Setting the Snapping Tolerance and Search Radius
Setting the Snapping Tolerance and Search Radius
Before we can edit vertices, it is very important to set the snapping tolerance and search radius to a value that allows us an optimal editing of the vector layer geometries. <P> Snapping tolerance <P> Snapping tolerance is the distance QGIS uses to search for the closest vertex and/or segment you are trying to connect when you set a new vertex or move an existing vertex. If you aren't within the snap tolerance, QGIS will leave the vertex where you release the mouse button, instead of snapping it to an existing vertex and/or segment. <P>
- A general, project wide snapping tolerance can be defined choosing Settings > mActionOptionsOptions. (On Mac: go to QGIS > Preferences, on Linux: Edit > mActionOptionsOptions.) In the Digitizing tab you can select between to vertex, to segment or to vertex and segment as default snap mode. You can also define a default snapping tolerance and a search radius for vertex edits. The tolerance an be set either in map units or in pixels. In our digitizing project (working with the Alaska dataset), the units are in feet. Your results may vary, but something on the order of 300ft should be fine at a scale of 1:10 000 should be a reasonable setting.
- A layer based snapping tolerance can be defined by choosing Settings (or File) > mActionOptionsProject Properties.... In the General tab, section Digitize you can click on Snapping options... to enable and adjust snapping mode and tolerance on a layer basis (see Figure <A HREF="#fig:snappingoptions">15</A>).
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[clip=true, width=14cm]editProjectSnapping
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<P> Search radius
<P> Search radius is the distance QGIS uses to search for the closest vertex you are trying to move when you click on the map. If you aren't within the search radius, QGIS won't find and select any vertex for editing and it will pop up an annoying warning to that effect. Snap tolerance and search radius are set in map units or pixels, so you may find you need to experiment to get them set right. If you specify too big of a tolerance, QGIS may snap to the wrong vertex, especially if you are dealing with a large number of vertices in close proximity. Set search radius too small and it won't find anything to move.
<P> The search radius for vertex edits in layer units can be defined in the Digitizing tab under Settings > mActionOptionsOptions. The same place where you define the
general, project wide snapping tolerance.
