1lrs(1) Grass User's Manual lrs(1)
2
6 A Linear Reference System (LRS) is a system where features (points or
7 segments) are localized by a measure along a linear element. The LRS
8 can be used to reference events for any network of linear features, for
9 example roads, railways, rivers, pipelines, electric and telephone
10 lines, water and sewer networks. An event is defined in LRS by a route
11 ID and a measure. A route is a path on the network, usually composed
12 from more features in the input map. Events can be either points or
13 lines (segments).
14 LRS is created from input lines and points in vector map. Points - MP
16 (mileposts) must have attached attributes specifying line and distance.
17 The distances from the beginning of the linear feature in real world
18 are specified by MP+offset. Typically, MP is in kilometers and offset
19 in meters.
20 The implementation of LRS in GRASS has some particularities.
22 Double referenced system
24 This feature gives a possibility to continue to use most of old mile‐
25 posts if only small part of linear object in real world has changed.
26 Example:
27 --- road (linear feature)
28 + MP (milepost, point, distance from the beginning in km)
29 Old situation:
30 +----+----+----+----+----+
31 0 2 3 4 5 6
32 New situation (for example a new bypass around the village)
33 ? ?
34 +----+
35 | |
36 | |
37 +----+----+ +----+----+
38 0 2 3 4 5 6
39 The segment between km 3 and 4 is now longer, it is now 3 km not 1 km
40 as in old version. It would be expensive to change also all MP >= 4,
41 but we cannot use km 4 twice. It is possible to use another notation
42 for the new segment, we reference the segment from the kilometer 3,
43 using only offset.
44 3+1000 3+2000
45 +----+
46 | |
47 | |
48 +----+----+ +----+----+
49 0 2 3 3+3000 5 6
50 4
51 This way, there is no ambiguity and minimal changes are needed. But
52 the MP 4 is no more the end of segment 3 - 4 but the end of segment
53 3+2000 - 3+3000. This information must be entered to the system and it
54 is done by optional MP attributes:
55 end_mp - end MP
57 end_off - end offset
59 In this case original MP on km 4 will have these attributes:
60 start_mp: 4
61 start_off: 0
62 end_mp: 3
63 end_off: 3000
64 Because each MP can keep 2 values (start, end) it is called 'double'
65 referenced LRS.
66 To avoid potential confusion, MP values are limited to integers only.
68 It would be ambiguous to have for example segments 3.500 - 3.500+200
69 and 3.600 - 3.600+200. The position 3+650 would fall into 2 segments,
70 correct would be 3.600+50. That means, that MP must be the beginning
71 of antonomous segment and all parts which becomes longer then before
72 must be referenced from the last not changed MP.
73 The MP start_mp and end_mp columns must be decimal, but v.lrs.create
75 takes only the decimal part, and adds its value to offset and prints a
76 warning.
77 It is highly recommended to work with polylines instead of segmented
79 vector lines. The command v.build.polylines creates this map structure.
80 LRS table structure
82 | AttributeTypeDescription | rsid integer reference segment ID, unique
83 in the table | lcat integer category of the line in the LRS map | lid
84 integer route ID (LID) | start_map double precision distance measured
85 along the line in LRS map from the beginning of the line to the begin‐
86 ning of the segment (absolute milepost distance) | end_map double pre‐
87 cision distance measured along the line in LRS map from the beginning
88 of the line to the end of the segment (absolute distance of subsequent
89 milepost) | start_mp double precision milepost number assigned to the
90 start of the segment | start_off double precision distance from
91 start_mp to the start of the segment measured along the physical object
92 | end_mp double precision milepost number assigned to the end of the
93 segment | end_off double precision distance from end_mp to end of the
94 segment measured along the physical object | end_type integer1: the
95 same as specified for from_ ; 2: calculated from map along the line
96 from previous MP; 3: defined by user
97 Available commands
99 v.lrs.create to create a linear reference system,
100 v.lrs.label to create stationing on the LRS,
102 v.lrs.segment to create points/segments on LRS, and
104 v.lrs.where to find line id and real km+offset for given
106 points in vector map using linear reference system.
107 Input lines for v.lrs.segment and v.lrs.label
109 v.lrs.create joins all connected lines of the same line ID into one
110 line, the LRS library and other modules using LRS expect this!
111 LR_get_nearest_offset in the LRS library checks duplicate segments only
112 by line_cat and map_offset, not by coordinates in map.
113 Duplicate positions
115 It can happen that one offset appears on 2 different lines:
116 ------1------- --------2------
117 +0.0 +1.0 +2.0
118 In this case, the module gives error because the position results in 2
119 points.
120 It can be also intended, for example a part of the road is shared with
122 another one, but MP are used only for one:
123 + road1/km15 + road1/km22
124 \ /
125 \ road1/km17 / road1/km20
126 +--------------+
127 / road2/km52 \ road2/km52
128 / \
129 + road2/km50 + road2/km54
130
133 Explanations of selected options:
134 llayer: vector layer in line map (usually 1; see vector‐
136 intro for "layer" concept)
137 player: vector layer in point map (usually 1; see vector‐
139 intro for "layer" concept)
140 rsdriver: Driver name for reference system table - DBMI
142 SQL driver (dbf, pg, mysql, sqlite, etc)
143 rsdatabase: Database name for reference system table -
145 DBMI SQL database name (e.g., "lrsdb")
146 rstable: Name of the reference system table - DBMI SQL
148 table name (e.g., "streamslrs")
149
151 R. Blazek, 2004, Introducing the Linear Reference System in GRASS,
152 Bangkok, GRASS User Conf. Proc.
153 R. Blazek, 2005, Introducing the Linear Reference System in GRASS,
154 International Journal of Geoinformatics Vol. 1(3)
155 v.build.polylines, v.lrs.create, v.lrs.segment, v.lrs.where,
157 v.lrs.label
158
160 Radim Blazek, ITC-irst/MPA Solutions Trento
161 Documentation update (based on above journal article and available
162 fragments): Markus Neteler
163 Last changed: $Date: 2007/06/28 16:30:32 $
165GRASS 6.2.2 lrs(1)