Embedded Files
Lab Overview
Lab Overview
In this lab, we learned how to import historic and/or paper maps into a digital form, within a geographic information systems (GIS) interface. The process of georectifying here is defined in the context of digitization, but georectification in general refers to aligning and systematically assigning a coordinate reference system (or spatial data) to an image. In this lab explicitly, we learned how to digitise a historic map for 1903 for the City of Montreal. Moreover, in this example, street lines (as saved in a raster) were isolated and converted into a network of polylines, which could exist separate from the other data in the paper map (as a form of data extraction from raster to vector).
Exercise 1: Preparing for analysis
Exercise 1: Preparing for analysis
Open up the ArcMap 10.8 software on the Remote desktop, and set the data source within the environment parameters of the .mxd file to be the source of your downloaded street data and the historic maps (to keep the data layers pulled in the same area as part of a data organisation strategy).
Using the add data feature in the table of contents pane, the vector layer for the current street was added before the raster layer for the map. As the first layer is used to assign the coordinate reference system for the entire project, this needs to be imported first. Otherwise, the map environment parameters need to be assigned within the environments pane for the .mxd file. The following Street layer “clipped-streets” and the map file “Montreal-map-early1900s” look like this in the Map.AI interface.
Figure 1. Street Layer Clipped to the Island of Montreal (as polylines)
Figure 2: Historic Street Map for Southern Shore of Montreal.
The paper map transparency was then adjusted for when superimposing on the current streets layer, preparing for georeferencing the data layer. Street labels were added to the vector layer for ease in identifying similar locations in the paper map. Labels were added in the layer properties window, and select the labels tab. The following map will look like this:
Figure 3: Labelled Street Polylines on the Island of Montreal.
Similar to the spatial analyst toolbox, in this lab we will go into the customise section and add in the relevant toolbar for our lab, “Georeferencing” .
Exercise 2: Image Rectification using the Georeferencing Toolbox
Exercise 2: Image Rectification using the Georeferencing Toolbox
Once the toolbar was opened, we reviewed the various functions such as adding points, aligning the referenced point to the image point, and alterations of the image (stretch, rotate, etc). Using the fit to display feature, the historic map for Montreal is superimposed on the Map.AI extent, allowing you to begin “rubersheeting” the file to match with the current street layers. Under georeferencing, the update display feature was turned off so that the layer points could be added and aligned without the map automatically loading into the new area.
Points were added for i) Sherbrooke St. and Redpath St., ii) Parc Avenue and Duluth Street., iii) Sherbrooke and Wood Avenue, and iv) Chemin Sainte Catherine and Avenue Pagnuelo. We then clicked “Update Display” in the georeferencing toolbar to align the paper map overtop of the current point links in the georeferenced Montreal street layer. After adding the initial links, 89 more connections were made to try and refine the locations and stretch the paper map accordingly. These are visible as the red/green crosshairs below:
Figure 4: Georeferenced Paper map on modern day streets in Montreal, QC.
Once the links appeared to make sense, we used the update georeferencing feature to lock in the spatial ordinance of all the points in the raster to the spatial data contained in the map interface.
We then followed the rectify function to create a raster for the layer in high resolution, with the appropriate spatial data attached to it. The rectify feature was then used to save and export the high resolution raster file and exported for use. The final map for montreal looks as follows:
Figure 5: Georeferenced historic paper map overtop of current day Montreal street polyline shapefile.
Conclusion:
Conclusion:
Overall, in this lab we learned how to work with both raster and vector data types, importing the files and working through the steps of georectiying a paper map. In doing so, spatial information is applied to a raster pyramid file. Beyond the context of this class, and even in the examples of my own work, digitization has helped in mapping out historical farm field boundaries for calculating soil loss equations for erosion in Southern Ontario, historic wetland extent mapping, and for examining historic indigenous treaty map boundaries as compared to modern territory boundaries.
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