Automatic generation of elevation data over Danish landscape

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Automatic generation of elevation data ages in 1:5,000, the first strip was scanned with north upwards, the second strip with south upwards, the third strip again with north upwards and the last strip with south upwards. In this project, the orientation parameters are therefore keyed in manually. 5.1.5 Pre-processing of primary data After the orientation of the images, the pre-processing is carried out. First the images are normalised, then an image pyramid and an object pyramid respectively are created. 5.1.5.1 Normalisation Each image was normalised, that is, an epipolar geometry was built up according to the principles described in Appendix 1, section A.1.4. These normalised images were then stored on the hard disk. 5.1.5.2 Image pyramid For each of the normalised images, an image pyramid was built. As standard, the image pyramid was built by the Gaussian function over 5 x 5 pixels. For images with a resolution of 15 μm, 9 levels more than the original image were built. For images with resolution 30 μm, the image pyramid was built with 8 levels, and for the 60 μm images with 7 levels more than the original image. (Cf. Appendix A, section A.1.7). The standard set-up for Match-T has been chosen for this project. 5.1.5.3 Object pyramid The object pyramid with interest points is built from the image pyramid. This is done according to the principles described in Appendix A, section A.1.8. Primary data, that is, the normalised images and the image and object pyramids are not changed by the later correlation or DEM generation. This process is therefore not influenced by changes such as, for instance, a different set up of parameters for the DEM generation. Therefore, primary data is re-used in the subsequent calculations with different mesh sizes. This is done to save calculation time. 5.1.6 The DEM generation Input arguments for the grid generation include indication of the calculation area defined by the lower left co-ordinate (lower left, II) and the upper right co-ordinate (upper right, ur). In addition, a starting point is indicated. This starting point must be located within the model. In this project, the co-ordinates for (II) and (ur) are chosen, so that the calculation areas are indicated by means of the corner points in such a way that neighbouring areas adjoin without overlap. This is done to avoid elevations being generated over the same area twice. This is valid for neighbouring models as well as between strips. By avoiding overlapping areas, possible problems arising from such double elevations will not have to be taken into consideration later in the project. The mesh size is chosen under the parameters for DEM generation. For this project, elevations are generated in a grid with a mesh size of 5 x 5 m, 12.5 x 12.5m and 25 x 25m respectively. As mentioned earlier, the primary data is not re-calculated, it is only the DEM generation itself which is determined for each mesh size. To achieve the optimum standard of comparison, the automatically generated grids must be coincident with the frame of reference which has a mesh size of 25 x 25m and 5 x 5m respectively. Therefore, the automatic elevations are generated in the same grid as the frame of reference. This means that for the automatically generated grids with the mesh size 25 x 25 m, the same co-ordinate endings are fixed as for the frame of reference, which means that the planimetric co-ordinates X an Y end on ***00.00, ***25.00, ***50.00 and ***75.00. For grids with a mesh size of 12.5 x 12.5 m, the co-ordinate endings are ***00.00, ***12.50,***25.00, ***37.50, ***50.00 etc. For this mesh size every other grid point will be coincident with a reference point. For automatically generated grids with a mesh size of 5 x 5 m, the planimetric co-ordinates end in ****5.00 or ****0.00. Here each point will be coincident with a reference point in the reference grid of 5 x 5 m, while there will be coincidence in every 5th reference point in the 25 x 25m grid. 60
5 Preparation for the grid generation 5.1.6.1 Correlation The standard value for the correlation window is 5 x 3 pixels. The threshold value of the correlation coefficient is as a standard value 0.75. The correlation coefficient is described in Appendix A, section A.1.1.1.1. In this project the landscape is considered as flat. The standard value for the correlation window and the correlation coefficient is selected and fixed. The choice of a flat terrain entails a parallax bound in the x-direction (because of the normalised images) of 3 pixels, see Chapter 2, section 2.4.3.1. 5.1.6.2 The finite-element reconstruction In the finite-element reconstruction, the values for the curvature, the torsion, the standard deviation etc. are fixed. The standard value of the curvature and torsion is 3, see the description of curvature and torsion in Chapter 2, section 2.4.3.2. The standard value for the standard deviation of the determined terrain points is 0.20m. The iteration process goes through 2 separate phases. The standard value of the first phase is 6 and for the last phase is 4. The last parameter used in the set up in this project is the threshold for the bias, the standard value is 35 %. All these standard values are fixed. The chosen values are presented in table 5.2: Function Parameter Value Comments The image and object pyramid Gaussian function over 5 x 5 pixels 9 levels Standard for 15 μm images The image and object pyramid Gaussian function over 5 x 5 pixels 8 levels Standard for 30 