In this CDS demonstration we learned how to Georeference and rectify maps to the correct latitudes and longitudes so that multiple maps can be layered together. We learned multiple ways to do this, including with and without having reference points in the layered map. Both ways gave us a semi-reliable way to place maps over each other and relate certain mappings to one another.
I don’t think that we will be using this in our blog, as we don’t have much of a need for map layering or rectifying maps to a specific area, but it could be very interesting to use this to show how areas change over time by georeferencing two maps with shown coordinate points layered together. This would probably only work if both maps could be referenced and rectified using the necessary coordinate points because the way to georeference and rectify the maps without coordinates requires marking landmarks against eachother, and unless these areas look the same, you might not have that chance.
It was very cool to see how accurately you could place a sort of random map on top of a complete up-to-date map and see where things lined up. It was kind of odd doing it with an early 2000s Notre Dame/South Bend map because of how much development had gone on in the time between then and now, especially since we take for granted how much was developed before our classes even got here.
So how does one go about georeferencing and rectifying maps to a common coordinate planes? Well the first step was to download and analyze maps. Its important to see what you’re trying to do with the maps and how you’re trying to say it. Also, it’s important to recognize whether the maps have coordinates you can use to place it on the plane, if not, you’ll need to find landmarks on both planes which you can use to bring the two together.
Once you’re done with the maps for the time, you need to set up ArcGIS. One of the more important steps is letting the system know what coordinate planes you intend on using so that the referencing works easier.
Next is the referencing of the maps themselves. The first case is using maps with the coordinate planes marked for you. Under the georeferencing tab, you have to set the points you plan on using. This is shown with the blue arrows at the bottom of the set map. These points will tell the map where to set the main image to when layering your new map on the plane. It’s important to get this as precise as possible so you are as close to a real map as possible.
Once you get the points set, you’ll place in the listed longitude and latitude for the point and layer it on top of your main coordinate plane. Apparently it works best with at least 3 points, but because we were pressed for time with our computer malfunctions in the class, it was decided that two points were good enough for work.
As I said, there is another way to place a map that includes using landmarks. Because the map above has set coordinate points, that was the easiest and most correct way to deal with placing it. Some maps don’t have coordinate points, unfortunately, so layering gets a little more difficult. In the map below, you can see the points used to mark our map (see the blue arrows again) as the center of the stadium and the edge of one of the lakes. Once rectified and fit to our basemap, you can see the rest of the map take shape, with the expressway and larger streets lining up between the base and layered maps.
After setting it all up and linking the maps, its up to you with what you do with your new coordinately correct maps, you can show how different maps stack up and differ from one another, or simply how old maps differ from the current base maps we take for granted. Either way, after georeferencing it really becomes your own cartographic adventure.