This week's assignment continued our investigation and incorporation of historic data into GIS, but this time the data are maps. We began by exploring the David Rumsey map collection (http://www.davidrumsey.com) and downloading a historic map of Macao based on surveys conducted by Cook and Bligh in the late 18th century. We then georeferenced this historic map based on imagery and topographic baselayers downloaded from ESRI. This was done by connecting control points from the historic map to corresponding locations on the baselayers. This was difficult for a couple of reasons. First, the historic map is of an unknown projection and clearly has errors. Second, the modern geography of Macao has changed much as a result of urbanization and land reclamation. It is thus difficult to know where best to place control points.
After placing thirteen control points and switching the transformation from first order polynomial to spline, I began to see a closer correspondence between the historic map and the modern baselayers. I was tempted to place more control points in the western portion of the historic map after seeing how the spline transformation altered it, but I believe Macao itself, the area of primary interest, is reasonably well-aligned.
This was an enjoyable exercise that made me more comfortable with georeferencing. I look forward to using these skills in the future.
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