Geographic information systems rely on georeferencing to establish accurate coordinates to the spatial data and help for mapping the satellite, aerial, and terrestrial imagery. SBL has handled several geo-referencing projects as a part of its 2D and 3D mapping service. Here is a case study of our recent historical aerial geo-referencing project of a city in Canada.
Georeferencing associates the raster image of a map or the physical map with spatial locations. Using georeferencing the mapping software can accurately position the image in its exact real-world location. The geo-referencing historical imagery project for the city is discussed here.
The authorities needed to preserve the current historical imagery in a digital format. For this purpose, SBL offered extensive georeferencing historical imagery services to preserve the imagery accurately for posterity.
The historical aerial imagery georeferencing project of a city in Canada
Converting the existing imagery to digital format can assure that it would be preserved accurately. Storing the imageries and protecting it from damages consumes lots of time, cost, and manpower from the authorities. Digital storage makes it convenient to archive the imageries in a centralized database. It reduces the storage and conservation efforts and expenses. Digital format also facilitates an easy search and access to the records.
Scope of the project
The project aimed to geo-reference an enormous volume of historical aerial images to further store it in digital format. It covered more than 6000 historical aerial photos between the period of the early 1940s and late 1990s. For geo-referencing the photos, we took the 2018 orthoimage dating of the city. SBL also used advanced softwares for geo-referencing that includes Quantum GIS and ArcGIS. Around 25 control points were used on an average. We maintained the RMSE lower than 1 for each of the aerial photos. The project was completed in a short turnaround time of three months and required 6 manpower resources.
Challenges and resolution
The biggest challenge of the project was the real-world changes and the absence of control points in many cases. The real-world changes were considerably huge for very old aerial photos. SBL handled these issues by adopting innovative techniques for those areas where control points were not available. For gathering control points in such cases, we used those georeferenced aerial photo series that immediately preceded the series.
The next challenge was to ensure the integrity of the edges between adjoining aerial images. To resolve this, SBL took control points at the edge of the photo and used the same control points for the adjoining photos. When the images passed through the quality control stage, we made checks to match the edges between adjoining photos.
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.
Benefits of the Project
The project was completed successfully within the predefined budget and time. We accurately preserved the historical imagery of the city in digital format.
Here is a comparison of the images collected during the execution of the project. Both images match exactly reflecting the accuracy of the maps we created.
Figure 1 – Georeferenced aerial photo
Figure 2 – Georeferenced photo (including control points)
The images do not show any discrepancies. This is evidence for the success of the historical aerial imagery georeferencing project.
SBL provides end-to-end geospatial services and has proven expertise in georeferencing historical imagery. We help institutions to create accurate 2D and 3D maps to use location intelligence in business decisions. Contact us to know more about our customized GIS solutions.