scale, such as 1:100,000. This may be taken to mean 1 inch on the chart is equal to 100,000 actual inches on the surface of earth, or it may be interpreted as 1 meter on the chart is equal to 100,000 meters on the surface of earth. The scale may also be a distance scale, such as 1 inch = I mile or 1 centimeter = 10 kilometers, as commonly seen on road maps. The military uses the comparative ratio scale. A chart covering a relatively small area is called a small-scale chart, while a chart covering a relative large area is called a large-scale chart. Typically, weather plotting charts range in scale from  1:1,000,000  for  3-foot  by  4-foot  charts  of individual  small  countries,  through  1:4,000,000  for charts  showing  sections  of  the  United  States.  Table  size charts for an entire hemisphere may be on a scale of 1:30,000,000. TYPES OF CHARTS There are nearly as many types of charts as there are applications for charts. The National Imagery and Mapping  Agency  Hydrographic/Topographic  Center produces a large portion of the charts routinely used by the  military.  Their  seven-part  catalog,  the  National Imagery and Mapping Agency (NIMA) Catalog of Maps, Charts, and Related Products, provides a listing of the various charts available, as well as ordering information. The catalog Part I,  Aerospace Products, Volume   1,   contains   listings   for   aeronautical charts-charts  displaying  air  routes  and  navigational aids used for flight route planning. Volume 2 contains listings for weather plotting charts-charts showing gross topography and land/water boundaries with station circles for established weather stations. Part II of the catalog contains listings for various hydrographic charts-charts used for surface and subsurface marine navigation. These charts show sounded water depths and ocean depth contour lines as well as positions of marine  navigation  aids.  Part  III  contains  several volumes  listing  various  topographic  charts-charts showing detailed height contours and structures on land areas  used  for  various  types  ofplanning.  Many  volumes are  classified. At  some  point  in  your  career,  you  will  use aeronautical   charts,   hydrographic   charts,   and topographic charts. Up until recently, every observer manually plotted many different types of observations on weather plotting charts.   Although you may still on occasion, manually plot data on a weather plotting chart, and therefore should be familiar with the various charts available, nearly all charts used by analysts and forecasters today are computer plotted. Refer to Part I, Volume II, of the NIMA catalog for a listing of the various  weather  plotting  charts  available  for  manual plotting  of  data. STATION IDENTIFIERS Meteorological  stations,  when  reporting  weather conditions, use either a WMO block/station number or station-identifier  letters.  The  station  identifier  letters are   usually   the   International   Civil   Aviation Organization  (ICAO)  assigned  station  identifier,  but may be an identifier assigned by the individual country. A  particular  station  may  use  its  WMO  block/station number to report synoptic observations, but use either the ICAO identifier or the nationally assigned identifier to report weather locally. National  Imagery  and  Mapping  Agency  weather plotting  charts  (DOD  WPCs)  have  predrawn  station circles for nearly every known permanent weather- reporting station. Each station circle is identified with the  WMO  station  number  and/or  the  ICAO  letter identifier, as shown in figure 4-10, view A. Mobile weather reporting stations, such as ships or temporary research sites, report location by latitude and longitude. A station circle must be added at the correct location whenever a mobile station’s report is plotted. WMO Block/Station Identification Numbers WMO block/station numbers are used to identify locations of permanent weather reporting stations. WMO   international   block/station   identification numbers are a series of six digits. The first two digits are the WMO block number, and the last four digits are the station number. In an area the size of a small city, the designated   Synoptic   Station—the   single   station assigned   the   responsibility   to   submit   synoptic observation  reports,  has  a  four-digit  station  number ending in a zero. Other weather observation sites in the city that are not designated synoptic reporting sites have other numbers as the last digit. For example, figure 4- 10, view B, shows the stations in the Norfolk, Virginia area.  Norfolk  International  Airport  (ORF), block/station  number  723080,  is  the  designated synoptic  reporting  station. Since the last digit of the synoptic station’s station number is always zero, the last digit is dropped when encoding.  Additionally,  weather  plotting  charts  only indicate the first three digits of the four-digit station number,  such  as  308,  instead  of  3080,  adjacent  to  a synoptic station circle. 4-10