Jun Liang, Geography @ UNC Lab1 Instruction Georeferencing a raster Add the layers residing in map coordinates (target data) and the raster you want to georeference. In the table of contents, right-click a target layer and click Zoom to Layer. From the Georeferencing toolbar, click the Layer dropdown arrow and click the raster layer you want to georeference. Click Georeferencing and click Fit to Display. This will display the raster in the same area as the target layers. You can also use the Shift and Rotate tools to move the raster as needed. Click the Control Points button to add control points. 2018/9/20 Jun Liang, Geography @ UNC

Steps for georeferencing To add a link, click the mouse pointer over a known location on the raster, then over a known location on the target data. You may find it useful to use a Magnification window to add your links in. Add enough links for the transformation order. You need a minimum of three links for a first-order transformation, six links for a second order, and 10 links for a third order. Click View Link Table to evaluate the transformation. You can examine the residual error for each link and the RMS error. If you're satisfied with the registration, you can stop entering links. Click Georeferencing and click Update Georeferencing to save the transformation information with the raster. This creates a new file with the same name as the raster but with an .aux file extension. 2018/9/20 Jun Liang, Geography @ UNC

Tips for georeferencing To display the Georeferencing toolbar, right-click the Tools menu, point to Toolbars, and click Georeferencing. You can delete an unwanted link from the Link Table dialog box. Press the Esc key to remove a link while you're in the process of creating it. You can permanently transform your raster after georeferencing by using the Rectify command. Click Georeferencing and click Rectify. This creates a new raster dataset in GRID, TIFF, or ERDAS IMAGINE format. 2018/9/20 Jun Liang, Geography @ UNC

Transformations Basics Transformations convert data from one coordinate system to another. They are often used to convert data from digitizer or scanner units to real world coordinates. The transformation functions are based upon the comparison of the coordinates of source and destination points, also called control points, in special graphical elements called displacement links. 2018/9/20 Jun Liang, Geography @ UNC

Transformations Basics By default, ArcMap supports three types of transformations: Affine, Similarity, and Projective. An affine transformation can differentially scale the data, skew it, rotate it, and translate it. The graphic below illustrates the four possible changes. x’ = Ax + By + C y’ = Dx + Ey + F where x and y are coordinates of the input layer and x’ and y’ are the transformed coordinates. A, B, C, D, E and F are determined by comparing the location of source and destination control points. 2018/9/20 Jun Liang, Geography @ UNC

Transforming the raster When you've created enough links, you can transform—or warp—the raster to map coordinates. Warping uses a mathematical transformation to determine the correct map coordinate location for each cell in the raster. Use a first order—or affine—transformation to shift, scale, and rotate your raster. Straight lines on the raster are mapped onto straight lines in the warped raster. Thus squares and rectangles on the raster are commonly changed into parallelograms of arbitrary scaling and angle orientation. 2018/9/20 Jun Liang, Geography @ UNC

Transforming the raster (Cont) A first order transformation will probably handle most of your georeferencing requirements. With the minimum of three links, the mathematical equation used with a first order transformation can exactly map each raster point to the target location. Any more than three links introduces errors, or residuals, that are distributed throughout all the links. In practice, add more than three links. Given only three, if one link is positionally wrong, it has a much greater impact on the transformation. Thus, even though the mathematical transformation error may increase as you create more links, the overall accuracy of the transformation will increase as well. 2018/9/20 Jun Liang, Geography @ UNC

Interpreting the root mean square error The degree to which the transformation can accurately map all control points can be measured mathematically by comparing the actual location of the map coordinate to the transformed position in the raster. The distance between these two points is known as the residual error. The total error is computed by taking the root mean square (RMS) sum of all the residuals to compute the RMS error. This value describes how consistent the transformation is between the different control points. While the RMS error is a good assessment of the accuracy of the transformation, don't confuse a low RMS error with an accurate registration. The transformation may still contain significant errors, for example, due to a poorly entered control point. 2018/9/20 Jun Liang, Geography @ UNC

Jun Liang, Geography @ UNC Lab 1 description You will be given 4 tiff files, and you need to geo-referencing them, and organize them in one Arcmap document. Because they are next to each other, you can check your work by examining edges between them, especially linear features. Data – I:/datashare/geog192/lab1 Output Requirement – Create a folder /geog192/lab1 under your H driver, and save your .mxd file there. You will need to email me the .mxd file before the due day. Note: I drive is geography’s AFS space. Path: /afs/isis/depts/geog H drive is your personal AFS space. Due day: Feb 3, 2005, before class. 2018/9/20 Jun Liang, Geography @ UNC