Historical Aerial Imagery Georeferencing Project
SBL is a leading provider of end-to-end geospatial services enabling the creation of 2D and 3D maps that help organizations capture accurate geographic coordinates and leverage the application of space and location in the business.
As a part of its 2D mapping services, SBL undertook an aerial image georeferencing project of old historical imagery for Laval in Southwestern Quebec, Canada. Laval is the thirteenth largest city of Canada covering Île Jésus and Îles Laval islands.
The project was undertaken to better utilize the available historical imagery and to preserve it in a usable digital format for posterity. Another valuable business case was the savings that could be made by reducing the storage and preservation costs of old archived photos which are now in an easily storable and preservable digital medium.
Approach and Methodology
A total of over 6000 historical aerial photos dating between the early 1940’s to the late 1990s were georeferenced using the orthoimage dating 2018 of the city.
The georeferencing was done using a combination of different software’s namely ArcGIS and Quantum GIS. The average number of control points used was around 25. RMSE was maintained at less than 1 for all the aerial photos.
The project was completed in 3 months’ time using a total of 6 manpower resources.
Challenges and Resolution
One of the biggest challenges SBL faced was in handling real-world changes. As the aerial photos became older the real-world changes between the aerial photos and the control orthoimage became even more pronounced. Control points were not available in a lot of cases. To handle these real-world change issues, SBL adopted some innovative techniques like using the immediately preceding aerial photo series which has been georeferenced to gather control points in areas where a deficiency of controls was affecting the quality of the georeferencing.
Another challenge was to make sure that there was integrity in the edges of the georeferencing between adjoining aerial photos. SBL ensured that control points were taken at the edge of each photo and that the same controls at the edges were used for adjoining photos also. During the quality control stage, special care was taken to match up the edges of adjoining aerial photos.
Key Benefits of the Project
SBL completed the project on time and within the allocated budget and the available historical imagery was successfully preserved in digital format. A comparison of the georeferenced aerial photos and georeferenced image with control points show the depth of 2D imaging and accuracy of maps that SBL was able to create.
Note the match between the two images that were collected during the execution of the project.
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Figure 1 – Georeferenced aerial photo
Figure 2 – Georeferenced image with control points
The lack of discrepancies between the images evidences the success of SBL’s Historical Aerial Imagery Georeferencing Project.