Methods of Georeferencing - Case Study Example

PHOTOGRAMMETRY SURVEYING By The of the The of the School The and where it is located The Date Introduction The exterior orientation of airborne sensors is a very critical venture and, thus, advice is inevitable to any National Mapping Agency. National Mapping Agency has a duty to deploy either the direct, indirect or the integrated sensor orientation method as each comes with cherished strengths as well as limitations when applied. In determining the exterior direction parameters and the exact attitude during the time of exposure, requires a careful imagery assessment, which is based on data from an airborne or satellite platform. In an endeavor to compute the Exterior Orientation Parameters then the direct, indirect and integrated sensor orientation techniques are employed. When Exterior Orientation parameters are directly determined using special positioning and sensors for orientation then this is referred to as Direct Georeferencing , whereas when Aerial Triangulation combined with extracted block of images which are well distributed is used to determine Exterior Orientation then this gives Indirect Georeferncing. Conversely, when the above two mentioned technique to effectively determines the Exterior Orientation then this is technically known as Integrated Sensor Orientation (ISO) (Tewari, 2007, p88). Nature of the direct approach to exterior orientation Direct Georeferencing commonly denoted by (DG) is used to refer to a system having capacity to directly associate data collected by a sensor remotely from the earth’s surface. The system does this by precisely taking the measure of the geographical position and the orientation of the remote gadget eliminating use of aged control points, which are based on the ground. This approach is best achieved when Integral Measurement Unit and Differential Global Position System are perfectly integrated together. When employing the direct method, the accuracy of ground object coordinate rely on the Global Positioning System for position and heavily on the Integral Measurement Unit for accurate orientation. When ground control points are not clear as is in the case of grounds covered with snow, in deserts and in coastlines then Direct Georeferncing can only be the best option. Occurences such as floods and forest fires amongst others demand emergency applications, as there is a greater need for faster orthophoto generation, which is subjected to limited time and resources. Direct Georeferencing/ method system utilizes a single strip orientation. In cases where Digital Elevation Model dominates direct method is proposed because it provides a sound solution in addition to being cost effective. With the Direct method, an orientation error exists which leads to an orientation error on the real ground in terms of the flying height. Global Positioning System at its best while using a dual frequency for a differential process will give a root mean square of around 5-10 cm.For effective end direct georeferencing then Fibred Optic Gyros and Laser Ring Gyros are adopted. Normally large scale mapping projects aimed at producing high accurate Exterior Orientation the Direct Georeferencing remains inevitable. With precise calibration of the direct georeferencing system then the system has the potential of producing accurate results despite the mitigation from the Global Positioning system solution power (Chamberlin, 1998, p82). Advantages of the direct approach to exterior orientation method Direct approach in Georeferencing has advantages in its way of operation. Firstly, direct method/Georeferencing does not require Aerial Triangulation in its mode of operation. The concerted efforts of the coupled Integral Measurement Unit and Differential Global Position System ensure that that Aerial Triangulation is not needed apart from when the entire calibration of the Direct Georeferencing system is being carried out. This calibration is very crucial for superb performance of the system during its application in some hazardous situations such as forest fires, floods and more notably when hurricanes strike. Secondly, direct approach promotes control free environment throughout its operation period except when issues of quality control arises. This quality of the free control environment puts this method of orientation in a better position to be applied freely especially when it is being applied to address the situation on the ground. Thirdly, direct mapping is perfect, and single photo orientation is exemplified indirect georeferencing. It is possible due to the superb combination of Integral Measurement Unit and Differential Global Position System thus enabling fine tie point solution on the ground. It is also worth noting that Direct Georeferencing utilizes the Inertial Measurement Units enables it to have a system, which is self, contained and fully independent. Direct Georeferencing also enjoys the acquisition of continuous data and thus putting it in a position to sample the data in a high rate.Direct Georeferencing employs the three positioning and altitude components for accuracy. Disadvantages of the direct approach to exterior orientation Direct Georeferencing comes with the following demerits. Firstly, direct approach has its accuracy immensely limited by the Geographical Positioning System solution. In as much as one would wish to get fine images on the points on the ground, the accuracy will never be assured as there will always exist an orientation error on the position depicted by the system. Secondly, Direct Georeferencing suffers from the demands of high accuracy Global Positioning System and the Inertial Measurement Unit systems to be in place. This demands come with a price tag as the cost of design; process and production is always immense. Cost of maintenance is always high for these sophisticated systems. Thirdly, the calibration of the systems parameters need to be done in a more precise way. It calls for a more tedious and vigorous job, which will in the end, add to the high cost of the system. On the other hand, maintaining those high precise calibrations is also tricky, and it demands a more careful handling of the systems. It explains the reason as to why skilled personnel are highly recommended when it comes to the Direct Orientation technique. In addition to these