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. 
<IMG WIDTH="514" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" SRC="img9.png" ALT="\begin{Tip} % latex2html id marker 606 [ht]\caption{\textsc{Data Integrity}} \qg... ...ve the integrity of your data, we offer no warranty in this regard. } \end{Tip}">

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<IMG WIDTH="239" HEIGHT="11" ALIGN="BOTTOM" BORDER="0" SRC="img11.png" ALT="\begin{Tip} % latex2html id marker 614 [ht]\caption{\textsc{Save Regularly}} \qg... ...and also confirms that your data source can accept all your changes. } \end{Tip}">

<IMG WIDTH="128" HEIGHT="11" ALIGN="BOTTOM" BORDER="0" SRC="img12.png" ALT="\begin{Tip} % latex2html id marker 619 [ht]\caption{\textsc{Concurrent Edits}} \... ...e at the same time as you. The last person to save their edits wins. } \end{Tip}">

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<IMG WIDTH="193" HEIGHT="11" ALIGN="BOTTOM" BORDER="0" SRC="img14.png" ALT="\begin{Tip} % latex2html id marker 627 [ht]\caption{\textsc{Vertex Markers}} \qg... ... click on the \tab{Digitizing} tab and select the appropriate entry. } \end{Tip}">
 All editing sessions start by choosing the mActionToggleEditingToggle editing option. This can be found in the context menu after right clicking on the legend entry for that layer.<A NAME="636"></A> Alternately, you can use the <A NAME="637"></A> mActionToggleEditingToggle editing button from the toolbar to start or stop the editing mode.<A NAME="640"></A> 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 and panning with the mouse wheel While digitizing you can press the mouse wheel to pan inside of the main window and you can roll the mouse wheel to zoom in and out on the map. For zooming 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: mActionCapturePointCapture Point, mActionCaptureLineCapture Line and mActionCapturePolygonCapture Polygon
mActionAddRingAdd Ring
mActionAddIslandAdd Island
mActionSplitFeaturesSplit Features
mActionMoveFeatureMove Features
mActionMoveVertexMove Vertex
mActionAddVertexAdd Vertex
mActionDeleteVertexDelete Vertex
mActionDeleteSelectedDelete Selected
mActionEditCutCut Features
mActionEditCopyCopy Features
mActionEditPastePaste Features

Adding Features <A NAME="677"></A>

Before you start adding features, use the mActionPanpan and mActionZoomInzoom-in/mActionZoomOutzoom-out tools to first navigate to the area of interest.

Then you can use the mActionCapturePointCapture point, mActionCaptureLineCapture line or mActionCapturePolygonCapture polygon icons on the toolbar to put the QGIS cursor into digitizing mode.

For each feature, you first digitize the geometry, then enter its attributes.

To digitize the geometry, left-click on the map area to create the first point of your new feature.

For lines and polygons, keep on left-clicking for each additional point you wish to capture. When you have finished adding points, right-click anywhere on the map area to confirm you have finished entering the geometry of that feature.

The attribute window will appear, allowing you to enter the information for the new feature. Figure <A HREF="#fig:vector_digitising">16</A> shows setting attributes for a fictitious new river in Alaska.

**Figure:** Enter Attribute Values Dialog after digitizing a new vector feature
[clip=true, width=8cm]editDigitizing

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Move Feature <A NAME="704"></A>

You can move features using the mActionMoveFeatureMove Feature icon on the toolbar.

Split Feature <A NAME="708"></A>

You can split features using the mActionSplitFeaturesSplit Features icon on the toolbar.

Editing Vertices of a Feature <A NAME="712"></A>

For both PostgreSQL/PostGIS and shapefile-based layers, the vertices of features can be edited.

Vertices can be directly edited, that is, you don't have to choose which feature to edit before you can change its geometry. In some cases, several features may share the same vertex and so the following rules apply when the mouse is pressed down near map features:

Lines - The nearest line to the mouse position is used as the target feature. Then (for moving and deleting a vertex) the nearest vertex on that line is the editing target.
Polygons - If the mouse is inside a polygon, then it is the target feature; otherwise the nearest polygon is used. Then (for moving and deleting a vertex) the nearest vertex on that polygon is the editing target.

You will need to set the property Settings>mActionOptionsOptions>Digitizing>Search Radius10 to a number greater than zero. Otherwise QGIS will not be able to tell which feature is being edited.

Adding Vertices of a Feature <A NAME="724"></A>

You can add new vertices to a feature by using the mActionAddVertexAdd Vertex icon on the toolbar.

Note, it doesn't make sense to add more vertices to a Point feature!

