Suppose a camera onboard a satellite is looking right down (nadir angle) and taking pictures of a flat scene. Also suppose the
camera is oriented such that the image columns represent lines of constant longitude and scan-lines show lines of constant
latitude. Therefore, the longitude-latitude lines appear as shown in Fig. 1a. Now, if the camera is rotated by 30 degrees about the
y-axis when taking a picture, the longitude-latitude lines will appear as shown in Fig. 1b. Angle of rotation of the camera about the
y-axis is known as the roll. If the camera is rotated by 60 degrees about the x-axis, the longitude-latitude lines appear as shown in
Fig. 1c. The angle of rotation of the camera about the x-axis is known as the pitch. If the camera is rotated about its optical axis by
30 degrees, the longitude-latitude lines will appear as shown in Fig. 1d. Rotation of the camera about its optical axis is know as
the yaw. Yaw is also known as the heading. An image obtained with camera roll-pitch-yaw equal to 30-60-30 degrees is shown in
Fig. 1e. Note that this is for the special case of a flat scene so that straight lines in the scene appear straight in the image.
An image obtained by an off-nadir angle shows the longitude-latitude lines distorted. The lines no longer intersect at 90-degrees.
As a result, an image obtained by an off-nadir angle camera will not align with a map of the area by a rigid transformation. To
preserve the geometry of a captured image even when the camera is not viewing the scene at the nadir angle, the image is
transformed by a process known as ortho-rectification. Ortho-rectification, converts an image obtained at an arbitrary
roll-pitch-yaw of the camera to one as if obtained when roll, pitch, and yaw were all 0. Ortho-rectifying the image of Fig. 1e using
roll, pitch, and yaw angles equal to -30, -60, and -30 degrees, respectively, the image shown in Fig. 1f is obtained. The only
difference between Figs. 1a and 1f is some intensity degradation caused by image resampling. Otherwise, the geometry of the
images is the same. Ortho-rectification stabilizes the camera as if always looking down and heading toward the North.
Ortho-rectified images are easier to register and analyze because they are not geometrically distorted. Correcting for an images
geometry by ortho-rectification, however, result in some intensity distortions in the image caused by the resampling process.
Typically, intensity distortions are very small compared to the geometry corrections gained; therefore, ortho-rectification is
generally believed to improve overall image quality. When the scene is not flat, orthorectification requires a DEM of the scene.
camera is oriented such that the image columns represent lines of constant longitude and scan-lines show lines of constant
latitude. Therefore, the longitude-latitude lines appear as shown in Fig. 1a. Now, if the camera is rotated by 30 degrees about the
y-axis when taking a picture, the longitude-latitude lines will appear as shown in Fig. 1b. Angle of rotation of the camera about the
y-axis is known as the roll. If the camera is rotated by 60 degrees about the x-axis, the longitude-latitude lines appear as shown in
Fig. 1c. The angle of rotation of the camera about the x-axis is known as the pitch. If the camera is rotated about its optical axis by
30 degrees, the longitude-latitude lines will appear as shown in Fig. 1d. Rotation of the camera about its optical axis is know as
the yaw. Yaw is also known as the heading. An image obtained with camera roll-pitch-yaw equal to 30-60-30 degrees is shown in
Fig. 1e. Note that this is for the special case of a flat scene so that straight lines in the scene appear straight in the image.
An image obtained by an off-nadir angle shows the longitude-latitude lines distorted. The lines no longer intersect at 90-degrees.
As a result, an image obtained by an off-nadir angle camera will not align with a map of the area by a rigid transformation. To
preserve the geometry of a captured image even when the camera is not viewing the scene at the nadir angle, the image is
transformed by a process known as ortho-rectification. Ortho-rectification, converts an image obtained at an arbitrary
roll-pitch-yaw of the camera to one as if obtained when roll, pitch, and yaw were all 0. Ortho-rectifying the image of Fig. 1e using
roll, pitch, and yaw angles equal to -30, -60, and -30 degrees, respectively, the image shown in Fig. 1f is obtained. The only
difference between Figs. 1a and 1f is some intensity degradation caused by image resampling. Otherwise, the geometry of the
images is the same. Ortho-rectification stabilizes the camera as if always looking down and heading toward the North.
Ortho-rectified images are easier to register and analyze because they are not geometrically distorted. Correcting for an images
geometry by ortho-rectification, however, result in some intensity distortions in the image caused by the resampling process.
Typically, intensity distortions are very small compared to the geometry corrections gained; therefore, ortho-rectification is
generally believed to improve overall image quality. When the scene is not flat, orthorectification requires a DEM of the scene.
 | |  |
| Ortho-rectification (flat scene) |
| Image Registration and Fusion Systems |
| (a) (b) (c) |
| (d) (e) (f) |
Fig. 1. Images obtained at (a) roll=pitch=yaw=0 degrees; (b) roll=30 degrees,;(c) pitch=60 degrees; (d) yaw=30 degrees; (e)
Roll=30 degrees, pitch=60 degrees, and yaw=30 degrees. (f) Ortho-rectification of image (e).
Roll=30 degrees, pitch=60 degrees, and yaw=30 degrees. (f) Ortho-rectification of image (e).
To obtain a license for this ortho-rectification software, please follow the link =>
