When I look at a map, I want to know how it relates to the reality of the terrain. One of the things I learned during my Master’s in Urban Design was to use AutoCAD. I enjoyed being able to created detailed figure-grounds, especially tracing over archival maps. The challenge however was, how would I ensure that they were projected properly? It was all great to have a really (what I thought at least) pretty map in 2D, a whole exercise to actually have to project the right way. I unsuccessfully tried to use Rhino etc to create maps that were projected right. It was only last semester that I found out that one could geo-rectify maps super easily (there is a list of tutorials you can use at the end of this blog post)! There are range of softwares and website that help with georeferencing.
The question I guess is why is geo-referencing important for my project? What will it add?
The simple answer is that geo-referencing a 2D map (especially a 2D map) spatializes it in a far more real way than looking at it and comparing it with a globe/3D map etc. Especially when it comes a historical map georeferencing lets the viewer get a better sense of what used to be and compare it to how things have changed.
Geoferencing, simply put works like this: the user identifies anchor/control points on both the 2D map and the properly projected map (often times archival maps have contour data and/or labels that can be helpful in figuring out these points), the user then marks those on both maps and voila! The software/website actually stretches the 2D map to match the projected map. Depending on the accuracy of both maps, the accuracy of the corrections and distortions will vary.
Georeferencing a historical map requires a knowledge of both the geography and the history of the place you are studying to ensure accuracy. The built and natural landscapes change over time, and it is important to confirm that the location of your control points — whether they be houses, intersections, or even towns — have remained constant. Entering control points in a GIS is easy, but behind the scenes, georeferencing uses complex transformation and compression processes. These are used to correct the distortions and inaccuracies found in many historical maps and stretch the maps so that they fit geographic coordinates.[1]
In a sense this treats the control points as tack pins that pin the historical map to a three dimensional surface. For a project such as mine, a georeferenced map makes it easier to see the ways in which the planners of Norris planned the town. It makes relationships with the nearby dam and urban areas more clear. And it also gives the user the ability to look at what has changed and/or the difference between planning and implementation. For a user, a well done georeferenced map also makes the experience a lot more interactive and meaningful.
Learning georeferencing:
I must admit that at first I didn’t think I would be able to do it. So I test-tired a low-resolution map of Norris and it worked really well! Heartened by that, I am currently finishing up the high-res map georeferencing. My next hurdle is putting it on to the website (I am still figuring that out!). I shall share the link as soon as its done.
Some of the links I found particularly useful and easy are listed below:
http://programminghistorian.org/lessons/georeferencing-qgis
http://www.kristenmapes.com/georectifiedmap1/
http://history2016.doingdh.org/map-warper-tutorial/
Happy georeferencing!
[1] Jim Clifford et al, Georeferencing in QGIS 2.0 (2013). Accessed December 25, 2016. http://programminghistorian.org/lessons/georeferencing-qgis.
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