disadvantages, it can be clearly stated that there is always that constant growth of errors with time in this technique. This continuous growing error has the potential to produce a mega divergence in the positioning error (Tewari, 2007, p27). Nature of Indirect approach to exterior orientation The approach is majorly applied when misalignments between the Global positioning System, Inertial Navigation System and the imaging system remain strange or unknown. Global positioning System and Inertial Navigation System get offset without their angles of drift indicated shown or indicated (Halpin & Morgan, 2008, p67). With the Indirect Georeferencing, the post calibration steps and the measurement errors are used in estimating the misalignment of the Global positioning System, and the Inertial Navigation Systems. In addition to this, the photogrammetric Triangulation function is heavily employed in order to get a good solution. This function models the sensors external orientation more importantly taking into great account the Global positioning System, and Inertial Navigation System measurements. Eventually, polynomial functions are arrived at which are subject to time defined. In other words, Aerial Triangulation combined with extracted block of images that are well distributed is used to determine Exterior Orientation. Indirect Georeferencing capitalizes on Aerial Triangulation that majorly involves the adjustment of a tie of points in an image block comprising of uniformly distributed Ground Control Points. Typically this is known as the triangular integration with a sensor as it involves introducing several perturbations fixed offsets in the right external orientation of the sensor. Estimations are carried out later, and the Root Mean Square found on checkpoints by effectively employing varied Ground Control Points and the Tie Points. In an instance or a situation where the Digital Elevation Model does not exist, then Indirect Georeferencing is highly applied as there is no need for more Ground Control Points and photos, which are overlapping (Tewari, 2007, p87). Advantages of the indirect approach to exterior orientation method Indirect approach in Georeferencing has advantages in its way of operation. Firstly, when additional Ground Control Points are used then the system finds redundant solutions. Though additional GCPs are highly recommended there is always the great task of making sure that the points are evenly distributed due to failure, which makes accuracy and effectiveness of the entire system to be compromised. Secondly, when it comes to the area of parameters of interior orientation, Indirect Georeferencing gives robust solutions. It is very critical when it comes to reliability and the ability of the system for the exterior orientation of the airborne sensors that the National Mapping Agency intent to deploy. It is also important to realize the benefit Indirect Georeferencing derives from the superb level of accuracy in terms of position, and the good velocity approximation which is coupled with perfect error models designed to be time independent (French & Burgess, 2007, p96). Disadvantages of the indirect approach to exterior orientation Indirect Georeferencings comes with a couple of disadvantages as follows. Firstly, it mainly experiences or rather depicts non-uniform distributed Ground Control Points and this adversely affects its mode of operation. For a good operating condition of the system then the basic Ground Control Points ought to be evenly distributed on the established scope of operation. Secondly, Indirect Georeferencing lacks the capacity to display or rather show the block of imagery. This incapacity renders this method devoid of the ability to show a perfect estimation of the expected Exterior Orientation Positioning of the ground (Halpin & Morgan, 2008, p60). Lastly, operating cost for the Indirect Georeferencing system very high and thus making it an expensive venture to explore. The cost of putting the structures in place, skilled labor to operate and manage the equipment, continuous improvement is too high. It poses a challenge to maintaining the precision and accuracy of the system because eventually demand the huge sum of money from the concerned body such as the National Mapping Agency. For example, the imaging components, Georeferencing components and on-board control unit inside the flight system of management demand capital attention in order to be placed and function effectively. Moreover, Indirect Georeferencing faces a challenge of losing the gaps in the trajectory, which have a tendency of causing gaps. This challenge is worsened due to the common experiences of data-sampling rate that is far too low. Thus, the inevitable ambient noise results from slow resolution time with reference to an established baseline by the designer (French & Burgess, 2007, p80). Nature of Integrated Sensor Orientation approach to exterior orientation This distinct approach combines the capabilities of both the Direct Georeferecing method or technique and the Indirect Georeferencing method in establishing the Exterior Orientation Positioning. The main task of Integrated Sensor orientation is to fine-tune the established d accuracy of the Exterior Orientation Parameters in an effort to boost not only the absolute accuracy but also the relative accuracy of the system. These exterior Orientation Parameters are initially generated by the Direct Georeferencing system and an initial estimation for Aerial Triangulation. It eventually leads to a tiny number of tie points in the overlapping are. On a larger scale, the ISO approach enjoys the advantage of using Exterior Orientation Parameters in solving the tie point process thus saving on blunders and tedious computational calculations, which could eventually encourage time wastage and probably lead to increased errors. When performing the Integrated Sensor Orientation it is of importance to see to it that the existing tie points are critically optimized with the goal of minimizing the processing time. Reduced processing time will automatically enhance reliability on the system. The parameters obtained from a neat comparison between the Aerial Triangulation and the orientations measured by the Global Positioning System are effectively evaluated with the aim of getting accurate potential. The extreme dependence