Aerial Survey Principles ● Basic concepts: Spatial resolution in digital imageries, stereoscopy, relief displacement and orthorectification and orthomosaic. ● Platforms ○ In addition ot low cost availability that enable the platform accessible ○ Advantage of being close to earth’s surface. ○ Enables high resolution as well as image quality since atmospheric interference is reduced. ○ Temporal resolution also achieved as result of low-cost and portability.allows discretionary deployment. ○ cost-benefit ratio of RPAS for aerial surveying increases as mapping coverage increases. determinants of this relationship are height and quality of the camera/sensors. ● Electromagnetic radiation ○ Light is a form of electromagnetic radiation. adiation travels at the speed of light visible light has wavelengths 0.4 between 0.7 micrometer ○ RPAS can acquire images and their spectral signatures with minimial degradation from atmospheric absorption ○ various kinds of surface materials can be distinguished from each other by these differences in reflectance because at certain wavelengths materials (e.g., concrete, sand) reflects more energy than green vegetation while at other wavelengths it absorbs more (reflects less) energy. These sensors are available on RPAS, allowing more detailed geospatial data to be gathered. ● Imaging angles ○ Nadir angle is the direction of the force of gravity in that direction. Opposite of nadir is zenith. ○ Ground Sample Distance: 1cm GSD means the distance between the center points of two adjacent pixels of an image is 1 cm. Greater the GSD, lower the resolution. Also increases as flight height increases. ○ Focal length of ○ a lens: distance between point where light rays converge and image sensor. Greater the focal length, greater the magnification and smaller FOV. Vice versa. ○ GSD=[size of each pixel] X [altitude] / [lens focal length] ○ Scale = [focal length]/[flying height] = d/D ■ D is the ground distance captured by the image plane width in the sensor,d. Relief displacement ● Single photograph produced based on single point perspective. Relief displacement occurs where the objects in the periphery appear to lean away. Relief displacement increases from centre of photo. Increases for taller objects ● V= nadir line passing vertical line passes through perspective center Stereoscopic viewing and RPAS mapping ● 3D vision is enabled as a result of partially overlapping images generated by each eye. ● Origin of stereovision can be exploited for aerial surveying and is important principle of stereo photogrammetry. ● Eyes are limited in their ability to obtain depth perception due to fixed distance between one to the other. Mirrored stereoscope is a tool for gaining depth perception of overlapping images. ● Bundle block adjustment is a photogrammetric methods that enables these parameters to be computed including tie point coordinates, camera locations and attitudes. ● Points clouds are generated and are used to generate Digital Elevation Models and Digital Terrain Models. These models enable volumetric calculations among other functions. ● An important role of the 3-D point cloud is orthorectification, a process enabling the relief displacement and oblique images to be corrected for constant scale within the image. ● The images are then put together to create a bigger map that is accurate for measurement called the orthomosaic. Things to study for exam ● ● ● ● ● Building blocks of digital image scale, such as pixels, ground sample distance, etc., and how they are related to focal length and flight height. Data required for flight plan computation Basic principles of stereoscopy Explain in your own words the sources of aerial image distortions and the purpose of an orthomosaic. General understanding of the advantages and limits of RPAS-based mapping capabilities compared to other aerial survey methods such as conventionally piloted aircraft and satellites.
