In QGIS, I've tried the following options to convert my GeoJSON file from MultiLineString to one or multiple LineStrings (translating the Danish version):
Vector -> Geometry Tools -> Polygons to Lines
Vector -> Geometry Tools -> Multipart to Single parts
Vector -> Geoprocess Tools -> Dissolve
Each option produces just the same MultiLineString.
How do I convert the MultiLineString to LineString(s) using either QGIS or another GIS software?
It shouldbe your second option, Multipart to Single Part. Notice, however, that this will only work with Multilinestrings whose individual Linestrings are spatially connected. To be spatially connected means that either the start or the end point of a line shall have the same coordinates as either the start or end point of another line, as such:
# Line A and Line B are connected
Linestring A (0 0, 1 1)
Linestring B (1 1, 2 3)
# Line A and Line B are disconnected
Linestring A (0 0, 1 1)
Linestring B (1 2, 2 3)
What Multipart to Single Part does is transform your one multiline feature into one single line feature, but if the individual linestrings are disconnected, it can't return one single feature, thus it returns a multiline. There are two things you can do from here:
1. Your Multiline really should be only one linestring feature
This means you'll have to edit your data. In QGIS, go to Properties -> Styles, add another line style on top of your current one, and change it to Marker Style. Then set it to last vertex only, and change the shape of it from a ball to a sideways triangle.
With this, you know the start and end of each individual line in your multiline feature. Then check for the arrows along your line, these are the intersections. Edit the layer, with snap enabled, and make sure each point with a triangle properly connects to the first point of the next line. Only the first and last points of the entire multiline should not need be connected to anything. Then try the Mulyipart to Single Part again.
2. I don't care about the number of features, I just want simple Linestrings
This means a conversion from one Multiline feature to many single Line features. QGIS doesn't do this (afaik), but PostGIS does. You should already have it installed together with QGIS, so create a database, upload your layer (easiest done with QGIS' DB Manager plugin), then execute:
SELECT (ST_DUMP(geom).geom) as geom
FROM your_table_name
The result should be a bunch of binary-code features. These are your Linestrings in WKB format. Below the result panel there is an option to load the result back into QGIS. Just choose a layer name and click Load. Notice, however, that this loaded layer is virtual, you'll need to save it as a shapefile or sometging if you want to retain it after you quit QGIS.
You can use GDAL command line tools "ogrinfo" and "ogr2ogr" . First check the layer name from your GeoJSON data with ogrinfo
ogrinfo test.json
INFO: Open of `test.json'
using driver `GeoJSON' successful.
1: test
Then you can check what geometry types are present in data on layer "test"
ogrinfo -dialect sqlite -sql "select distinct ST_GeometryType(geometry) from test" test.json
INFO: Open of `test.json'
using driver `GeoJSON' successful.
Layer name: SELECT
Geometry: None
Feature Count: 2
Layer SRS WKT:
(unknown)
ST_GeometryType(geometry): String (0.0)
OGRFeature(SELECT):0
ST_GeometryType(geometry) (String) = POINT Z
OGRFeature(SELECT):1
ST_GeometryType(geometry) (String) = MULTILINESTRING Z
Finally select the multilinestrings and save them into a new GeoJSON file as single part linestrings
ogr2ogr -f geojson -dialect sqlite -sql "select * from test where ST_GeometryType(geometry)='MULTILINESTRING Z'" -explodecollections -nln lines new.json test.json
Check the result with ogrinfo:
ogrinfo new.json -al
INFO: Open of `new.json'
using driver `GeoJSON' successful.
Layer name: lines
Geometry: Line String
Feature Count: 1
Extent: (13.317425, 58.521537) - (13.438575, 58.629173)
Layer SRS WKT:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
name: String (0.0)
OGRFeature(lines):0
name (String) = (null)
LINESTRING Z (13.4147787 58.5979263 280,13.4145641 58.597658 278
Seems to work and what used to be MULTILINESTRING Z is now LINESTRING Z. Ogr2ogr with option -explodecollections creates one linestring for each part of multilinestring and copies the attributes which is probably what you want. In this example the multilinestring had only one linestring member and no explosion really happened.
If your data contained both linestrings and multilinestrings, modify the SQL
where ST_GeometryType(geometry)='MULTILINESTRING Z'
OR ST_GeometryType(geometry)='LINESTRING Z'
If you prefer to use a software with GUI, you can consider to use OpenJUMP which is a nice little program for editing vectors.
OpenJUMP can handle all sort of geometries on the same layer. You can see 4 points and on multilinestring here. Notice the symbol with two linestrings.
Select the geometry and use the geometry conversion tool.
All done. Notice the linestring symbol and WKT presentation of the converted geometry. OpenJUMP can save the result back to GeoJSON. QGIS can also open the OpenJUMP's native JML format with GDAL through "Add vector layer".
