LRS (Linear Referencing System)
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)
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.
0 2 3 4 5 6
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:
0 2 3 3+3000 5 6
In this case original MP on km 4 will have these attributes:
- end_mp - end MP
- end_off - end offset
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
|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 |
- 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,
- 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.
It can happen that one offset appears on 2 different lines:
In this case, the module gives error because the position
results in 2 points.
+0.0 +1.0 +2.0
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
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")
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
Radim Blazek, ITC-irst/MPA Solutions Trento
Documentation update (based on above journal article and available fragments): Markus Neteler
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