  will  take,  as  an  illustrative example, the situation shown in figure 5-16. This figure shows  two  horizontal  control  points,  consisting  of monuments A and B. The term “monument,” incidentally, doesn’t  necessarily  mean  an  elaborate  stone  or  concrete structure. In structural horizontal control, it simply means any  permanently  located  object,  either  artificial  (such  as  a driven length of pipe) or natural (such as a tree) of known horizontal location. In figure 5-16, the straight line from A to B is a control base line from which the building corners of the structure can be located. Corner E, for example, can be located by first measuring 15 feet along the base line from A to locate point C; then measuring off 35 feet on CE, laid off at 90° to (that is, perpendicular to)  AB.  By  extending  CE  another  20  feet,  you  can locate  building  corner  F.  Corners  G  and  H  can  be similarly located along a perpendicular run from point D, which is itself located by measuring 55 feet along the base line from A. PERPENDICULAR BY PYTHAGOREAN THEOREM The  easiest  and  most  accurate  way  to  locate points on a line or to turn a given angle, such as 90°, Figure  5-16.—Locating  building  corners. 5-11