Georeferencing in ArcGIS

Transcription

1 Georeferencing in ArcGIS

2 Georeferencing In order to position images on the surface of the earth, they need to be georeferenced. Images are georeferenced by linking unreferenced features in the image with corresponding features in other datasets that have known coordinate system. When georeferencing an image that has already been produced in a certain projection (e.g., an old scanned hard copy map), the ArcMap data frame must be set to the same projection. As raster data, images have projection coordinates assigned to two of their corners and the position of each pixel is then derived, based on the row and column position of that pixel in the image matrix, while considering that the pixel size is maintained across the entire image.

3 Georeferencing vs. Orthorectification Georeferencing should not be confused with orthorectification: Orthorectification is a process of creating a planimetrically correct image (e.g., an aerial photo), where the inherent distortions and displacements in the image, such as the horizontal displacement, have been rectified and the features on the image are properly positioned planimetrically. Orthorectification is a more accurate process of determining locations for image features and should be used especially for images of areas with large topographic relief. Orthorectification requires elevation/terrain data. Georeferencing produces satisfactory results for flat areas and, if orthorectification is not available, can be used for areas of rough terrain. In such circumstances, positional error may be reduced by adding more control to such areas during Ground Control Point (GCP) collection.

4 Georeferencing Images in ArcMap Reference Layer 3) If using a polygon layer as reference, a hollow symbology is most likely appropriate. Image 1) Add data in two stages. The reference layer must be added to the frame first. Then the uncorrected image can be added. 2) The image will be georeferenced to the map projection set in the data frame (which will, by default, be that of the first layer brought into the project). Change the data frame projection if needed.

5 Georeferencing Images in ArcMap 4) Spatial features on the reference layer and the image are matched by the Fit to Display, Pan and Rotate functions on the Georeferencing toolbar (Customize Toolbars Georeferencing), and the general Zoom In/Out functions performed on the reference layer Fit to Display Pan and Rotate This process requires finding an area on the reference layer that can be viewed on the corresponding image. Next, choose Fit To Display. The objective then is to iteratively adjust the size and location of the reference layer features so they approximately match the corresponding features in the image. If needed, Pan and Rotate functions may also be useful in accomplishing this task. Now the main part of the georeferencing process namely adding control points can begin.

6 5) Once the reference layer and the image are closely-matched (spatially), control points can be added (1 st leftclick on the image point, 2 nd leftclick on the corresponding reference layer point) Note: Control points should be spread around the image. The third control point should form a triangle (and not a line) with the first two points. Georeferencing Images in ArcMap GCP buttons RMS Error Link Table button 6) After at least 4 (for 1 st Order Polynomial) control points (pairs) are established, the Link Table can be opened to check the RMS error. (At least 6 points are needed if using a 2nd Order Polynomial transformation.) Control Points Link Table Control points can be saved into a text file and reloaded if needed. Control points with high residual errors can be selected and deleted/replaced if necessary (press Delete Link button while a point is selected), to bring down the overall RMS error.

7 Entering Direct X,Y Coordintes If the x and y coordinates for a specific image point are known, they can be entered directly. For this, left-click on the image point first, then right-click and choose Input X and Y. Next, enter the x and y coordinates. Keep in mind that the coordinates have to be in the same map projection and datum as the datum frame/reference layer.

8 Georeferencing Images in ArcMap The RMS error is a Root Mean Square sum of all residual errors, each of which represents the difference between the point s new position (calculated by the chosen transformation [e.g., 1 st Order Polynomial] model using all GCP s) and its specified position on the reference layer (details about this relationship are shown in the Link Table). RMSE n ( X 2, X estimated, i ) ( Yactuali, Yestimated, i ) i 1 actuali i 1 n n 2 where X actual and Y actual are reference image coordinates, and X estimated and Y estimated are raw image coordinates at time/place i. The RMS error is a good measure of the transformation s accuracy, but should be accompanied, at a minimum, by a visual assessment of the registration.

9 Georeferencing Images in ArcMap 7) Once a satisfactory registration is achieved, along with a low enough RMS error (zoom to the image layer and confirm that features align), you can save the GCP data in the Link Table. 8) Lastly, the image can be georeferenced by choosing Rectify to create a new file with the projection of the reference layer. Note that using Update Georeferencing ** will provide/modify a spatial reference to the existing file. **Note that if you choose Update Georeferencing, a spatial reference is instantly provided and the data in the link table will be removed!!!

10 Rectifying Georeferenced Images The image can be rectified and saved as a new image, with the same projection. The new image can be used for analysis and can be saved in a format recognized by a wider range of image software. Resample Type: If the georeferenced image contains discrete values (e.g. thematic values) use Nearest Neighbour, otherwise, for images with continuous data (e.g., aerial photography, satellite imagery), use Bilinear Interpolation or Cubic Convolution. 9) Open the drop-down Georeferencing menu and click on Rectify. Depending on further analysis, set NoData to a value that is outside of the value range existing in the image (e.g for 8-bit data), or to zero. 10) Specify desired settings and parameters.