In this version of QGIS, vertices can only be added to an existing line segment of a line feature. If you want to extend a line beyond its end, you will need to move the terminating vertex first, then add a new vertex where the terminus used to be.

Moving Vertices of a Feature <A NAME="729"></A>

You can move vertices using the mActionMoveVertexMove Vertex icon on the toolbar.

Deleting Vertices of a Feature <A NAME="733"></A>

You can delete vertices by using the mActionDeleteVertexDelete Vertex icon on the toolbar.

Note, it doesn't make sense to delete the vertex of a Point feature! Delete the whole feature instead.

Similarly, a one-vertex line or a two-vertex polygon is also fairly useless and will lead to unpredictable results elsewhere in QGIS, so don't do that.

Warning: A vertex is identified for deletion as soon as you click the mouse near an eligible feature. To undo, you will need to toggle Editing off and then discard your changes. (Of course this will mean that other unsaved changes will be lost, too.)

Add Ring <A NAME="738"></A>

You can create ring polygons using the mActionAddRingAdd Ring icon in the toolbar. This means inside an existing area it is possible to digitize further polygons, that will occur as a 'whole', so only the area in between the boundaries of the outer and inner polygons remain as a ring polygon.

Add Island <A NAME="742"></A>

You can mActionAddIslandadd island polygons to a selected multipolygon. The new island polygon has to be digitized outside the selected multipolygon.

Cutting, Copying and Pasting Features <A NAME="746"></A> <A NAME="747"></A> <A NAME="748"></A> <A NAME="749"></A> <A NAME="750"></A> <A NAME="751"></A>

Selected features can be cut, copied and pasted between layers in the same QGIS project, as long as destination layers are set to mActionToggleEditingToggle editing beforehand.

Features can also be pasted to external applications as text: That is, the features are represented in CSV format with the geometry data appearing in the OGC Well-Known Text (WKT) format.

However in this version of QGIS, text features from outside QGIS cannot be pasted to a layer within QGIS. When would the copy and paste function come in handy? Well, it turns out that you can edit more than one layer at a time and copy/paste features between layers. Why would we want to do this? Say we need to do some work on a new layer but only need one or two lakes, not the 5,000 on our big_lakes layer. We can create a new layer and use copy/paste to plop the needed lakes into it.

As an example we are copying some lakes to a new layer:

Load the layer you want to copy from (source layer)
Load or create the layer you want to copy to (target layer)
Start editing for target layer
Make the source layer active by clicking on it in the legend
Use the mActionSelectSelect tool to select the feature(s) on the source layer
Click on the mActionEditCopyCopy Features tool
Make the destination layer active by clicking on it in the legend
Click on the mActionEditPastePaste Features tool
Stop editing and save the changes

What happens if the source and target layers have different schemas (field names and types are not the same)? QGIS populates what matches and ignores the rest. If you don't care about the attributes being copied to the target layer, it doesn't matter how you design the fields and data types. If you want to make sure everything - feature and its attributes - gets copied, make sure the schemas match.

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Deleting Selected Features <A NAME="768"></A>

If we want to delete an entire polygon, we can do that by first selecting the polygon using the regular mActionSelectSelect Features tool. You can select multiple features for deletion. Once you have the selection set, use the mActionDeleteSelectedDelete Selected tool to delete the features. There is no undo function, but remember your layer isn't really changed until you stop editing and choose to save your changes. So if you make a mistake, you can always cancel the save.

The mActionEditCutCut Features tool on the digitizing toolbar can also be used to delete features. This effectively deletes the feature but also places it on a ``spatial clipboard". So we cut the feature to delete. We could then use the mActionEditPastepaste tool to put it back, giving us a one-level undo capability. Cut, copy, and paste work on the currently selected features, meaning we can operate on more than one at a time.

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Snap Mode <A NAME="782"></A> QGIS allows digitized vertices to be snapped to other vertices of the same layer. To set the snapping tolerance, go to Settings>mActionOptionsOptions->Digitizing. (On Mac: go to QGIS > Preferences, on Linux: Edit > mActionOptionsOptions.) Note that the snapping tolerance is in map units or pixels.

Saving Edited Layers <A NAME="792"></A>

When a layer is in editing mode, any changes remain in the memory of QGIS. Therefore they are not committed/saved immediately to the data source or disk. When you turn editing mode off (or quit QGIS for that matter), you are then asked if you want to save your changes or discard them.

If the changes cannot be saved (e.g. disk full, or the attributes have values that are out of range), the QGIS in-memory state is preserved. This allows you to adjust your edits and try again.

Editing an Existing Layer

Editing an Existing Layer

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