Georeferencing: moving analog maps into modern-day GIS
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Washington/Canadian-Media: The technology in the process of Georeferencing or adding digital spatial reference information to an otherwise non-spatial image is explained by Meagan Snow, Geospatial Data Visualization Librarian in the Geography and Map Division of the Library of Congress (LoC), LoC reports said.
The addition of spatial reference information to a scanned map image facilitates the alignment of the map image correctly with the geographic features it was built to represent.
This enables a user to layer any other spatial data file alongside (or on top of) their map image.
Snow makes use of the following 1967 map of the US Capitol grounds as an example.
This map shows properties under the jurisdiction of the Architect of the Capitol in 1967. The Madison Building of the Library of Congress, home to the Geography & Map Division is missing from this map. The comparison of this 1967 map to today’s Capitol Hill Complex reveals how the area has changed over time.
Maps that are scanned as image files, explains Snow and meets the criteria for what is called raster data: data composed of a continuous grid of cells (or pixels).
The fact that spatial data can commonly be stored in a raster format enables scanned map images to be loaded directly into GIS software without any file conversions needed.
It is the presence of “spatial reference information” in GIS software that enables the Geographic data layers to align correctly when viewed through the software allows a user to manually add control points between the non-spatial scanned map image and a pre-existing GIS data layer that already has spatial reference information and displays correctly in GIS software.
Georeferencing tools (available in all of the most widely used GIS software options) provided by the software package they are using allows a user to place a control point by selecting a specific point on the scanned map image and then selecting the exact same point on the GIS layer. Once the user adds a couple of control points, the scanned map image will begin to align with the existing data layer scale.
But for the rest of the map to be aligned, a user must continue to add control points, making sure they are well-distributed across the map image until it is determined that the two layers are aligned properly. Here’s what the map looks like after 21 control points have been placed.
The georeferencing can be saved after the completion of the process enabling the scanned map image to go to the right place in the world whenever it is loaded into GIS software.
In the lower right-hand corner of the map, we can now see the current aerial footprint of the Madison Building where it was missing.
A georeferencing a scanned image has many uses, the primary reason being it allows a map user to view the map in geographic context with any number of other spatial data sources and further enables the user to compare maps created at different scales or in different times periods.
The users are also facilitated to see an older map juxtaposed against current aerial imagery or spatial data as well as to use the scanned map image as a basis for the creation of new spatial datasets.