Home
| Index | Software | Mashup HowTo | Georeferencing | Reprojecting | QGIS #1:Digitizing | #2: Projection & Alignment
Note 1: 24 June 2008
As of summer 2008, QGIS is now at version 0.10.0, nicknamed "Io". I have used it to create shapefiles of land use in Kabul, overlaid on a scanned map and various satellite photos saved as GeoTIFFs. I start with this UsageNote about digitizing because I figure you could read all sorts of theory and intros, but you would prefer to immediately use the program.
Above: opening screenshot of QGIS on the Ubuntu 8.04 system. I have arranged the toolbars as I like them; your default first view will have a different toolbar arrangement. 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:
When you left-click on the color box in the "Layer Properties" box, it pops up another dialog from which you can pick colors:
QGIS can edit shapefiles directly, interactively. To start digitizing, highlight the shapefile 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 must) 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.
Those of you who have used CAD software or Adobe Illustrator will know the value of turning layers on or off, dragging them up or down, and turning on or off the snapping capabilities of a layer. Once the snapping tolerances are adjusted, I have found QGIS to be the best onscreen digitizing interface I have ever used.
NEXT: setting your projection and projection options to make sure that all the layers show up.