μm images The image and object pyramid Gaussian function over 5 x 5 pixels 7 levels Standard for 60 μm images DEM generation Window size for convolution 5 x 3 pixels Standard window size DEM generation Parallax bound in row direction 3 pixels Standard value for flat terrain DEM generation Threshold for correlation coefficient 0.75 Standard value Finite-element Curvature and torsion 3 Standard value Finite-element Standard deviation for grid points 0.20 m Standard value Finite-element First iteration process 6 Standard value Finite-element Second iteration process 4 Standard value Finite-element Threshold value for bias 35 % Standard value Table 5.2: The standard set-up for Match-T for all calculations. The above parameters are fixed in all automatically generated grid calculations. As described in Chapter 4, there are digital images in three different scales and three different resolutions. In addition, calculations are planned with three different mesh sizes, that is, 27 calculations in all. 5.2 Grid calculation After setting up Match-T, the calculations are done. In Match-T there are three modules where the grid calculation can be followed online. These modules are called: � Monitor for individual programme steps � Graphic online/offline � Online statistics 5.2.1.1 Monitor for individual programme steps While the calculation itself is running, the process can be followed in the window, ”Monitor for individual programme steps”. Here the individual steps for normalisation, pyramid build-up and DEM generation can be followed numerically and in bar form. In this window, the currently used time and the estimated use of time are indicated for each process. Likewise, there is a possibility of interrupting the calculation. In this project the module is only used to follow the calculation and will not be mentioned further. 5.2.1.2 Graphic online/offline Graphic online/offline is a visualisation function which offers the opportunity to follow the correlation and grid generation process online, where the correlated points and grid points are shown. This function must 61
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Automatic generation of elevation d
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Abstract This thesis consists of tw
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Table of contents: Table of content
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9 Theory and practice Scale Resolut
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Conclusion and perspectives 10 Conc
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Conclusion and perspectives problem
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Appendices Version 1.1 Appendix A:
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Table of contents B.2.1.5 Evaluatio
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Appendix A
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A.1 ABM and FBM Appendix A: Correla
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The correlation coefficient may the
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A.1 ABM and FBM If the correlation
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A.1 ABM and FBM level values (edge
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A.1.2.3.2 The Förstner operator A.
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This is done by an adjustment by th
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A.1.2.3.5 Quality evaluation of the
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K 1 A.1 ABM and FBM By means of the
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A.1.4 FBM in one dimension A.1 ABM
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A.1 ABM and FBM Image pyramid Objec
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Appendix B
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B.1 Description of the data materia
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Figure B.1.2: The flight paths for
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B.1 Description of the data materia
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B.2 Data capture B.2 Data capture T
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Figure B.2.3: Example of the rover
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y = 0.011 m z = 0.011 m B.2 Data ca
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B.2 Data capture The accuracy requi
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B.2 Data capture The standard devia
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Appendix C
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C.1 Description of the analysis pro
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C.1.3.3 Calculation options C.1 Des
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Editing programme
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Editing programme
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Bundle adjustment E.5 Aerotriangula
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E.6 Aerotriangulation for images in
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Graphic online/offline Graphic onli
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Appendix G: Sizes of grid files G.1
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G.1 Sizes of grid files Appendix H
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Code specification Code specificati
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Automatic generation of elevation data over Danish landscape

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