# How to turn all lines into one direction?

How can I turn all features of my polyline (postgis and/or shapefile) into one direction - it doesn't matter what direction. I know how to switch the direction of lines but I have no idea how to bring all lines (I have thousands of them) into one single direction.

EDIT:

A line can have only two directions, the one or the other like 0 or 1. I want them all to have the direction 0 OR 1. For example the left line in the image is in direction 1 the middle line is in direction 0 and the right one is in direction 1. Now I want the middle line to direction 1 OR the right and left line to direction 0. So that all lines have the same direction 1 OR 0.

``````geom;given_direction;wanted_direction
line;1;1
line;0;1
line;1;1
line;1;1
line;0;1
line;1;1
``````
• (Possible Duplicate) Did you try this: gis.stackexchange.com/questions/9261/… Commented Feb 22, 2014 at 19:52
• this is switching the direction of every line to its opposite but I want all lines into ONE direction. (Or I am wrong?) Commented Feb 23, 2014 at 12:10
• you have to know which ones you wanna change, otherwise, it'll change the entire collection.
– OLS
Commented Feb 23, 2014 at 12:15
• If line 1, 3, and 4 is in direction 'A' and line 2 and 5 in direction 'B' I need all changed to direction 'A' OR 'B'. Commented Feb 23, 2014 at 12:23
• Can you please add a sample file? This way it might be easier to help. Commented Mar 19, 2014 at 8:18

I assume with "one direction" you mean that every segment of a collection of polylines should show into one direction. Also there is a need, that the segments in the collection are in the right order following a single path.

Given this, things are easy: You are searching for pretty much the same thing, I did a couple of months ago. The StackOverflow Community helped me out with this... look here.

Things are getting harder if you have to collect the connected polylines first: A possible algorithm could be:

1. Take first polyline A.
2. Save Start Point in a variable
3. Save End Point in a variable
4. Take next polyline B
5. compare A.endPoint with B.startPoint: if equals then merge A and B to A and start over with 3.
6. comapre A.startPoint with B.endPoint: if equals then merge B and A to A and start over with 2.
7. compare A.endPoint with B.endPoint: if equals then merge A and B.reverse() to A and start over with 3.
8. compare A.startPoint with B.startPoint: if equals then merge B.reverse() and A to A and start over with 2.
9. Do this until no unordered polylines are left.

In this algorithm it doesn't matter if you merge all Coordinates into one big linestring, or into a 2 dimensional sorted array which is in fact a multilinestring.

It is not possible to have a single orientation ("from points" connected to "end points" for all your lines) of all your lines if you have loops, which is probably your case because you work with a road network. Therefore you should decide of a rule that would help you set the orientation (for instance, switch all lines where the X coordinate of the end point is larger than the X coordinate of the from point or (better) do this based on the network distance, or the elevation...).

Based on the above statement, I suggest that you only solve you orientation for pseudo nodes. A quick fix is to use Vector -> Geometry Tools -> Singleparts to multipart in order to remove those pseudo-nodes. Otherwise Jürgen Zornig method is best (and you can adapt based on iterative selection because your lines are unlikely to be ordered.)

ST_LineMerge can do this job, if the lines are collected in a `MULTILINESTRING` and share a common start/end vertex.

## Example 1 (linestrings with the same direction)

``````SELECT ST_AsText(ST_LineMerge(
ST_GeomFromText('MULTILINESTRING((-29 -27,-30 -29.7,-36 -31,-45 -33),(-45 -33,-46 -32))')
)
);
st_astext
--------------------------------------------------------------------------------------------------
LINESTRING(-29 -27,-30 -29.7,-36 -31,-45 -33,-46 -32)
(1 row)
``````

## Example 2 (linestrings with different directions)

``````SELECT ST_AsText(ST_LineMerge(
ST_GeomFromText('MULTILINESTRING((-29 -27,-30 -29.7,-36 -31,-45 -33),(-46 -32,-45 -33))')
)
);
st_astext
--------------------------------------------------------------------------------------------------
LINESTRING(-46 -32,-45 -33,-36 -31,-30 -29.7,-29 -27)
(1 row)
``````

First, you have to know which lingstrings you wanna reverse, and than you need to reverse them. I don't know how to do it via QGIS but if you have database side (POSTGRES / POSTGIS) than you can use ST_Reverse

You can also convert the linestring to array and reverse it

``````list.reverse()
``````