HACHURES—A  method  of  portraying  relief  by short,  wedge-shaped  marks  radiating  from  high elevations and following the direction of slope to the  lowland. HALF  SECTION—A  sectional  view  that  passes halfway through the object; used when the shape of  one  half  is  identical  to  the  other  half. HATCHING—Sections  lines  that  are  drawn  on the  internal  surface  of  sectional  views;  may  be used  to  define  the  kind  or  type  of  material  of which  the  sectioned  surface  consists. HEIGHT   OF   INSTRUMENT—1.   (Spirit leveling)  The  height  of  the  line  of  sight  of  a leveling  instrument  above  the  adopted  datum. 2. (Stadia surveying) The height of the center of the  telescope  (horizontal  axis)  of  transit  or telescopic  alidade  above  the  ground  or  station mark. 3. (Trigonometrical leveling) The height of the  center  of  the  theodolite  (horizontal  axis)  above the  ground  or  station  mark. HELIOTROPE—An   instrument   composed   of one  or  more  plane  mirrors  so  mounted  at  the station  being  sighted  upon  that  the  sun’s  rays  can be  reflected  to  any  one  observing  the  station. HEPTAGON—A   polygon   of   seven   sides. HEXAGON—A   polygon   of   six   sides. HIDDEN  LINES—Thick,  short,  dashed  lines indicating the hidden features of an object being drawn. HORIZONTAL  ANGLE—The  angle  formed  by two  intersecting  lines  on  a  horizontal  plane. HORIZONTAL   CONTROL—Control   that determines  horizontal  positions  only,  with  respect to  parallels  and  meridians  or  to  other  lines  of reference. HORIZONTAL  DATUM—In  plane  surveying, the  grid  system  of  reference  used  for  the horizontal  control  of  an  area;  defined  by  the casting and northing of one station in the area, and the azimuth from this selected station to an adjacent  station. HORIZONTAL  DIRECTION—A  direction  in  a horizontal  plane. HORIZONTAL   DISTANCE—A   distance measured   along   a   level   line.   It   is   commonly thought  of  as  the  distance  between  two  plumb lines.  The  distance  may  be  measured  either horizontally or inclined, but the inclined distance is  always  reduced  to  its  horizontal  length. HORIZONTAL  LINE—A  line  tangent  to  a  level surface,  or  a  line  lying  on  a  horizontal  plane. HORIZONTAL  PLANE—A  plane  perpendicular to  the  direction  of  gravity. HORIZONTAL   POSITION—The   grid   position of  a  horizontal  control  point. HORIZONTAL   REFRACTION—A   natural error  in  surveying  that  is  the  result  of  the horizontal bending of light rays between a target and  an  observing  instrument.  This  refraction  is usually  caused  by  the  differences  in  density  of  the air along the path of the light rays, resulting from temperature  variations. HORN  CENTER—Device  used  to  prevent  the compass needle from making holes in a drawing. HUB—A wooden stake or pipe set in the ground with a tack or other marker to indicate the exact position.  A  guard  stake  protects  and  identifies  the hub. HYDROSCOPIC   MOISTURE—The   films   of adsorbed moisture that may be present in airdried soil.  Hydroscopic  moisture  may  be  driven  off  by oven-drying.  (See  also  adsorbed  moisture.) ILLUSTRATION  BOARD—Smooth  white  paper with  a  cardboard  backing,  used  for  signs  or  charts or  mounting  of  maps,  photos,  or  drawings  that require  a  strong  backing. IMAGE   PLANE—See   PLANE   OF   PROJEC- TION. INDIA   INK—Drawing   ink   consisting   of   a pigment (usually powdered carbon) suspended in an  ammonia-water  solution. INFINITE   DISTANCE—An   indefinite   unmea- surable distance; for example, parallel lines are said  to  converge  at  infinity. INSCRIBED   FIGURE—A   figure   that   is completely  enclosed  by  another  figure. AI-7