A Traditional Line Map - Essay Example

?Running head: ASSESSING WHETHER ORTHOPHOTOS ARE A SUITABLE SUBSTITUTE FOR A TRADITIONAL LINE MAP Assessing Whether Orthophotos are a Suitable Substitute for a Traditional Line Map Insert Name Insert Course Title Insert Instructor’s Name 09 August 2011 Assess whether orthophotos are a suitable substitute for a traditional line map Introduction When a camera is used to take a photograph of the earth, the photo is recorded as a perspective projection. If the photo is taken with the camera precisely facing the center of the earth, then the projection is in vertical perspective and when taken from other positions the projection, it is a tilted perspective projection. Projection from a 3D surface to a 2D medium is generally of two types - central perspective and parallel projection. 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 orthographic 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 “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 viewing, 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 image rectification. Parametric means use transformations based on information on the interior and exterior orientation of the sensor using technologies such as differential rectification, sensor specific model rectification and RFM rectification. In the rectification process for aerial photography, the camera model is composed of an inner orientation model that relates the scanned image coordinates to the camera reference frame given by the fiducial marks, and an exterior orientation model that describes the 3-D perspective geometry of a frame camera.9 Inner orientation The inner orientation defines the geometric parameters of the imaging process by creating a process through which the image-forming bundles of rays for each photo is reconstructed in its original geometry.10 Inner orientation changes the pixel coordinate system to the camera coordinate system. Thus it establishes the relationship between the camera model and the aerial photograph image. This process uses the tie points located between the aerial photograph and the camera fiducial marks and the camera focal length. Prior to inner orientation the calibration data is feed into the system to enable the process to measure the fiducial points. Exterior orientation Exterior orientation is used to relate the aerial photo to the target coordinate system using the ground control points (GCPs), with reasonable rectification being achieved using approximately twelve control points spread over the image.11 Hence, exterior orientation determines the position and angular orientation parameters associated with the image. The exterior orientation is divided into two steps: relative orientation and absolute orientation. The relative orientation process orient one image to another by establishing angular relationship between two consecutive photographs images to the initial status when the image was captured leading to creation of stereomodel.12 In this process, the Y-parallax is eliminated at six standard points since the software evaluates six or more conjugate parallax points on each image within a model and computes a rotation angle and coordinates of the oriented exposure station.13 Using ERDAS IMAGINE for the given assignment, the following steps were followed to accomplish inner and exterior orientation processes. After data preparation and image correction, the Orientation in the Camera Model Properties window was clicked to view the available options for orientating the image. Since none was provided for the sample, all the fields were left as ’Unknown’. Secondly, it moved onto the Projection and clicked Add/Change Projection to see the list of available projections. While in the Projection Chooser box, the Custom Tab was clicked and from the dropdown list for each field the Projection Type was changed to ‘UTM’ , the Spheroid Name to ‘Clarke 1866’ and Datum Name to ‘NAD27’. The UTM Zone was changed to ‘11’ by typing in the value and finally it was confirmed that the NORTH or SOUTH windows displays ‘North’. Thirdly, OK was clicked in the Projection Chooser dialog box to confirm the input parameters. Additionally, the projection information entered displayed under the Current Reference Map Projection in the Camera Model Properties window. Using the dropdown list that was generated ‘Meters’ was selected as the Map Units. Then the Apply button was clicked after it became active. Lastly, Save As in the Camera Model Properties window was clicked to save the model to a local folder that prompted entering of ‘ps-napp’ as the name for the geometric model with the ‘.gms’ extension being added automatically to the model filename. Absolute orientation Absolute orientation is an extension