Assess whether orthophotos are a suitable substitute for a traditional Essay

Assess whether orthophotos are a suitable substitute for a traditional line map - Essay Example Central perspective projection involves sensing the entire field within a view from a single point in space.1 The single point in space can be the geometric center of the camera lens and this projection is mainly associated with the vertical aerial photographs taken with a frame camera. In central perspective projection, the horizontal position of pixels on the photograph is wrong or misplaced, and is only correct at the nadir position if the topography has a relief and the magnitude of positional shift or relief is a function of relief, the altitude of the sensor H and the focal length of the camera f.2. In a central aerial perspective, the viewpoint is high compared to the object or landscape, whereas in central ground perspective, the viewpoint might be made to equal the height of an upright person.3 Due to its nature, central perspective projection is mainly associated with aerial photographs. Topographic maps are quite different from aerial photographs since they have an orthogr aphic projection that involves viewing earth directly below from poly-perspectives that are parallel to each other. This view eliminates relief displacement, leading to all positions to be indicated correctly. The object’s dimensions are projected directly, making orthographic projections to retain shape and proportion, thus the drawings are accurate, precise, appear flat and lack 3D quality of perspective drawings.4 The orthographic projection distorts both area and direction; thus, when earth globe is viewed, the projection shows a single hemisphere in perspective.5 Orthographic projection is mostly associated with orthophotos. Orthorectification is â€Å"the process of transforming a central perspective image into an orthogonal image by removing positional displacement caused by topographic relief from the input image, in addition to providing the ground coordinates for all pixels†6. The effect of other parameters present during image capture including angle of view ing, height of the position and earth rotation are also eliminated from the rectified image like what happens in standard image georeferencing. Hence, orthorectification reduces geometric errors normally found within photography and imagery by taking raw digital imagery and applying a digital elevation model (DEM) and triangulation to create an orthorectified image within which each pixel possesses geometric fidelity.7 Orthorectified images are commonly referred to as orthoimages and they have a uniform scale that is deficient of any relief displacement. Orthorectification is normally used for large scale hyperspatial resolution images or aerial photographs. It’s very suitable for mountainous terrains and for remote sensing materials that are used to construct 3D models of the scene. Processes for rectification Before image orthorectification takes place, the relationship between image coordinates (r, c) and the ground coordinates (E, N, Z) must be established and spaceborne satellite imagery of this relationship is based on the exterior and interior orientation parameters of the sensor, with the aid of 3D GCPs.8 Image rectification is achieved through either nonparametric or parametric approaches. Nonparametric means use transformations similar to those used by 2D polynomial-based