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Using Georeferencing Tools for Navigation and Research of Aerial Photography

GIS technology provides users with analytical tools at a high level of geographical precision. The combination of aerial photography and georeferencing technology brings traditional air photo interpretation to a higher level. Georeferencing is the process of aligning spatial data (layers that are shape files: polygons, points, etc.) to an image file such as an historical map, satellite image, or aerial photograph. When an image is georeferenced to the earth's surface, the user can overlay additional geospatial datasets. It may be land use information, census data, recent air photo imagery, current streets, property boundaries, etc. If a particular feature on a photo is not clear, one can confirm its identification with geospatial data (for example using topographic vector data to identify a wetland in the image). With the GIS tools available, a user can compare the past and the present with very close precision. If the user is interested in studying the loss of forestry for example, one can overlay an historical aerial photograph on top of a more recent one to study the changes and discover precisely where new trees have been planted.

For users of historical air photos, using current datasets such as streets, or landmarks like retail locations will assist the user in navigating the older photo. Streets are an excellent source for bearings and will help identify the location of the photo. Adding geospatial data such as buildings and points of interests to historical photos of farmland will clearly demonstrate land use changes over time.

The images below are some examples of how GIS technology can aid the user in learning about the changes in land and its usage.

Kitchener, Ontario : 1945

Georeferenced air photos can be overlaid on top of one another for accurate comparisons
Kitchener, Ontario : 2006

These two images reveal changes
in land over time

Image : Google Earth
Kitchener, Ontario : 1945

Comparing these two images reveal very
little change in land over time
Kitchener, Ontario : 2006

Image : Google Earth
City of Waterloo, 1930
City of Waterloo, 1930

Adding geospatial layers such as current buildings, ground water and parks helps analyze changes in both human-made and natural features

Data : City of Waterloo

This image shows a 1930 photo of the City of Waterloo with current zoning overlay. The pink boundaries represent Residential, the green is Open Space (fields, forests), and the yellow is Industrial.

This image in using 2001 Canada Census Data to show population for each community district. We can use census data to learn about community growth over the years.

About Georeferencing Aerial Photographs

Any paper aerial photography can be scanned and georeferenced to be used in a GIS program or in online programs such as Google Earth/Maps. Georeferencing is completed in GIS software programs such ESRI’s ArcGIS, TatukGIS Editor, free products like DIVA-GIS, MapWindow GIS, QGIS, and Mapmaker. Georeferencing an image requires the user to identify several points on the photo. The user will initially need to locate a landmark to get geographical bearings. Street intersections make excellent reference points as they have specific points that need to be connected or ‘referenced’ to the same intersection point on the street vector file. Many users find using vector streets or rivers helps identify the location of the photos. The GIS program records the coordinates of each point and thereby tags the photo and applies a specific spatial reference system to it.

- Published October, 2008

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