Georeferencing allows you to add spatial data to a scanned flat map.  Using software like ArcGIS, an image is scanned and imported, and spatial data is added by lining the image up with an existing source (for example, a city boundary shapefile and a streets shapefile) by assigning anchor points to each map.  The scanned image can be manually stretched, scaled and shifted to line up as much as possible with the base map, and each anchor point will further align the scanned image.  When the scanned image and the base map line up to the user’s satisfaction and with minimal error, the data is exported into a single file that now contains a georeferenced image – i.e., the scanned map now has spatial data and can be incorporated into any number of mapping applications. The 2nd order polynomial total Root Mean Square error is below 20 for 9 control points. This was adequate minimal error for this exercise.

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After georeferencing the historic 1891 Tacoma map to a Tacoma boundary shapefile and streets shapefile, I digitized all narrow gauge railroads and landmarks from this historic map. I use Esri ArcGIS Online to visualize these historic railroads and landmarks.

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