On the 23rd of November, our class met in the Center for Digital Scholarship at the NYUAD Library, to discuss and exercise on georeferencing. Georeferencing is the process of matching the coordinates of a map or aerial photo with an existent set of coordinates (e.g. an old map can be matched – overlapped, superimposed – with the occupying reality of the old map’s geographic coordinates).
Matt Sumner a Data Services Librarian at NYUAD lead the ArcGIS workshop concomitantly with the class discussions on georeference, data comparison, and limitations of such a process.
What a georeferencing software does to maps is very similar to what an OCR software does to images: it makes them “digitally useful.” This means that the resulted digital map can be engaged in other digital exercises: it can be compared with other digitized maps or, as I said above, overlapped with them. Doing so helps creating a visualization of the land transformations in time. We can see whether the borders between two countries have stayed the same, for example. Or, we can look at an evolution of land distribution over time (for climate change research purposes). Using a georeferencing software (such as ArcGIS) helps doing all that. During the class, Matt showed us which are the steps for doing so and georeferenced an old Soviet map of the UAE.
The maps we were working with (already scanned and ready to be used) were easy to work with because each of the four corners of them had the geographic coordinated written down. From there, all we had to do was to match the current coordinates with the corners of the images. However, in case we didn’t have the coordinates available, what we would have had to do was to find reference points (a building, a street, the center of the city) that could help with the overlapping. After both the current map and the old map were imported, the workspace looked like this:
In order to place the map on the contour, we had to create four referencing points which would be placed at the coordinates specified at the corners of the image. Then, once these were created, the last step was to match each point with its corresponding corner and …
Voila! With very little disagreement (because each corner pulls the image towards itself), the two objects overlapped! Now it’s time to look how “things” changed over time.
The Soviet map was created in 1978 with materials from 1975, while the red contour is not older than ten years. We can clearly notice some changes (and imagine if we had the physical map and not just a contour how many changes we could have spotted!); we can see how the lakes have diminished in size, as well as the fact that the coastline has expanded.
Matt had more scans like this (and he has also placed all the other necessary reference points before), so, in a few minutes while me and my classmates were discussing about the interesting things we learnt, he created this:
As a fun exercise, and because we were lucky to have a copy of it, we also georeferenced a map of the city of Abu Dhabi dating back somewhere between 1964 and 1971. The map provides a simplified shape of the city, together with important landmarks and streets which helped us georeference it (because we had no coordinates for it).
We’ve learned some interesting facts about mapping in general and about georeferencing during the ArcGIS workshop. Some things that I found note-worthy are:
- people believe what a map says. Although, we have seen that this might not always be 100% accurate;
- in georeferencing, we have to make compromises and choose between fitting the entire map in a shape and having “disagreements” with the contour, or perfectly fitting a part of it and neglecting the rest;
- a map can be seen as a “purposeful simplification of reality.”