generally  marked  with  permanent  markers.  These markers  could  be  in  the  following  forms: .  A  bronze  disk  set  in  concrete l  An  iron  pipe  filled  with  concrete .  A  crosscut  on  an  existing  concrete  structure or  on  a  rock  outcrop .  A  hole  drilled  in  concrete  and  filled  with lead or a metal rod driven into the ground with a  center-punched  mark  to  designate  the  exact point All permanent survey station markers should be  referenced  so  they  can  be  replaced  if  disturbed. Methods of referencing points are discussed later in this training manual. Surveyor’s  tacks,  spikes,  and  nails  are  often driven  into  growing  trees,  bituminous,  or  other semisolid surfaces as permanent markers. A nail will be more conspicuous if it is driven through a  bottle  cap,  a  washer,  a  plastic  tape,  or  a “shiner.” A shiner is a thin metal disk much like the  top  or  bottom  of  a  frozen  fruit  juice  can. A  SPAD  is  a  nail  equipped  with  a  hook  for suspending  a  plumb  bob.  It  is  driven  into  an overhead  surface,  such  as  the  top  of  a  tunnel.  The plumb  bob  will  locate  on  the  floor  the  point vertically below the point where the spad is driven. Points on concrete or stone surfaces are often marked with an X cut with a hammer and chisel. Another way to do this is to cut holes with a star drill and then plug them with lead. A much more durable form of marker is made of  a  length  of  metal  pipe—usually  called  iron  pipe regardless of the actual metal used. Lengths run from about 18 to 24 in. Sawed-off lengths of pipe have  open  ends;  pipes  cut  with  a  shear  have pinched  ends  and  are  called  pinch  pipe.  There  are also manufactured pipe markers, some of which are  T-shaped  rather  than  cylindrical  in  cross section.  A  commercial  marker  may  consist  of  a copper-plated steel rod. All commercial markers have caps or heads that permit center punching for  precise  point  location  and  stamping  of  the identifying   information. A  still  more  durable  form  of  marker  is  the concrete monument. A short length of brass rod is  often  set  in  the  concrete  to  mark  the  exact location  of  the  point.  Concrete  monuments  that are used as permanent markers by various federal survey  agencies  have  identifying  disks  set  in concrete,  like  those  shown  in  figure  11-57. Figure 11-57.-Various types of federal marking disks. Points  on  concrete  or  masonry  surfaces  may be  permanently  marked  by  setting  lengths  of cylindrical brass stock into holes plugged with lead or grout. Brass stock markers set in pavement are commonly  called  coppers.  Manufactured  brass disks, similar to the ones shown in figure 11-57, may be set in grouted holes in street pavements, sidewalks,  steps,  or  the  tops  of  retaining  walls. Points on bituminous surfaces maybe marked by driving in pipe, railroad spikes, or case-hardened masonry   nails,   commonly   called   PK   nails.   A center  punch  for  marking  a  precise  location  on metal  stock  or  metal  caps  is  a  common  item  of equipment  for  a  surveyor. MARKING   MATERIALS KEEL,  or  LUMBER  CRAYON,  is  a  thick crayon  used  for  marking  stakes  or  other  surfaces. Common  marking  devices  that  contain  a  quick- drying  fluid  and  a  felt  tip  are  also  popular  for marking  stakes.  All  of  these  types  of  graphic marking  materials  come  in  various  colors. In  addition  to  keel,  paint  is  used  to  mark pavement surfaces. Paint may be brushed on or sprayed from a spray can. To make the location of  a  point  conspicuous,  use  a  circle,  a  cross,  or a triangle. Identification symbols, such as station 11-48