1lrs(1) GRASS GIS User's Manual lrs(1)
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7 A Linear Referencing System (LRS) is a system where features (points or
8 segments) are localized by a measure along a linear element. The LRS
9 can be used to reference events for any network of linear features, for
10 example roads, railways, rivers, pipelines, electric and telephone
11 lines, water and sewer networks. An event is defined in LRS by a route
12 ID and a measure. A route is a path on the network, usually composed
13 from more features in the input map. Events can be either points or
14 lines (segments).
15 LRS is created from input lines and points in vector map. Points - MP
17 (mileposts) must have attached attributes specifying line and distance.
18 The distances from the beginning of the linear feature in real world
19 are specified by MP+offset. Typically, MP is in kilometers and offset
20 in meters.
21 The implementation of LRS in GRASS has some particularities.
23 Double referenced system
25 This feature gives a possibility to continue to use most of old mile‐
26 posts if only small part of linear object in real world has changed.
27 Example:
28 --- road (linear feature)
29 + MP (milepost, point, distance from the beginning in km)
30 Old situation:
31 +----+----+----+----+----+
32 0 2 3 4 5 6
33 New situation (for example a new bypass around the village)
34 ? ?
35 +----+
36 | |
37 | |
38 +----+----+ +----+----+
39 0 2 3 4 5 6
40 The segment between km 3 and 4 is now longer, it is now 3 km not 1 km
41 as in old version. It would be expensive to change also all MP >= 4,
42 but we cannot use km 4 twice. It is possible to use another notation
43 for the new segment, we reference the segment from the kilometer 3, us‐
44 ing only offset.
45 3+1000 3+2000
46 +----+
47 | |
48 | |
49 +----+----+ +----+----+
50 0 2 3 3+3000 5 6
51 4
52 This way, there is no ambiguity and minimal changes are needed. But
53 the MP 4 is no more the end of segment 3 - 4 but the end of segment
54 3+2000 - 3+3000. This information must be entered to the system and it
55 is done by optional MP attributes:
56 • end_mp - end MP
58 • end_off - end offset
60 In this case original MP on km 4 will have these attributes:
61 start_mp: 4
62 start_off: 0
63 end_mp: 3
64 end_off: 3000
65 Because each MP can keep 2 values (start, end) it is called ’double’
66 referenced LRS.
67 To avoid potential confusion, MP values are limited to integers only.
69 It would be ambiguous to have for example segments 3.500 - 3.500+200
70 and 3.600 - 3.600+200. The position 3+650 would fall into 2 segments,
71 correct would be 3.600+50. That means, that MP must be the beginning
72 of antonomous segment and all parts which becomes longer then before
73 must be referenced from the last not changed MP.
74 The MP start_mp and end_mp columns must be decimal, but v.lrs.create
76 takes only the decimal part, and adds its value to offset and prints a
77 warning.
78 It is highly recommended to work with polylines instead of segmented
80 vector lines. The command v.build.polylines creates this map structure.
81 LRS table structure
83 Attribute Type Description
84 rsid integer reference segment ID, unique in the table
86 lcat integer category of the line in the LRS map
88 lid integer route ID (LID)
90 start_map double precision distance measured along the line in LRS map from the begin‐
92 ning of the line to the beginning of the segment (absolute
93 milepost distance)
94 end_map double precision distance measured along the line in LRS map from the begin‐
96 ning of the line to the end of the segment (absolute dis‐
97 tance of subsequent milepost)
98 start_mp double precision milepost number assigned to the start of the segment
100 start_off double precision distance from start_mp to the start of the segment measured
102 along the physical object
103 end_mp double precision milepost number assigned to the end of the segment
105 end_off double precision distance from end_mp to end of the segment measured along
107 the physical object
108 end_type integer 1: the same as specified for from_ ; 2: calculated from map
110 along the line from previous MP; 3: defined by user
111 Available commands
114 • v.lrs.create to create a linear referencing system,
115 • v.lrs.label to create stationing on the LRS,
117 • v.lrs.segment to create points/segments on LRS, and
119 • v.lrs.where to find line id and real km+offset for given points
121 in vector map using linear referencing system.
122 Input lines for v.lrs.segment and v.lrs.label
124 v.lrs.create joins all connected lines of the same line ID into one
125 line, the LRS library and other modules using LRS expect this!
126 LR_get_nearest_offset in the LRS library checks duplicate segments only
127 by line_cat and map_offset, not by coordinates in map.
128 Duplicate positions
130 It can happen that one offset appears on 2 different lines:
131 ------1------- --------2------
132 +0.0 +1.0 +2.0
133 In this case, the module gives error because the position results in 2
134 points.
135 It can be also intended, for example a part of the road is shared with
137 another one, but MP are used only for one:
138 + road1/km15 + road1/km22
139 \ /
140 \ road1/km17 / road1/km20
141 +--------------+
142 / road2/km52 \ road2/km52
143 / \
144 + road2/km50 + road2/km54
145
147 Explanations of selected options:
148 • llayer: vector layer in line map (usually 1; see vectorintro
150 for "layer" concept)
151 • player: vector layer in point map (usually 1; see vectorintro
153 for "layer" concept)
154 • rsdriver: Driver name for LRS table - DBMI SQL driver (dbf, pg,
156 mysql, sqlite, etc)
157 • rsdatabase: Database name for LRS table - DBMI SQL database
159 name (e.g., "lrsdb")
160 • rstable: Name of the LRS table - DBMI SQL table name (e.g.,
162 "streamslrs")
163
165 R. Blazek, 2004, Introducing the Linear Reference System in GRASS,
166 Bangkok, GRASS User Conf. Proc.
167 R. Blazek, 2005, Introducing the Linear Reference System in GRASS, In‐
168 ternational Journal of Geoinformatics, Vol. 1(3), pp. 95-100
169 v.build.polylines, v.lrs.create, v.lrs.segment, v.lrs.where, v.lrs.la‐
171 bel
172
174 Radim Blazek, ITC-irst/MPA Solutions Trento
175 Documentation update (based on above journal article and available
176 fragments): Markus Neteler
177 Main index | Topics index | Keywords index | Graphical index | Full in‐
179 dex
180 © 2003-2023 GRASS Development Team, GRASS GIS 8.3.1 Reference Manual
182GRASS 8.3.1 lrs(1)