Photogrammetry and Remote Sensing Flashcards

An aerial photograph is taken with the camera pointing vertically down. On the photograph, a tall radio tower's top appears further from the center of the image than its base. This phenomenon, where the top of a vertical feature is displaced radially outward from the center of a vertical photograph, is known as:

Answer: Relief displacement

Relief displacement is the effect on a vertical photograph where tall objects appear to lean away from the principal point (the photo's center). The displacement is radial from the center and increases with the height of the object and its distance from the center.

In the context of processing LiDAR data, which of the following BEST describes the process of creating a "bare-earth" digital elevation model (DEM)?

Answer: Classifying and filtering out points that represent vegetation and buildings.

A bare-earth DEM represents the ground surface without features like trees, buildings, or other structures. This is achieved by classifying the raw LiDAR point cloud to identify ground points and then filtering out all non-ground points before creating the surface model.

Which of the following correctly pairs an active remote sensing technology with a passive remote sensing technology?

Answer: Active: LiDAR; Passive: Panchromatic satellite imagery

Active remote sensing systems, like LiDAR and RADAR, emit their own energy and record the reflected return. Passive systems, like satellite cameras that capture panchromatic or multispectral imagery, record naturally available energy, which is primarily reflected sunlight.

An aerial photograph is taken over relatively flat terrain using a camera with a focal length of 152 mm. The aircraft is flying at an altitude of 2280 meters above the ground. What is the approximate scale of the resulting photograph?

Answer: 1:15,000

The scale of a vertical aerial photograph is the ratio of the camera's focal length (f) to the flying height above the ground (H). Scale = f / H. First, ensure units are consistent. f = 152 mm = 0.152 m. H = 2280 m. Scale = 0.152 m / 2280 m = 1 / 15,000.

Which of the following best describes the key characteristic of an orthophoto that distinguishes it from a standard unrectified aerial photograph?

Answer: It has been geometrically corrected to have a uniform scale.

A standard aerial photograph has scale variations and relief displacement due to terrain and perspective geometry. An orthophoto is a photograph that has been geometrically corrected ('orthorectified') to remove these distortions. The result is an image with a uniform scale, similar to a map, where true distances can be measured directly.

In photogrammetry, the apparent shift in the position of an object when viewed from two different points of observation in an overlapping pair of aerial photographs is known as:

Answer: Stereoscopic parallax

Stereoscopic parallax is the displacement of an object's position in two photographs taken from different camera stations. This difference in position is directly related to the object's elevation, and it is the fundamental principle that allows for the perception of depth and the extraction of 3D topographic information from stereo pairs of images.