Challenges of Georeferencing Oblique Air Photos As Opposed to Using Nadir Imagery
Aerial photographs utilized in remote sensing will fall into either the nadir or the oblique category. Nadir aerial imagery, sometimes called vertical aerial photography, is captured using a camera looking directly down on the subject in an orthogonal manner and maintains a consistent scale (Jensen, 2016; Nex, n.d.; NCGICC, 2015; Okin, 2022a). Oblique aerial photographs are taken using a camera positioned at a look angle which views the subject matter from a skewed angle and varies in scale. Both Nadir and Oblique imagery are useful for various remote sensing purposes; however, the unique challenges that georeferencing oblique images present must be acknowledged.
One of the primary issues resulting from oblique photography is the distortions produced by extreme topography or tall features and obstructions resulting from the rectification of the image. The attempt to rectify an oblique photograph to a map projection or another image using ground control points (GCPs) often results in the distortion of “particularly tall” and above-ground-level features (Okin, 2022a, 13:06). This distortion is a result of the rectification method as it counters the skew originally present in the oblique image due to the angle of its capture. Nadir images do not encounter these issues due to their orthogonal orientation. Therefore, they do not typically exhibit obstruction of features nor distortions resulting from the geometric correction.
Another common concern involves the lack of projection information and orientation direction accompanying oblique aerial imagery (Okin, 2022a). The implementation of GCPs in oblique georeferencing or rectification is reliant on the analyst’s knowledge of the subject area, access to take manual coordinates of the location, or their ability to determine the coordinates features in the image using software such as Google Earth with the acceptance that some error may be present. An additional consideration for the usefulness of an oblique image regarding the intended purpose and the ability to generate GCPs is the various levels of obstructed subject matter generated by topographic or man-made features with great height due to and depending on, the angle at which the image is captured. A nadir aerial image may experience the same lack of projection and orientation information that oblique aerial images present.
Consideration should be taken regarding the scale and contrast level variations between two oblique aerial images alongside the variations between oblique and orthogonal images (Nex, n.d.; NCGICC, 2015; Okin, 2022a). Similar to the directional orientation of the image, variations in scale and contrast can create additional difficulty for analysts attempting to rectify an oblique image using another oblique or an orthogonal image. Just as the direction of the image impacts the identification of features, scale, and contrast can contribute to an inability to determine landmarks for coordinate production and a lack of comparability between images (Verykokou & Ioannidis, 2018). Correction for contrast inconsistencies could be corrected by stretching one of the image histograms to match that of the other image’s histogram (Okin, 2022b).
One solution for the challenges presented by oblique aerial imagery is Pictometry (Jensen, 2016). Pictometry allows for aerial photography to be utilized effectively by adding a vertical (orthogonal) aerial photograph and oblique photographs from each cardinal direction. This method ensures that “horizontal and vertical measurements of structures and the terrain can be made while viewing oblique photography” (Jensen, 2016, pp. 105-106).
References
Jensen, J. (2016). Introductory Digial Image Processing: A Remote Sensing Perspective (4th ed.). Pearson Education.
NCGICC. (2015). Guidance for Acquisition of Oblique Aerial Imagery and Software System. North Carolina Geographic Information Coordinating Council. https://it.nc.gov/media/593/openLinks to an external site.
Nex, F. (n.d.). OBLIQUE AERIAL IMAGES: POTENTIALITIES, APPLICATIONS AND BEST PRACTICES. 31.
Okin, G. (2022a). Changing geographic projections and geometric correction. BruinLearn. https://bruinlearn.ucla.edu/courses/141360/pages/411-dot-3-6-changing-geographic-projections-and-geometric-correction-13-44?module_item_id=5376640
Okin, G. (2022b). Contrast stretching: Other Stretches. BruinLearn. https://bruinlearn.ucla.edu/courses/141360/pages/411-dot-2-3-contrast-stretching-other-stretches-9-40?module_item_id=5376617
Verykokou, S., & Ioannidis, C. (2018). Oblique aerial images: A review focusing on georeferencing procedures. International Journal of Remote Sensing, 39(11), 3452–3496. https://doi.org/10.1080/01431161.2018.1444294Links to an external site.
