How do you get a paper map into your GIS? Part 2
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In previous videos I’ve spoken about points, lines and polygons, where you get GIS data from, and how to get a paper map into a GIS using a digitizer. In this video I talk about how to get a paper map into a GIS using a scanner.
Scanning converts your map features into image pixels. The higher the resolution of the scanned image (more dots per inch (DPI)), the smoother and more accurately defined the data will appear. As the DPI increases, so does the file size. You need to experiment with DPI on a map-by-map basis, but a scan resolution of 300-400 DPI is usually sufficient. Scanning is the way I bring data into my GIS projects these days. The way I process the scan depends on the complexity of the map and nature of the project.
There are three broad approaches to processing a scanned map in a GIS. At some point each involves georeferencing the scan (placing it into the same coordinate system as the other maps in your GIS)…
- Use the scan as a backdrop: You could just use the scanned georeferenced image as a backdrop to other maps in your GIS. One advantage of this is that the cartographic tricks contained in the original map don’t get lost in the GIS. Although OK, this is not really a GIS approach because a scanned image is “dumb” meaning that it can’t be queried with a mouse!
- On-screen digitize: Just use the scanned georeferenced image as a backdrop to other maps in your GIS and then on-screen digitize from it.
- Vectorize:Vectorizing simply involves converting scanned data into vector data. Once converted within a GIS environment, the data is identical to a GIS map created using a digitizing tablet. Vectorization sounds simple and appealing but it rarely is. Often datasets require a lot of editing after conversion. Take the case of a map with roads and rivers…
- They get confused with each other because both roads and rivers look the same to the vectorization software interpreting the scan.
- Where roads and rivers cross each other, a gap gets created in one-or-other of the themes. This problem occurs even with sophisticated software that could color-separate blue rivers and black roads
One way around these two problems is to trace each map theme (roads, rivers, lakes, etc) onto individual pieces of tracing paper (preferably stable-base Mylar). You then scan, georeference and vectorize each individual scan.
Next video: Hmmmm, I'm open to suggestions! Dont forget to signup for my FREE eBook if you haven't already!!!
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