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Support : Dialog Search Aids : File 89 Geographic Coordinates Fields File 89, GeoRef: Geographic Coordinates Fields
Since September 1977, coordinates of latitude and longitude have been assigned to some records to define geographic areas discussed in a document. The geographic coordinates permit searching of quadrangular geographic areas. Geographic coordinates are searchable with two types of prefixes. Searching with the boundary-defining prefixes (N1=, N2=, S1=, S2=, E1=, E2=, W1=, W2=) allows specification of the precise boundaries of a quadrangle. Searching with the intermediate prefixes (LN=, LS=, LE=, LW=) permits retrieval of a specific quadrangle, even when it is contained inside a larger area. For some geographic terms from GeoRef's controlled vocabulary, coordinates of latitude and longitude are listed in the GeoRef Thesaurus and in the online thesaurus. For other geographic areas, coordinates are assigned according to information given in the document or are derived from an atlas. Coordinates are assigned at the discretion of an indexer and are not automatically added when a geographic descriptor is assigned. Therefore, for the most comprehensive retrieval, search geographic terms as well as coordinates. The following example shows that only 34 records have been indexed with the exact coordinates for the Gulf of Alaska, while many records are indexed with the Descriptor "Gulf of Alaska":
Use the (S) operator to link coordinates in the Select statement in order to specify that they belong in the same set of coordinates for a single area. Note that although the Descriptor field includes several geographic terms, only one set of coordinates (those for the specific area discussed in the document) is assigned. Searching by coordinates is the most useful when the area of interest is not equivalent to an area that can be defined by a specific geographic name that would be in the Descriptor field. Geographic Coordinates are displayable by the individual display codes and also by using the LL display code. Latitude. Latitude indicates the number of degrees north or south of the equator (0 degrees). Latitude is indicated by a character string of seven characters: a letter N or S to indicate direction (i.e., north or south of the equator) and a six-digit number indicating the number of degrees, minutes, and seconds. Latitude coordinates are also cascaded to the full degree (i.e., first two digits): N451430 is North, 45 degrees, 14 minutes, 30 seconds; it is also cascaded to N45. Longitude. Longitude indicates the number of degrees east or west of the prime meridian (0 degrees). Longitude is indicated by a character string of eight characters: a letter E or W to indicate direction (i.e., east or west of the prime meridian) and a seven-digit number indicating the number of degrees, minutes, and seconds. Longitude coordinates are also cascaded to the full degree (i.e., first three digits): W1211752 is West, 121 degrees, 17 minutes, 52 seconds; it is also cascaded to W121 .Latitude and longitude define a quadrangle. Quadrangular geographic areas are defined by two coordinates of latitude and two coordinates of longitude. Coordinates are specified in order, starting from the southeastern- most point of the quadrangle (i.e., the lower right corner of the quadrangle). At the southeastern corner, Latitude 1 is specified. Moving north to the northeastern corner, Latitude 2 and Longitude 1 are specified. Moving west to the northwestern corner, Longitude 2 is specified. The figure below illustrates the points at which latitude and longitude coordinates are assigned:  Boundary-defining coordinates and intermediate coordinates. A quadrangle is assigned two sets of coordinates. Boundary-defining coordinates identify the four boundaries of a quadrangle. Intermediate coordinates identify boundaries of a quadrangle and every full degree between boundaries. One set of prefixes is used to search the boundary-defining coordinates and another set of prefixes is used to search the intermediate coordinates: Boundary-defining Prefixes Intermediate Prefixes N1= Latitude one, North LN= Latitude, North N2= Latitude two, North LS= Latitude, South S1= Latitude one, South LE= Longitude, East S2= Latitude two, South LW= Longitude, West E1= Longitude one, East E2= Longitude two, East W1= Longitude one, West W2= Longitude two, West The following figure shows the two sets of coordinates assigned to a particular quadrangle. All records with this quadrangle are retrieved when any one of the coordinates shown is selected:  Search examples using