Historical maps can be fantastic sources of information, but they are rarely accurate enough to layer over current images without going through the process of georeferencing and rectifying. This map shows a georectified map of Macao in 1785 layered over a recent image.
To do this, there must be at least 2 layers, and control points are added between the historic map and the modern one, using points that can be identified on both maps. In this lab, we also added a topographic map to better identify specific locations. As more points are added, the historic map shifts in location so that the 2 maps line up as much as possible. In this case, 10 points were added before the map was rectified using a type of sampling called Cubic Convolution, which is used for data like images and photographs.
The purpose here is to highlight the changes in landscape and environment over time. Other data can be examined using this method, such as population trends and settlement patterns. It can be used at a variety of scales, from neighborhoods and cities, to larger regions.
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