This page is a basic introduction to on-screen digitizing in QGIS. This page was originally created on 24 June 2008, with screenshots from QGIS version 0.10 “Io,” running on Ubuntu Linux. QGIS has advanced through numerous updates since this page was created, but the basic concepts remain the same.
I used QGIS to create shapefiles of land use in Kabul, overlaid on a scanned map and various satellite photos saved as GeoTIFFs.To add GeoTIFFs and other sorts of raster data (satellite photos, scanned maps, etc), go to: Layer -> Add a Raster Layer…; to add shapefiles go to Layer -> Add a Vector Layer…
Above: screenshot of QGIS with my working files of Kabul loaded. Note that I say files; what you see are 8 vector files (.shp format) overlaid on a raster file (GeoTIFF format). Each file, either a Shapfile or GeoTIFF, shows up as a layer on the left column. Its visible data shows up in the main pane. The ‘Properties’ of the shapefiles are set so that these layers are 40% transparent. This helps while digitizing. When you right-click on the layer name in the Legend pane, you can select the “Properties” dialog from the drop-down menu that appears:
You can also adjust the color and weight of lines and edges, and the color of polygon-fills. Later versions of QGIS enable you to make very refined adjustments, but a detailed discussion of Layer Properties is beyond the scope of this tutorial. The point here is that you can see through translucent layers to manually digitize over raster images.
Unlike Illustrator or other vector graphics programs, you cannot simply start clicking away in the view window to input graphic data. To start digitizing, highlight the shapefile (layer) in the Legend pane, and then click the blue pencil icon to toggle into editing mode:
To make polygons, choose “Capture Polygon”: (I don’t think that is a very intuitive label for the function, but now you know)
When creating polygons, you can (and should) adjust the Snapping Options of the layer. That is a good thing, because the snapping controls can really fine-tune your on-screen digitizing. But it takes a lot of fussing to get it adjusted. First go to Settings -> Project Properties… and within that dialog box, click the “General” tab; then click the “Snapping Options…” sub-dialog box:
Within this dialog, you need to do three things:
1. Check the box at the left of each layer to enable the snapping options on that layer.
2. Under “Mode”, choose whether to snap to vertex, to segment, or to vertex and segment.
3. Under “Tolerance”, set the sensitivity. Note that I have set it to a very small number! Adjust to your needs.
Once the snapping tolerances are adjusted, I have found QGIS to be the best onscreen digitizing interface I have ever used.
MOST IMPORTANT!!! Once you have added or edited data in a layer, you need to SAVE YOUR WORK by clicking the Save button (now on the Digitize toolbar) or by exiting Edit Mode by clicking the Blue Pencil icon. Why? Because each layer is actually a freestanding shapefile, and this step actually modifies that separate file. Simply clicking [Ctrl +S] in QGIS will only save the display and projection properties of the QGIS workspace.
Like in CAD and graphics programs, the layers can be moved up and down, and turned on and off. You can also turning on/off the snapping settings for each layer.