AW: [postgis-users] Linear Referencing: Create Measure

[Düster Horst]

Hallo strk

Thak you very much for your response. The required function should measure
lines or multilines. For instance I need to measure a river network. The
measures of the rivers are implicit predefined during the creation process
of the river networkdata and don't represent the real world in all cases.
The predefinition bases on the two attributes "from" and "to" measure. The
"from" value represents the lowest measure value and the "to" value highest
measure value of any linestring in case of our rivernetwork. The units of
our predefined measures is km and the datatype is float8.  
The work to do is to interpolate the measure of every vertex point of any
linestring/multilinestring with respect to "from" and "to". With this
approach it is also possible to create measures in percent of linelength.
You have to set from=0 and to=100.

A second case could be to calculate the measures based on the length of the
relevant linestring from zero to length. Therefore you don't need to
predefine from and to.

In both cases you have to differ between linestrings and multilinestrings.
In case of continuous linestrings you can interpolate over the whole line.
In case of multilinestrings you have to consider possible gaps in the
multilinestring. I'll attach the code of a former version of Christoph
Spoerri's lrs_create_measure. Unfortunately the lrs_create_measures
functions don't work correct yet.  

I think the function call could be:
line_create_measure(geometry,[float8],[float8])

line_create_measure(wkb_geometry,from,to) or
line_create_measure(wkb_geometry,0,1) would solve the first case
line_create_measure(wkb_geometry) would solve the second case.

I hope this will be a base for a further discussion.

With best regards
 
Dr. Horst Düster 
GIS-Koordinator, Stv. Amtschef

Kanton Solothurn 
Amt für Geoinformation
Abteilung SO!GIS® Koordination
Rötistrasse 4
CH-4501 Solothurn 

Tel.: ++41 (0)32 627 25 32 
Fax: ++41 (0)32 627 22 14

horst.duester at bd.so.ch 
www.sogis.so.ch



> -----Ursprüngliche Nachricht-----
> Von: strk at refractions.net [mailto:strk at refractions.net]
> Gesendet am: Donnerstag, 29. Juni 2006 23:57
> An: 'postgis-users at postgis.refractions.net'
> Betreff: Re: [postgis-users] Linear Referencing: Create Measure
> 
> Maybe we can add such a function, but semantics must be exactly
> defined. How are 'from' and 'to' defined ? How are they applied
> to different types of geometries ?
> 
> --strk;
> 
> On Thu, Jun 29, 2006 at 11:43:44AM +0100, Düster Horst wrote:
> > I'm faced with the problem to create MGeometries for using 
> with the "Linear
> > Referencing" functions of PostGIS. I like to create the 
> measures defined by
> > two table attributes meas_from and meas_to. In further 
> versions of Christoph
> > Spoerri's LRS functions there exists a function
> > lrs_create_measure(geometry,from,to). As the result the 
> M-Values were
> > interpolated based on a start and a stop value of each line. The new
> > functions don't offer such a functionality. I would be 
> greatful for any hint
> > how to create measures along a line referenced by a from 
> and a to value of
> > this line.
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> 

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