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LRS (Linear Referencing System)

DESCRIPTION

       A  Linear  Referencing  System  (LRS)  is a system where features (points or segments) are
       localized by a measure along a linear element.  The LRS can be used  to  reference  events
       for  any  network  of  linear  features,  for  example roads, railways, rivers, pipelines,
       electric and telephone lines, water and sewer networks. An event is defined in  LRS  by  a
       route  ID  and  a  measure.  A  route is a path on the network, usually composed from more
       features in the input map.  Events can be either points or lines (segments).

       LRS is created from input lines and points in vector map.  Points -  MP  (mileposts)  must
       have  attached  attributes specifying line and distance.  The distances from the beginning
       of the linear feature in real world are specified  by  MP+offset.   Typically,  MP  is  in
       kilometers and offset in meters.

       The implementation of LRS in GRASS has some particularities.

   Double referenced system
       This  feature  gives  a possibility to continue to use most of old mileposts if only small
       part of linear object in real world has changed. Example:
       --- road (linear feature)
        +   MP (milepost, point, distance from the beginning in km)
       Old situation:
       +----+----+----+----+----+
       0    2    3    4    5    6
       New situation (for example a new bypass around the village)
                 ?    ?
                 +----+
                 |    |
                 |    |
       +----+----+    +----+----+
       0    2    3    4    5    6
       The segment between km 3 and 4 is now longer, it is now 3 km not 1 km as in  old  version.
       It  would  be  expensive  to change also all MP >= 4, but we cannot use km 4 twice.  It is
       possible to use another notation for the new segment, we reference the  segment  from  the
       kilometer 3, using only offset.
             3+1000  3+2000
                 +----+
                 |    |
                 |    |
       +----+----+    +----+----+
       0    2    3  3+3000 5    6
                      4
       This  way,  there is no ambiguity and minimal changes are needed.  But the MP 4 is no more
       the end of segment 3 - 4 but the end of segment 3+2000 - 3+3000. This information must  be
       entered to the system and it is done by optional MP attributes:

           ·   end_mp - end MP

           ·   end_off - end offset
       In this case original MP on km 4 will have these attributes:
       start_mp:  4
       start_off: 0
       end_mp:    3
       end_off:   3000
       Because each MP can keep 2 values (start, end) it is called ’double’ referenced LRS.

       To  avoid  potential  confusion,  MP  values  are  limited  to integers only.  It would be
       ambiguous to have for example segments 3.500  -  3.500+200  and  3.600  -  3.600+200.  The
       position 3+650 would fall into 2 segments, correct would be 3.600+50.  That means, that MP
       must be the beginning of antonomous segment and all parts which becomes longer then before
       must be referenced from the last not changed MP.

       The  MP  start_mp  and  end_mp  columns  must  be decimal, but v.lrs.create takes only the
       decimal part, and adds its value to offset and prints a warning.

       It is highly recommended to work with polylines instead of  segmented  vector  lines.  The
       command v.build.polylines creates this map structure.

   LRS table structure
       Attribute                                                    Type                                                         Description

       rsid                                                         integer                                                      reference segment ID, unique in the table

       lcat                                                         integer                                                      category of the line in the LRS map

       lid                                                          integer                                                      route ID (LID)

       start_map                                                    double precision                                             distance  measured  along  the  line  in  LRS  map  from the
                                                                                                                                 beginning of the  line  to  the  beginning  of  the  segment
                                                                                                                                 (absolute milepost distance)

       end_map                                                      double precision                                             distance  measured  along  the  line  in  LRS  map  from the
                                                                                                                                 beginning of the line to the end of the  segment   (absolute
                                                                                                                                 distance of subsequent milepost)

       start_mp                                                     double precision                                             milepost number assigned to the start of the segment

       start_off                                                    double precision                                             distance  from start_mp to the start of the segment measured
                                                                                                                                 along the physical object

       end_mp                                                       double precision                                             milepost number assigned to the end of the segment

       end_off                                                      double precision                                             distance from end_mp to end of the  segment  measured  along
                                                                                                                                 the physical object

       end_type                                                     integer                                                      1:  the same as specified for from_ ; 2: calculated from map
                                                                                                                                 along the line from previous MP; 3: defined by user

   Available commands
           ·   v.lrs.create to create a linear referencing system,

           ·   v.lrs.label to create stationing on the LRS,

           ·   v.lrs.segment to create points/segments on LRS, and

           ·   v.lrs.where to find line id and real km+offset for  given  points  in  vector  map
               using linear referencing system.

   Input lines for v.lrs.segment and v.lrs.label
       v.lrs.create  joins all connected lines of the same line ID into one line, the LRS library
       and other modules using LRS expect this!  LR_get_nearest_offset in the LRS library  checks
       duplicate segments only by line_cat and map_offset, not by coordinates in map.

   Duplicate positions
       It can happen that one offset appears on 2 different lines:
       ------1-------     --------2------
       +0.0            +1.0              +2.0
       In this case, the module gives error because the position results in 2 points.

       It can be also intended, for example a part of the road is shared with another one, but MP
       are used only for one:
        + road1/km15         + road1/km22
         \                  /
          \ road1/km17     / road1/km20
           +--------------+
          / road2/km52     \ road2/km52
         /                  \
        + road2/km50         + road2/km54

NOTES

       Explanations of selected options:

           ·   llayer: vector layer in line map (usually 1; see vectorintro for "layer" concept)

           ·   player: vector layer in point map (usually 1; see vectorintro for "layer" concept)

           ·   rsdriver: Driver name for LRS table - DBMI SQL driver  (dbf,  pg,  mysql,  sqlite,
               etc)

           ·   rsdatabase: Database name for LRS table - DBMI SQL database name (e.g., "lrsdb")

           ·   rstable: Name of the LRS table - DBMI SQL table name (e.g., "streamslrs")

SEE ALSO

       R.  Blazek,  2004,  Introducing  the Linear Reference System in GRASS, Bangkok, GRASS User
       Conf. Proc.
       R. Blazek, 2005, Introducing the Linear Reference System in GRASS,  International  Journal
       of Geoinformatics, Vol. 1(3), pp. 95-100

       v.build.polylines, v.lrs.create, v.lrs.segment, v.lrs.where, v.lrs.label

AUTHOR

       Radim Blazek, ITC-irst/MPA Solutions Trento
       Documentation  update  (based  on  above  journal article and available fragments): Markus
       Neteler

       Last changed: $Date: 2014-04-20 12:47:59 +0200 (Sun, 20 Apr 2014) $

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