In this blog, I first chose a map of Northfield between 1849 and 1859, when Minnesota was designated as a territory from Minnesota Digital Library. Then, I used Allmaps Editor to match the location points in the old map to the new one. Specifically, I chose some notable points like Central Park and locations near the river. Then, the Allmaps editor helped me automatically overlap the old map and the new map at a precise place. You can see my result through Allmaps Viewer. Also, Allmaps contains XYZ tiles.
Once rectified, the map becomes available in different formats, like a WMS link for web embedding or a GeoTIFF for professional GIS analysis. This flexibility is useful for various applications. For instance WMS link can let users can freely zoom in and out and pan the map to interact with author’s narrative content, which is much more vivid than static images.
One possibility I came up with is to Superimpose the corrected historical map with modern satellite images to create a time slider for urban development. Also, we can mark the location information mentioned in the literature on the corrected map and conduct spatial analysis. A logical next step would be to rectify maps from different time periods and compare them in a series.
Howeverm this tool still has its disadvantage. Since, the urban management changed so quickly during these decades, historical maps were often inaccurate by modern standards, and manually picking control points introduces more error. In areas where the coastline changes or the topography has completely altered, it is particularly difficult to find reliable reference points. You can see in my picture, the coastline is not perfectly matched.
Hence, georectifying isn’t suitable for all research. It is ineffective for maps that do not require geographical accuracy, such as symbolic or religious maps. Moreoever, it less appropriate for research demanding extremely high precision or for maps covering vast regions like continents.
Tags: Georectifing map, Digtal Humanities, Allmaps
