qgisqgis-2qgis-openlayers-plugin
I am new to QGIS. I drew a line depicting a small length of road. But I drew it in segments and then at last right clicked at the end point and gave id=1.
I extracted the nodes, their geometry. But what I need is the length of individual segment, its starting node coordinates, ending node coordinates.
for eg:
I need data in the attribute table as id;start_X;start_Y;end_X;end_Y;length
I have a large dataset of road network to be done in the same way.
The current version of QGIS (QGIS 3.0+) has the option to Force segments to be exactly of a given length, except for the last one using v.split. v.split is accessed from the Processing Toolbox under GRASS --> Vector
One of the easier ways to do this is with a spatial database such as PostGIS. Load your raster into the database using raster2pgsql (which is installed with PostGIS) and then load your rivers with shp2pgsql (also installed with PostGIS).
Then, you can run a simple query that samples the elevation model at the end points of all your lines:
select
st_value(r.rast, st_startpoint(geom)) as a,
st_value(s.rast, st_endpoint(geom)) as b
from
rivers, dem r, dem s
where
st_contains(st_convexhull(r.rast), st_startpoint(geom)) and
st_contains(st_convexhull(s.rast), st_endpoint(geom));
I've named the DEM twice in the from clause because the ends of the line may be contained by two different tiles in the raster, and each tile will have to be matched independently. That's why there are two st_contains calls.
The great thing about PostGIS is that you can automate this kind of processing over millions of records with a minimum of effort.
Note that this works if the line geometry is LineString. If they're MultiLineStrings, you'll have to refine the query, or use st_dump to turn them into LineStrings.
Best Answer
You can use Explode Lines in the Processing toolbox to turn each segment into an individual line. Have a look here:
Simple automated way to split all lines in layer at vertices using QGIS?
You can then open the attribute table of the layer with the newly split lines and use the field calculator to calculate the length into a new attribute with the...
...expression.
Getting the start and stop nodes is trickier, but you can perhaps use the WKT from geom function and then split the result on commas. If you use the expression:
...you get a result that looks something like this:
So if you split that on the comma, either in Excel (if you export the table and rejoin it in QGIS) or with a regular expression in the Field Calculator, you might be able to get start and end nodes.
However, I'm not sure if WKT preserves the order of the nodes (start vs end node etc), but someone else around here can probably confirm whether that's the case.
Edit: Also, if you select all your lines, open the attribute table, copy all, and then paste it into Excel, the first column will be the WKT "Linestring(312423.4353425..." representation of the geometry. That way you can extract the start and end nodes in Excel.
Have a look here:
Is there any way to extract coordinates of a road segment using QGIS?