of the center coordinates generated from the Aerial Triangulation, and the distinct misalignment corrections take into consideration in an effort to check the most effective accuracy of the Integrated Sensor Orientation systems. If this is done then the coupling between Direct Georeferencing and the Indirect Georeferencing ( Integrated Sensor Orientation ) will give a more accurate determinate object which is closely related to the photogrammetric determination of tie points. There is always a need for a good calibration of the system in order to enhance the reliability. Therefore, the context calibration of the entire system have to be carried in order to establish the exact special shifts and the whole misalignments of the sensor components and determination of the spatial shifts and disorientations between the sensor components such as the camera and the interior orientation together with the space effects emanating from the system is very crucial. Although Integrated Sensor Orientation practically consumes time as a stream of images from a block, have to be indentified from tie points, the potential search area tie points can be drastically narrowed down and thus automatically enhancing the accuracy and the reliability of the system (Chamberlin, 1998, p86). Advantages of Integrated Sensor Orientation approach to exterior orientation Integrated Sensor Orientation (ISO), approach in Georeferencing as advantages in its way of operation. Firstly, it combines every advantage from the above Direct and Indirect Georeferencing systems. By combining the characteristics of both systems makes it relatively more accurate and possess the power to reduce drastically the time required for data processing. Once the intensified tie point identification can be optimized and thus less time of computing the expected results. Secondly, ISO enjoys the potential of utilizing less accurate data from Global Positioning System and the Inertial Measurement Units because of reduced errors associated with them. This capability of utilizing reliable data enables Integrated Sensor Orientation method of mapping of airborne sensors cut a notch high from the rest of the methods as it ensures that optimal results are arrived at by the end of the day. In an endeavor to optimize the expected results then, it may use the Per Von Gruber value. This value option has the potential of dividing a photo into nine PPVG values which when changed drastically limits the number of possible tie points emanating from several regions especially when doing Point narrowing process and mistakes or error detection. In addition to these advantages, the Integrated Sensory Orientation enjoys the easy of analyzing the redundant and complementary data as they are presented separately meaning that their respective errors are represented as a separate venture. The process makes this georeferencing technique effective to use (French & Burgess, 2007, p90). Disadvantages of Integrated Sensor Orientation approach to exterior orientation Integrated Sensor Orientation Georeferencing system or method comes with a couple of disadvantages as follows. First, there is the constant requirement of a block of imagery for the georeferencing process. This demand poses a threat to the time factor as well as the accuracy of the system. Blocks comprising of strip imagery leads to difficulties in pinpointing of the image especially where there is overlapping tie point centers. Secondly, Integrated Sensor Orientation suffers a shortcoming in the line of application especially in projects, which require orthophoto generated from Digital Orientation Parameters. This demerit limits this technique and makes it literally hard to apply some real life situations that do not involve the parameter generation using Digital Elevation Models. Since Integrated Sensor Orientation utilizes Inertial Measurement Units, and the Differential Global Position System suffers from the lack of significant limitation just as it suffers from the danger of lacking the precise time of synchronization. These demerits pose as a threat to the efficient operation of the entire system. Conclusion The three methods of Georeferencing in details and in a more broad perspective to enable the National Mapping Agency to make wise decision regarding which amongst Direct Gearerefencing technique, Indirect Georeferencing technique and the Integrated Sensor Orientation is to be deployed. It is of importance to note that these three approaches come with strengths and weaknesses and thus the dire need for the National Mapping Agency to understand which amongst the three is best suited for the deployment for the exterior orientation of the airborne sensors. Having this information is critical for the Agency so that an effective operating is deployed which will render reliable results especially during the time of need. In case of massive floods, fierce forest fires and not forgetting the unclear cost line and grounds covered with snow, the agency has to deploy the best Georeferencing technique or system, which squarely depends on the initial capital for the purchasing as well as it, will for the installation and maintenance. Integrated Sensor Orientation utilizes the benefits generated from both the Direct and Indirect technique, which makes it easy to minimize the errors from the two systems. Conversely, the efficiency is always a point of concern as it involves time consumption. This explains the reason as to why the Integrated Sensory Orientation is only limited to stereo data collection as opposed to the other systems such as Direct Georerefencing, which can utilize existing Digital Orientation Parameters to produce the expected products with limited tie point overlap. References Halpin, T. A., & Morgan, A. J. (2008). Information Modeling And Relational Databases. Burlington, Ma, Elsevier/Morgan Kaufman Publishers. Http://Site.Ebrary.Com/Id/10244754. Tewari, A. (2007). Atmospheric And Space Flight Dynamics: Modeling And Simulation With Matlab And Simulink. Boston, BirkhäUser. Chamberlin, D. D. (1998). A Complete Guide To Db2 Universal Database. San Francisco, Morgan Kaufmann Publishers. French, F., & Burgess, C. (2007). In The Shadow Of The Moon A Challenging Journey To Tranquility, 1965-1969. Lincoln, University Of Nebraska Press. 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