Data Sources
From GisWiki
There are many useful datasets available for minimal cost, either on CD or available for download. Data is available in a variety of formats, fortunately free software tools to reformat the data are available if needed.
Without getting into any detail on projection issues (a very complex subject), these data may also be in differing projections and use coordinate systems based on different datums. Software is also available to reproject these data, in the event of your GIS software not supporting on-the-fly reprojection.
Geographic data is available in two broad categories, raster and vector. Raster data comprises a grid, such as the pixels in an aerial photograph or the values in a gridded elevation model. Vector data consists of spatial features, such as points, lines and polygons. A spatial vector feature mat have non-spatial attributes, eg, a road may have associated data such as surface material, number of lanes, name, etc.
In some cases raster data can be used to derive vectors, and vice versa. For instance, an aerial photograph could be used to derive lines representing roads, polygons representing lakes or points representing lighthouses. Alternatively, a vector dataset of elevations (heights) can be used to create a gridded elevation model, which in turn can be used to derive elevation contours.
