In the Merge shapefiles dialog, you have the option to choose whether you wish to merge all shapefiles in a folder or pick individual shapefiles to merge:
Depending on how your shapefiles are stored, you can do either of the following:
Keep Select by layers in the folder unchecked to merge all shapefiles in a directory
Check Select by layers in the folder to select individual files to merge
If the previous option is enabled, choose the shapefile type (Point, Line, or Polygon).
Set the input directory/files by clicking on Browse.
Name the output shapefile by clicking on Browse.
Choose whether you wish to select Add result to map canvas.
With the Polygon Centroids tool, which can be found by navigating to Vector | Geometry Tools, you can generate points that will be located at the center of polygons. Simply provide the input polygon layer and name the output. In the following example, centroids of the Neighborhoods_pdx.shp shapefile have been generated. This tool preserves all the attributes during the conversion. With the data in point form, you can generate a heat map, compute densities, or measure distances:
Converting polygons to lines and lines to polygons
With the Polygons to Lines and Lines to Polygons tools, which can be found by navigating to Vector | Geometry Tools, you can convert between these two geometry types. There are many reasons why this conversion may be desirable. In the following example, we are provided with a slope layer in a polygon format. Here, we will convert it to a line geometry so that it can be styled as contour lines. All the attributes will be maintained during either conversion. Conversions from line to polygon are necessary if the area needs to be calculated. An example of this is parcel data from a CAD program that is provided to you as a line layer.
There are two tools for generating polygons around individual points in a layer: Voronoi and Delaunay triangulation. Voronoi polygons represent the area of influence around each point. These are named after the Russian mathematician Georgy Voronoy, who invented the algorithm. They are also referred to as Thiessen polygons and are named after Alfred Thiessen, who independently created the same algorithm. You can use the Voronoi polygon in QGIS by navigating to Vector | Geometry Tools | Voronoi Polygons. The resulting polygon represents the area closer to the point used as the input than to any other points in the layer. The Delaunay triangulation tool can be found by navigating to Vector | Geometry Tools | Delaunay Triangulation.
Delaunay triangulation creates a series of triangular polygons. The method creates a triangle in such a way that a circle drawn through the three nodes of the triangle will contain no other nodes. This is the same technique that is used to generate triangulated irregular networks (TINs).
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