As mentioned in my previous posts about visualization, cartography and analysis, QGIS is easy-to-install, integrates with OpenGeo Suite, and has reliable support offerings, making it a viable alternative to proprietary desktop GIS software such as Esri ArcGIS for Desktop. While editing spatial data is not something that everyone does, it is definitely an important component of desktop GIS. So how does QGIS perform when it comes to editing data?
Strength: Creating Shapefiles
To start off with, the simple act of creating a new shapefile is actually a bit easier in QGIS than in other software. For example, in ArcGIS the typical workflow is to open a folder catalog to locate a folder, then right-clicking and choosing New > Shapefile. The workflow in QGIS is to simply click the New shapefile layer button (a), put in some of the details, and then save it in a standard Save As dialog. Adding new features to the new layer is straightforward in QGIS. When the new shapefile is added you click the toggle editing button (b) and then add a new feature with the Add feature button (c).
Once digitizing of the new feature is complete, you must right-click to finish the feature. This triggers a pop-up window that displays the attributes of the new shapefile and allows you to fill in the attributes. This prompting for attributes after each completed feature could be cumbersome for some workflows, especially those that involve many features that will all have the same attributes, but it is more intuitive for smaller, simpler jobs. A little sleuthing, however, uncovers a setting for suppressing this pop-up under Settings > Options > Digitizing.
Strength: Copying & Pasting
Copying and pasting of features from one dataset to another is actually quite easy in QGIS. In fact, it’s so easy that you may be able to accomplish it without consulting any how-tos (an important productivity enhancer in my opinion). It’s intuitive: click the layer that you will be selecting features from in the table of contents, select the features in the map or in the table and hit Ctrl-C (⌘C on OS X), then select the layer you want to copy to, click the edit button (pencil icon), then hit Ctrl-V (⌘V on OS X). As a bonus, you can also copy and paste WKT strings to and from QGIS.
Strength: Snapping and Topological Editing
Snapping is also easy, intuitive, and familiar, though new users might be surprised that it is turned on by default. This may cause some confusion if the snap tolerance is too high, making it seem like new features can’t be digitized without sharing a vertex with an existing feature. The snapping tolerances can be adjusted in the Snapping options found in the Settings menu (a). When editing existing polygons, it is often useful to make sure that shared boundaries move together. To accomplish this in QGIS, check Enable topological editing in that same snapping dialog (b). Once checked, it will detect shared boundaries and move them together.
Strength: Working with Vectors
In proprietary GIS software, converting lines to polygons is typically only possible in advanced versions of the software but QGIS makes it easy to accomplish. It is fairly straightforward to use the Lines to polygons tool that comes out-of-the-box in QGIS under the Vector menu. Tracing existing features is also possible via the QGIS plugin called AutoTrace.
Mixed Results: Feature Templates
Feature templates are short-cuts that allow you to digitize easily by self-populating “type” attributes in a layer (e.g., road, sidewalk, tunnel). Feature templates aren’t currently available in QGIS. If you digitize many different feature types at once then QGIS may slow your workflow as you’d have to manually input each attribute as you finish each feature.
Hidden Gem: CadTools
Some of the advanced feature construction tools that you might be familiar with such as points at invisible intersections, regular arcs, Bézier curves, and trace aren’t available in the core QGIS software but they are available in plugins. One of the most notable plugins for advanced feature construction is CadTools. CadTools provides 13 advanced tools for workflows that require high accuracy at large scales.
While I set out with fairly low expectations regarding QGIS editing, my impression of the QGIS editing tools has been outstanding. If the basic tools aren’t enough, the plugins come to the rescue with advanced functionality. As with visualization, cartography and analysis, it’s clear that the QGIS developers are cognizant of the demands that we GIS professionals put on our software in terms of needing exacting, fool-proof, and robust editing tools and they have made great progress in meeting those needs.