5 WORKING WITH VECTOR DATA Search radius Search radius is the distance QGIS uses tosearch 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 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. The search radius for vertex edits in layer units can be defined in the Digitizing tab under Settings >, Options . The same place where you define the general, project wide snapping tolerance. 5.4.2 Topological editing Besides layer based snapping options the General tab in menu Settings ->, Project Properties. . . also provides some topological functionalities. In the Digitizing option group you can x Enable topological editing and/or activate x Avoid intersections of new polygons . Enable topological editing The option x Enable topological editing is for editing and maintaining common boundaries in polygon mosaics. QGIS ‘detects’ a shared boundary in a polygon mosaic and you only have to move the vertex once and QGIS will take care about updating the other boundary. Avoid intersections of new polygons The second topological option called x Avoid intersections of new polygons avoids overlaps in polygon mosaics. It is for quicker digitizing of adjacent polygons. If you already have one polygon, it is possible with this option to digitise the second one such that both intersect and qgis then cuts the second polygon to the common boundary. The advantage is that users don’t have to digitize all vertices of the common boundary. 5.4.3 Editing an Existing Layer By default, QGIS loads layers read-only: This is a safeguard to avoid accidentally editing a layer if there is a slip of the mouse. However, you can choose to edit any layer as long as the data provider supports it, and the underlying data source is writable (i.e. its files are not read-only). Layer editing is most versatile when used on PostgreSQL/PostGIS data sources. QGIS 1.0.0 User, Installation and Coding Guide 46 5.4 Editing Tip 12 DATA INTEGRITY It is always a good idea to back up your data source before you start editing. While the authors of QGIS have made every effort to preserve the integrity of your data, we offer no warranty in this regard. Tip 13 MANIPULATING ATTRIBUTE DATA Currently only PostGIS layers are supported for adding or dropping attribute columns within this dialog. In future versions of QGIS, other datasources will be supported, because this feature was recently implemented in GDAL/OGR >, 1.6.0 All editing sessions start by choosing the Toggle editing option. This can be found in the con- text menu after right clicking on the legend entry for that layer. Alternately, you can use the Toggle editing button from the toolbar to start or stop the editing mode. Once the layer is in edit mode, markers will appear at the vertices, and additional tool buttons on the editing toolbar will become available. Zooming with the mouse wheel While digitizing you can use the mouse wheel to zoom in and out on the map Place the mouse cursor inside the map area and roll it forward (away from you) to zoom in and backwards (towards you) to zoom out. The mouse cursor position will be the center of the zoomed area of interest. You can customize the behavior of the mouse wheel zoom using the Map tools tab under the Settings >, Options menu. Panning with the arrow keys Panning the Map during digitizing is possible with the arrow keys. Place the mouse cursor inside the map area and click on the right arrow key to pan east, left arrow key to pan west, up arrow key to pan north and down arrow key to pan south. You can also use the spacebar to temporarily cause mouse movements to pan then map. The PgUp and PgDown keys on your keyboard will cause the map display to zoom in or out without interrupting your digitising session. You can perform the following editing functions: • Add Features: Capture Point , Capture Line and Capture Polygon • Add Ring • Add Island QGIS 1.0.0 User, Installation and Coding Guide 47 Source.