Definitions
(*) Photogrammetry is the science of obtaining measurements from photographs. It allows us to extract valuable information about objects, structures, and landscapes by analyzing overlapping images.
(**) Orthorectification is a process of removing distortion from an image caused by the curvature of the Earth and changes in terrain. It involves correcting the perspective of the image to align it with a map or a coordinate system, resulting in a more accurate representation of the Earth's surface.
In simpler terms, imagine taking a picture of a city from an airplane window. The buildings closer to you appear larger, while those further away appear smaller. Additionally, the curved shape of the Earth causes the image to appear distorted. Orthorectification corrects for these distortions so that the image accurately reflects the size and location of objects on the Earth's surface.
(***) The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. Simply put, the image is taken from directly above.
(**) Orthorectification is a process of removing distortion from an image caused by the curvature of the Earth and changes in terrain. It involves correcting the perspective of the image to align it with a map or a coordinate system, resulting in a more accurate representation of the Earth's surface.
In simpler terms, imagine taking a picture of a city from an airplane window. The buildings closer to you appear larger, while those further away appear smaller. Additionally, the curved shape of the Earth causes the image to appear distorted. Orthorectification corrects for these distortions so that the image accurately reflects the size and location of objects on the Earth's surface.
(***) The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. Simply put, the image is taken from directly above.
(****) Ground Sample Distance (GSD) is the distance between the centers of two adjacent pixels measured on the ground. In simple terms, it is the measure of a pixel in an image and how a pixel relates to a physical distance on the ground. It is usually described in centimeters per pixel (cm/px). The lower the Ground Sample Distance, the higher accuracy and more detailed your survey will be. The range for UAV photogrammetry typically falls between 1.5 to 2.5 cm/px (.6 to 1 inch). For example, a GSD of 5 cm/px means that one pixel in the image represents a 5 x 5 centimeter square (25 cm2) on the ground. A smaller square translates to more detail within the image.