How to visually sort the order of Points with QGIS, GDAL, ...?

Question

The following Points layer is the basis of a rendering in a Android application.

Android allows to draw a shape from the provided points. I cannot use LineStrings.

However, the order of points exported from QGIS seems to be odd.

Is there any way to visually sort the order of points in QGIS or using the GDAL tools? The order of feature ids in the final export format (such as CSV) needs to be kind of a roundtrip as visualized in the following picture. If there are multiple paths or a "hole" it is perfectly okay to have multiple point layers.

Export experiments

I tried different steps to get hold of the latitude/longitude coordinates:

1. QGIS: Export points layer as CSV, add layer create options: GEOMETRY=AS_YX
2. Convert into Android specific format JSON format using regular expressions.

This has the disadvantage that the feature id is still included. The order of points however is not as desired.

1. ogr2ogr -f CSV output.csv input.shp -lco GEOMETRY=AS_YX
2. Convert into Android specific format JSON format using regular expressions.

Here the feature id is already omitted. The order of points however is not as desired.

1. QGIS: Export points layer as GML
2. ogr2ogr -f "CSV" -select "" output.csv input.gml -lco "GEOMETRY=AS_YX"
3. Convert into Android specific format JSON format using regular expressions.

The order of points however is not as desired. The order of points is different to what the former exports/conversions output.

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The original layer is also available as a LineString layer.

Answer 1

Because the points for each id are already sorted, you can solve with:

ogr2ogr -dialect SQLite -sql "SELECT MakeLine(geometry), id FROM Muenster GROUP BY id" Muenster_lines.shp Muenster.shp

Then you can easily join the single lines and obtain the result path, as shown below:

EDIT

I think that it's not possible to sort all the points without defining a route based on a network topology. At the moment, you can only "order" the points with the same id calculating their FID, a special field of OGR SQL. For instance:

ogr2ogr -where "id='5967589'" Muenster_5967589.shp Muenster.shp
ogr2ogr -f CSV -sql "SELECT FID FROM Muenster_5967589" -lco GEOMETRY=AS_XY Muenster_5967589.csv Muenster_5967589.shp

This is the content of Muenster_5967589.csv:

X,Y,FID,
7.6175577,51.9619021,0
7.617543,51.9620712,1
7.617544,51.9623006,2
7.617577,51.9624964,3
7.6176213,51.9626584,4
7.6177773,51.9631274,5
7.617828,51.9634665,6
7.6178088,51.9638744,7
7.6177495,51.9647474,8
7.6176424,51.9652417,9

Answer 2

The same result is achieved with the Point2One plugin (and you can sort the points)

Result:

Answer 3

I tried this in QGIS:

1. Load the polyline data from OSM
2. Reproject it to UTM 32N
3. Vector -> Geoprocessing -> Buffer with a value of 10 (metres) for Buffer distance.
4. Vector -> Geoprocessing -> Dissolve using dissolve all will lead to one polygon
5. Vector -> Geometry Tools -> Multipart to single part creates one polygon for every LEZ with a hole in the middle
6. Vector -> Geometry Tools -> Polygons to Lines
7. Vector -> Geometry Tools -> Lines to Polygons
8. Vector -> Geoprocessing -> Dissolve using dissolve all will remove the inner ring
9. Vector -> Geometry Tools -> Multipart to single part creates one polygon for every LEZ without hole
10. Using Field Calculator, create unique ids with a new field filled with $id
11. Install the MMQGIS plugin
12. MMQGIS -> Import/Export -> Geometry Export to CSV

For checking:

• Import as Delimited text layer
• Install Points2One plugin
• Join the points to a line with Points2One

This works for Münster, but may fail on other cities (like Frankfurt, where a segment of the border is missing).