Aerial Photography: History, Ontario, and York University
Overview of processes involved in archiving and digitizing aerial photos
Section A: About Aerial Photography
What are Aerial Photographs?
Aerial Photographs are images taken by cameras mounted on aircrafts or other flying objects. These objects include fixed-wing aircrafts, helicopters, balloons, blimps, drones and even birds (historically)!
Image Credit: Air Mechanic Cecil Haliday of the Royal Flying Corps demonstrates a C type aerial reconnaissance camera fixed to the fuselage of a BE2c aircraft. (n.d.). photograph. Retrieved from https://www.warhistoryonline.com/world-war-ii/british-intelligence-transformed-aerial-reconnaissance-world-war-two-m.html
Background
Photography was first developed in the 1840's. Air photos can be dated back to the early 1900's when they were first conceptualized. Initially, humans tried strapping cameras with a timer onto birds, taking (often poor) images as they flew. Eventually, people started carrying along cameras onto balloons and other aircrafts. This permitted more control over what ground area the photos would capture.
Aerial photos developed rapidly through the 1930's and 1940's as a method to map the Earth’s surface, and for reconnaissance purposes in World War l and ll.
Aerial photos have been a source of topographic depiction. Stereoscopic viewing of overlapped, successive aerial photos permits deriving contour lines in the landscape. This three-dimensional visualization has been used to assist in producing topographic maps through the use of a stereoscope, historically. With the advancement in technology, the same principles are used in a generating three-dimensional image from two-dimensional images from a technique known as Structure from Motion (SfM). This technique simply needs multiple images of an area from different angles and a relatively high degree of overlap. Specialized software (e.g. Pix4D) can automatically detect matching features within the images and construct a 3-D image.
Today, aerial photographs are used in environmental projects that require high spatial and temporal resolution, which satellites may not offer.
Comparison of different spatial resolutions in the built environment using aerial photograph (0.2 m), HyMap (3.6 m), Sentinel-2 (10 m), and Landsat-8 (30 m).
Image Credit: Urban Mapping and Change Detection. (2017). photograph. Retrieved from http://eoscience.esa.int/
The typical spatial resolution for free satellite imagery ranges from 10 – 30 m whereas aerial photography can provide fine detail of less than 20 cm depending on the altitude the aircraft flies. A major advantage of aerial photography is that the user defines the time factor (high temporal resolution) which allows to study short lived phenomena and avoid cloud cover that may persist in satellite imagery.
Comparison of common satellites and their respective resolutions (spatial, spectral, temporal).
Image Credit: Dust Event Detection. (n.d.). photograph. Retrieved from https://gec.cr.usgs.gov/dustdetection/
Image Credit: Structure from Motion principle. (2016). photograph. Retrieved from Theia-sfm.org
Why are Historical Air Photos Important?
Over the past century, air photos have served as a remarkable historical landscape record. Aerial Photography permits a better understanding and visualization of environmental change over time, and therefore, may offer a better understanding of trends into the future.
Aerial Photographs are used to supplement historical research projects. For example, successive images of coast lines offer a method to monitor changes in a single location through time. Gradual trends and erosion rates can be determined by comparing images in a time series known as change detection. Changes in land cover can be analysed using satellite imagery as well through classification techniques (such as supervised and unsupervised) using remote sensing software. This information can assist with planning effective restoration practices.
Image Credit: Changes in land use, land cover, and subdivision expansions. (n.d.). photograph, Environment Canterbury. Retrieved from https://canterburymaps.govt.nz/news/view-our-historical-imagery-gallery/
Taking Air Photographs
The Camera
A camera is mounted on an aircraft specifically designed for capturing a rapid sequence of air photos while limiting geometric distortion. Photos are taken of a region of interest (ROI) on the ground, which is determined by the camera’s field of view, the aircraft's flight path, and altitude.
The Flight Path
An aircraft's flight path is normally a series of parallel straight lines. Along each individual flight line, photos are taken rapidly, ensuring a 50 – 60% overlap between successive photos. This overlap permits stereoscopic, three-dimensional viewing.
Aerial survey of Little Llangothlin Lagoon, NSW, Australia
Image Credit: Flight path across a lake. (2016). photograph. Retrieved from https://peerj.com/articles/1831/#fig-1
Types of Air Photographs
Most air photos are vertical photographs, where the camera points straight down and the image captured is of the ground surface directly below the aircraft. Alternatively, oblique air photos are taken of an area not directly below the camera, but rather, are on an angle. This process requires methods of geometric correction to correct for distortions by a process of matching ground control points on the image and a base map to accurately assign pixels a location so that it can be re-sampled correctly.