geographic coordinates. The following searches use coordinates that define a quadrangle encompassing the Gulf of Alaska to illustrate how retrieval can be broadened or narrowed, depending upon which prefixes and search techniques are used. The coordinates defining this quadrangle are: Latitude: N540000; N611500; Longitude: W1350000; W1630000. Searching a quadrangle with precise boundaries. Searching with boundary-defining prefixes provides the most narrow, precise retrieval. Use N1= or S1= to specify Latitude 1; N2= or S2= to specify Latitude 2; E1= or W1= to specify Longitude 1; and E2= or W2= to specify Longitude 2. Use the (S) operator to link the four boundary-defining prefixes. Retrieval includes only areas whose exact boundaries are indexed by the coordinates that are selected. Larger areas that may contain the defined area are NOT retrieved:
Searching quadrangles with a range of boundary possibilities. This type of search will not retrieve areas with boundary values larger or smaller than the specified values, nor will it retrieve areas that encompass, overlap, or share a boundary with the defined areas. This strategy retrieves quadrangles that have a bottom boundary between N50 and N55, a top boundary between N60 and N65, a right boundary between W130 and W135, and a left boundary between W160 and W165:
Searching a quadrangle and all quadrangles that contain it. Intermediate coordinates may identify either boundaries or points within a quadrangle. Use LN= or LS= to specify Latitude 1 and Latitude 2. Use LE= or LW= to specify Longitude 1 and Longitude 2. Use the (S) operator to link the four intermediate coordinates. When intermediate coordinates are used to define an area, retrieval includes all quadrangles that encompass the four coordinates specified. The defined quadrangle is retrieved even when it lies inside a larger area, as shown in the following example:
Item 1 specifies the precise area defined by the search (Gulf of Alaska); while item 2 specify larger areas that encompass the Gulf of Alaska (Alaska, Canada, and North America). Broadest retrieval. This example illustrates the most general type of search using geographic coordinates. Intermediate coordinates are used to specify a range of values for latitude and a range for longitude. Retrieved quadrangles must include at least one value from the specified latitude range and one from the longitude range. Thus, search results include the area defined by the coordinates selected, as well as all larger quadrangles that contain the defined area, quadrangles that overlap the defined area by at least one point, or quadrangles that share a boundary with the defined area.
Area that crosses the prime meridian. This example shows a search using the intermediate coordinates to search an area in the North Sea that includes longitude coordinates east and west of the prime meridian:
Note that using the intermediate coordinates retrieves the defined area and all larger quadrangles that contain the defined area. The following search shows an example of using the boundary-defining prefixes to search an area that crosses the prime meridian. Longitude 1 is east of the prime meridian and Longitude 2 is west. Search results include only quadrangles with boundaries that fall within the range of values specified in the Select statement:
Areas that cross the equator. This example uses the coordinates for Brazil to illustrate searching an area that crosses the equator. Latitude 1 is in the Southern Hemisphere; Latitude 2 is in the Northern Hemisphere. Intermediate coordinates retrieve the defined area and all areas that may contain the defined area:
The following search again uses the coordinates for Brazil to illustrate searching an area that crosses the equator. Latitude 1 is in the Southern Hemisphere; Latitude 2 is in the Northern Hemisphere. Boundary-defining prefixes retrieve only areas with boundaries that match the coordinates specified in the Select statement:
Areas that cross the International Date Line (180 degrees east and west). This example uses the coordinates for the Aleutian Islands to illustrate searching for an area that crosses hemispheres at the International Date Line.
Note that using intermediate coordinates retrieves the defined area and all larger areas that contain it. The next example uses coordinates for the Aleutian Islands to retrieve a defined quadrangle that crosses hemispheres at 180 degrees east and west. Search results include only quadrangles that match the coordinates specified in the Select statement:
Last modified on 15 Dec 2011. |
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