of exterior orientation process that requires knowledge of high quality ground control points (X, Y and Z coordinates) to create 3D ground coordinates within the relatively oriented stereo pairs.14 Hence, absolute orientation relates a model to the ground coordinate system through establishment of a relationship between the models coordinates and the true ground coordinates (horizontalization and scaling of the model).15 Thus, absolute orientation is the late stage in the orientation process. Using ERDAS IMAGINE for the given assignment the following steps were followed to accomplish absolute orientation process. From the last step of orientation as indicated earlier in the paper, on closing the current window, the GCP Tool Reference Setup dialog box opened. It was followed by navigation to the default sample data directory (\examples) and “ps_camera.gcc” was selected, which had been derived from USGS 1:24,000 scale topographical maps using a digitizer. Next OK was clicked in the Reference GCC File dialog box placing a link box cursor in the main Viewer and then a second Viewer and the GCP Tool dialog box were opened. While in the GCP Tool the Solve Geometric Model icon was clicked to solve the coordinate transformation and calculation of the RMS error and residuals. Then the Control Point Error for the X and Y values displayed besides the icons on the GCP tool. Lastly, GCP Tool using more than three GCPs run the model and more than six GCPs made the model stable and accurate. Using ERDAS IMAGINE the above steps can be followed for orientation of different aerial photos with only changes being mainly on the quoted parameters. Advantages and disadvantages of Orthorectification Compared to georectified images, orthorectified photography is more likely to produce accurate image especially in regions where there is a high relief across the area and the digital georeferenced aerial photographs provides an excellent analytical medium which can be used effectively in combination with other types of georeferenced survey information.16 Orthorectification produces highly processed images which have all distortions from earth curvature and rotation, motion, angle of view and relief displacement removed.17 Additionally, orthorectified images have a uniform scale that makes it easy to determine area, angles and distance within the map easier. It main disadvantage is that this process is suitable for mountainous terrains and remote sensing materials that are used to construct 3D models of the scene. Also it suitable for large images since if small images are captured at higher altitude than topographical relief they produce insufficient displacement in the images thus they cannot be processed using this process. Thus, orthophotos are suitable replacement to traditional line map only in mountainous terrains and areas with high relief displacement such cliffed coastal line. Otherwise, the traditional line map remains relevant especially in flat terrain regions, as they offer accurate information in relation to these areas compared to orthophotos. Bibliography AS Rosenberg, An evaluation of a UAV guidance system with consumer grade GPS receivers, ProQuest, Eisenhower, 2009. B Leupen, Design and analysis, 010 Publishers, 1997, Rotterdam. Channel Coastal Observatory, Digital Aerial Photos, Southeast Regional Coastal Monitoring Programme, 2003, retrieved 9 November 2003, http://www.channelcoast.org/southeast/survey_techniques/airborne_remote_sensing_topographic_surveys/?link=digital_aerial_photos.html CP Lo & AKW Yeung, Concepts and techniques of geographic information systems, Prentice Hall, New York, 2002. Gen Contours, Photogrammetry, Gen Contours Pvt. Ltd, 2009, retrieved 8 August 2011, http://www.gencontours.com/photgrammetry.html J Gao, Digital Analysis of Remotely Sensed Imagery, McGraw Hill Professional, New York, 2009. M Mitton, Interior Design Visual Presentation: A Guide to Graphics, Models, and Presentation Techniques, John Wiley and Sons, New Jersey, 2003. M Neteler, H Mitasova, Open source GIS: a GRASS GIS approach, 2nd Edition, Springer, Boston, 2004. M Zalewski, DM Harper & RD Robarts, Ecohydrology & Hydrobiology international journal Ecohydrology & Hydrobiology (E&H), Index Copernicus, Vol. 6, Nos. 1-4, 2006, retrieved 8 August 2011, http://books.google.com/books?id=I-E67wh8xzcC&pg=PA99&dq=orthorectification&hl=en&ei=-dA_TsHJM86M-wapg-DMAg&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDMQ6AEwATgK#v=onepage&q=orthorectification&f=false RA. Schowengerdt, Remote sensing, models, and methods for image processing, Academic Press, California, 1997. S Erle, R Gibson & J Walsh, Mapping hacks: tips & tools for electronic cartography, O'Reilly Media, Inc., North Sebastopol, 2005. Read More