[GIS] Line layer not displaying correctly in QGIS

layerslineqgis

I am having issues displaying a .shp file correctly in QGIS 2.12.3.

Sometimes when I add it to a map, it displays like the red lines in the photo below (which is how it should be). However, sometimes it displays as the gold lines in the other photo below (notice there are several lines missing in the top-center section).

I am afraid there is an issue with the geometry of the layer, but I haven't figured out how to diagnose it or fix it.

Note that the layer was created by joining together three different line layers. Originally these layers were not in the same projection, so I couldn't join them together. I thought I was able to change the layers to the same projection and join them together and thus successfully display the correct layer, but I fear I may have done something wrong.

Note that the red lines below only represents one of the original layers that was joined. So the remaining two component layers aren't always appearing as part of the combined layer as they should.

Best Answer

Since you noticed you did something wrong, you have to re-do the join.

Most probably you used Set Layer CRS or Assign Projection, which does NOT reproject correctly.

Use Save As ... to a different filename and a common CRS for all layers. Then remove the original layers, and turn On-the-fly-reprojection OFF to see if they still align. If yes, and both share the same column names in the attribute table you can safely merge them in one file with

Select all features in the first layer
Edit -> Copy features
Switch the second layer to edit mode
Edit -> Paste features

BTW By now, QGIS 2.16.1 is the current version, I suggest to upgrade.

Related Solutions

QGIS – Merging Lines with and without Overlaps

@oskarlin, I tried to recreate a similar example of your layers. However, I do not know whether your Layers consist of line segments (polylines) or continuous lines (monolines), therefore there are two cases that define the workflow.

CASE 1. When Layers (polylines) consist of segments, i.e. the polyline is split into sections at each vertex or intersection node, see the image below.

'Layer_2', as well as 'Layer_1', includes two fields, namely "id" and "Value".

For merging two shapefiles, you need to proceed with Vector -> Data Management Tools -> Merge vector layers. Moreover, there are other solutions available, e.g. the MMQGIS plugin does the job (MMQGIS -> Combine -> Merge layers) or SAGA GIS Merge Layers Module from the Processing Toolbox.

It will provide you with a new polyline layer that includes geometries and attributes of polylines, 'Layer_1' and 'Layer_2' accordingly, see the graphics below. 2. Then in a new shapefile create a column that defines the overlap value of your layers, e.g. string data type with a length of 1. 3. In Expression dialogue execute the following expression

    CASE
       WHEN sum(num_geometries($geometry), group_by:= geom_to_wkt($geometry)) > 1 THEN 'Y'
       ELSE 'N'
    END

'Y' will give you all overlapping lines, and 'N' lines that do not overlap.

P.S. If there is a difference in any part of the geometry, this expression fails to match them. @Kazuhito comment

As you can notice from the image below, each Layer has only two line segments where exists an overlap. Moreover, you can easily distinguish between line segments based on values from the Attribute Table.

At this last step, you will need to delete the duplicates based on geometry, however, I am not sure whether you have to remain the "Value" field in case it plays a primary role in your project.

When it does not, you can achieve the desired result with Delete duplicate geometries tool, GRASS v.clean tool with enabled rmdupl option from the Processing Toolbox, or the MMQGIS plugin(MMQGIS -> Modify -> Delete Duplicate Geometries).

CASE 2. When your layers contain continuous lines, see the image below.

It's just a matter of running a few more tools.

First, add a field to your original layers with an identifier that will mark features as being from that original layer when they are all merged.
Second, merge the layers. (Check possibilities above)
Then use the Union geoprocessing tool (Vector -> Geoprocessing Tools -> Union) to split features into overlapping and non-overlapping parts. The non-overlapping parts of lines will still be one multipart feature so if you want each segment to be separated, you'll need to run the Multipart to Singleparts... tool (Vector -> Geometry Tools -> Multipart to singleparts).
Finally, create the overlapping field and populate it using the above instructions.

P.S. Answer was completed with the help of @Gabriel C..

References:

QGIS – Dissolving Features Based on Another Layer

You can use the tool Overlap analysis:
Processing toolbox > Vector Analysis > Overlap analysis

This will give you the total area and percentage of each parcel covered by buildings.

Best Answer

Related Solutions

QGIS – Merging Lines with and without Overlaps

QGIS – Dissolving Features Based on Another Layer

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