Image Credit: Airphoto Geometry. (n.d.). photograph. Retrieved from http://www.engr.usask.ca/classes/GEOE/218/notes/airphoto_reading/apg.htm
Where to Find Archived Aerial Photographs
- City of Toronto Aerial Photograph Website
https://www.toronto.ca/city-government/accountability-operations-customer-service/access-city-information-or-records/city-of-toronto-archives/whats-online/maps/aerial-photographs/
2. Natural Resources Canada (Government of Canada)
https://www.eodms-sgdot.nrcan-rncan.gc.ca/index_en.jsp
3.Ontario Ministry of Government and Consumer Services
4. York University Geographic Resource Centre
The Geographic Resource Centre (GRC) is where all the air photos our team is currently working on, and have already archived, are stored. To search for available air photos, please search the catalogue provided below. To request a physical copy of the air photo, the GRC is room S403 of the Ross Building at York University Keele campus.
https://grc.geog.laps.yorku.ca/
5. York University Map Library
The Map library at YorkU contains digitized versions of archived air photos. To search the archive for aerial photographs, the latitude and longitude of each photo's centroid has been placed on an interactive map and able to be looked up in York University's Map Library.
The list of air photos archived include sources from:
- Ministry of Mines and Resources Canada - Toronto Island only
- Forest Resource Inventory (FRI) aerial photography - Ontario Ministry of Natural Resources
- Ministry of Transportation Ontario - along Hwy 401 corridor only
- Ontario Ministry of Natural Resources - along Hwy 401 corridor and main roads
- Ontario Dept. of Lands and Forests
- The Municipality of Metropolitan Toronto Land Information Services
http://researchguides.library.yorku.ca/c.php?g=679618&p=4793558.
Section C: Scanning and Spatially referencing Aerial Photographs
Scan Specifications
All archived, aerial photos from both the GRC and Hodgins collections have been scanned at 600 dpi, in tiff, black & white format.
Finding Photograph Centroids
Besides the aerial photographs, maps were also provided in the collections. Over these maps are drawn lines, representing the flight lines the aircraft followed while taking the images. Beside the drawn lines over the maps are numbers that coincide with the flight lines numbers of the image bundles. In combination, these factors assist in finding the location of where the thousands of aerial photographs were taken.
The maps, drawn lines over the maps, and flight line numbers beside the lines, all assist in finding the locations of where the aircraft was when the photographs were taken. To utilize this information, Google Earth was primarily used to spatially register the photographs. Significant spatial features labelled on the maps are used as reference in Google Earth. For example, if Star Lake is present underneath a flight line of images attempted to be spatially registered, Star Lake will be searched for in Google Earth. If Star Lake and the surrounding area appears similar to the figures in the photographs being spatially referenced, the attempt was successful!
Then, to find the latitude and longitude coordinates of each photo’s centroid, the photo must be measured for length and width in order to accurately locate its centre. This centre location is then located in Google Earth, and once found, the appropriate latitude and longitude coordinates are recorded for that photo’s centre.
Potential Further Processing
Once all centroids of all aerial photographs in a bundle have been spatially referenced, the next step is to properly/completely spatially reference the images. To accomplish this, multiple Ground Control Points (GCPs) for each image must be identified and spatially referenced. A minimum of 10 GCPs are recommended for each photo, spread out relatively evenly over the entire image. The best GCPs are the ones easily identified, and associated with features that experience minimal erosion, and do not move, over time. They should be points directly on top of the ground surface (not tree-tops), and should be easily identifiable in both Google Earth and the photo itself, such as small islands or other sharp edges.
Using a software such as the OrthoEngine tool in PCI Geomatics in combination with Google Earth, the images can be spatially referenced individually, and then combined in ArcGIS. When the GCPs are all very distinguishable and each photo has been properly spatially referenced, the images can be “stitched” together to create a mosaic of multiple photographs, and can serve to recreate an image of what the landscape looked like when the images were taken.
Section D: The Outcome
The GRC collection: All it's cataloged Aerial Photographs
This map mainly focuses on Southern Ontario. When zoomed out, more air photo locations become visible.
GRC Collection Air Photo Centriods for Southern Ontario, CA
Web Application for the GRC Collection Air Photos
ArcGIS Web Application
This web application allows for user based analysis:
- Chart Statistics -View chart statistics based on year range (bar graph, pie chart, line graph).
- Query Tool - Formulate queries and zoom to feature based on year.
- Coordinate Conversion - Search area by coordinate or convert within coordinate systems.
- Summary Function- Summarize air photo year range based on selected region/cluster (count of photos in cluster, average year, minimum year, maximum year).
- Measurement Tool- Measure by area, distance, or display coordinates of a selected point on the map.
- Distance and Direction - View the distance and direction from a start and end point defined by the user using selected features (line, circle, ellipse, rings).
- Analysis Functions- Extract Data: Select and download data (layers or clipped features) as CSV, KML, Geodatabase, or Shapefile. Summarize Within: Finds features within a specified region and summarizes attributes based on the input layer.
- Screening Tool - Define an area of interest by searching a place, drawing on the map (point, polyline, polygon), or by coordinates for analysis through setting a buffer or tolerance to view air photos within the specified criteria.
- Swipe Tool - Slide the swipe tool to reveal areas of the map with available air photos.
