Digitizing History
The ability to georeference historical documents and images allows for incorporation of imagery into map space. Once transformed into a map space, features within these documents may be digitized, turning imagery into usable spatial information.
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Landmarks and narrow gauge rail roads in Tacoma, Washington circa 1891 were digitized after georeferencing a historic map. These features are now available for further analysis, such as rail line 'Network Analysis,' or spatial studies of 19th century Tacoma.
Using historical maps to digitize features
Georeferencing
Images and historic documents typically do not contain spatial referencing information. Digital imagery may have latitude, longitude and altitude information for the point location of the image, but the extent, and features within the document must be referenced.
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Georeferencing is the process of giving an image or raster assigned coordinate points to locations within it. This allows rasters (or image) to exist in a map space, and be transferred between coordinate systems. The process transforms images and documents into spatially useful data.
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Ground Elevations & Building Heights
Prior to georeferencing, the map document was scaled, rotated and shifted until map features were aligned to their spatially correct areas.
Once scaled, control points on the map were connected to their corresponding locations on the image. In this case, roads were particularly useful when their names had not changed in the last century. Water bodies were also useful.
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As points were georeferenced, residual errors are calculated, which give a measure of the distance from where control points were assigned, and where they ended up using first order polynomial transformations. In this analysis, the total root mean error (RMS) was kept below 45.
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A second order polynomial transformation allowed for an RMS of below 20.
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Some distortion remained, particularly noticeable on the outer reaches of the map space, but was within reason for this project.
Digitizing Landmarks & Railways
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Features can be digitized using a variety of tools. Polygons were used for landmarks, and polylines for rail roads.
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Lines were drawn such that they were the best approximation of the center of each hand line.
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Curved lines were given extra care to ensure minimal pixelation in the final shapefile.
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15 landmark features were digitized
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10 Rail lines were digitized. Rail roads were given names based on what they were named within the map document, or by referencing street or line termination cities.
Digitized landmarks along Tacoma's Historic Waterfront District.
Digitized narrow gauge rail raods along Tacoma's Historic Waterfront District.
Complete Digitization
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Digitized features were given name attributes, and rail roads were given origin and destination attributes.
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The complete data sets are now shapefiles, available for use in GIS and in any coordinate system.
To explore this data using ArcGIS software please click on the link below and download this analysis as a map package.
Resources/References
Cartographer/GIS Analyst: Tracie S. Barry Contact: barryts@uw.edu
Created: May 19, 2019
Software: ArcMap 10.6.1
Projected Coordinate System:NAD 1983 HARN StatePlane Washington South FIPS 4602 Feet
Data Sources: UWT GIS Certificate Program