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This 20 page Class Notes was uploaded by Sierra on Friday March 4, 2016. The Class Notes belongs to FNR 210 at Purdue University taught by Ningning N. Kong in Spring 2016. Since its upload, it has received 14 views. For similar materials see Natural Resource Information Management in Agriculture and Forestry at Purdue University.
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Date Created: 03/04/16
Creating data for the Martell Forest & GeoRectifying 1 Martell Forest Information you have at the start point: ◦ Reference layers Existing reference maps. ◦ Background maps Aerial photo, scanned map, etc. What you need to do: ◦ Georectify the background maps ◦ Create & Digitize the information you know, and save into GIS. 2 Why? For maps without spatial reference information. Examples: ◦ Scanned maps – maps from books, historical maps, field scratch ◦ Aerial photos & Satellite images Most of the raster datasets that you obtain from the government or commercial sources are already georeferenced, and they should be ready for you to use. 3 Georectification: the process of aligning geographic data to a known coordinate system so it can be viewed, queried, and analyzed with other geographic data. This process creates additional information within the file itself or in supplementary accompanying files that specifies how GIS software should properly place and draw the data. 4 5 To convert your raster data into GIS, you need reference data that covers the same area as your target area. Reference data is a layer with a known spatial reference. All the transformations in the georeferencing are based on the assumption that the reference data is perfectly accurate. 6 7 In ArcMap, add the raster dataset that you want to align with your map. Add control points that link your raster dataset positions to known positions in map coordinates. Save the control points information when you are satisfied with the alignment. Permanently transform the raster dataset. 8 9 Control points: Locations that can be accurately identified on the scanned map/aerial photo and in real-world coordinates on the reference data. The control points are used to build a transformation that will shift the raster dataset from its existing location to the spatially correct location. Link: The connection between one control point on the scanned map and the corresponding control point on the aligned target data. 10 Example: road or stream intersections, the mouth of a stream, rock outcrops, the end of a jetty of land, the corner of an established field, street corners, or the intersection of two hedgerows. Control point is better to be selected based on artificial land markers. 11 Points should be located evenly across the image (sometimes difficult in natural areas) The first pair of points results in a shift in the image relative to the coordinate system. The second pair results in a change in scale across the image The 6third pair results in the production of affine transformation Subsequent pairs produce RMS (Root Mean Square) error calculations 12 View Link T able 13 Transformation Transformation Technique Required Best fit links Shift (zero order Slides already georeferenced data to its When previously georeferenced data needs polynomial) correct location 1 to be placed in its correct location Affine (1st order Rotates, shifts, and scales a raster to fit its For simple data alterations that fit the raster polynomial) proper location on earth 3 to its proper location on earth; optimized for global accuracy Higher order 6 (2nd) For complex data alterations that fit the polynomial (2nd and Bends, curves, and wraps a raster to fit iOR 10 raster to its proper location on earth; proper location on earth (3rd) optimized for global accuracy, 3rd) First performs a polynomial transformation For complex raster data transformations for global accuracy, then adjusts the that fit the raster to its proper location on Adjust control points locally, ensuring both good3 earth; optimized for both global and local global and local fit accuracy Transforms the source control points When control points are known and/or Spline exactly to the target control points, 10 precisely registered; optimized for scanned ensuring local accuracy imagery Preserves the projective properties of 4 When lines need to remain straight but not Projective embedded objects parallel; optimized for oblique imagery 14 The number of links you need to create depends on the complexity of the transformation you plan to use to transform the raster dataset to map coordinates. Adding more links will not necessarily yield a better registration. You can achieve a better georeferencing result if you evaluate the links, eliminate ones that do not match, and define the most accurate transformation for your data. 15 You can save the control points information and use it later. 16 All the transformations you did in Step 1-3 are temporary. You need to transform the scanned map permanently. Convert the scanned map to match the reference map. 17 Measures the residual locational error between the true coordinates and the transformed coordinates. 18 Minimum number of links – transformation equation can fit perfectly. However, using only the minimum number of control points is not recommended because you can overlook control point errors. Placing more than the required number of links introduces the possibility of errors. These errors indicate how well the transformation was able to "fit" your link point in the image to the desired map location. 20