![]() GCPs that are digitized in a Viewer are stored with the raster data that are opened in the Viewer. The source and reference coordinates of the GCPs can be entered in the following ways: GCPs in IMAGINE Files. Landmarks that can vary (for example, the edges of lakes or other water bodies, vegetation, and so forth) should not be used. For small-scale imagery, larger features such as urban areas or geologic features may be used. GCPs for large-scale imagery might include the intersection of two roads, airport runways, utility corridors, towers, or buildings. The more dispersed the GCPs are, the more reliable the rectification is. From the GCPs, the rectified coordinates for all other points in the image are extrapolated. Entering GCPsĪccurate GCPs are essential for an accurate rectification. Even though only one set of GCPs is associated with an image file, one GCP set can include GCPs for a number of rectifications by changing the point IDs for different groups of corresponding GCPs. Such GCPs are called corresponding GCPs.Ī default point ID string is provided (such as GCP #1), but you can enter your own unique ID strings to set up corresponding GCPs as needed. The point ID is a name given to GCPs in separate files that represent the same geographic location. In the Cell Array of GCP data that displays in the Multipoint Geometric Correction tool or GCP Tool, one column shows the point ID of each GCP. ![]() If a GCP set exists for the top layer that is displayed in the Viewer, then those GCPs can be displayed when the Multipoint Geometric Correction tool (IMAGINE ribbon Workspace) or GCP Tool (Classic) is opened. The GCP set is stored in the image file along with the raster layers. GCPs in ERDAS IMAGINEĪny ERDAS IMAGINE image can have one GCP set associated with it. ![]() ![]() For example, in image-to-image registration, map coordinates are not necessary. These coordinates are not limited to map coordinates. The term map coordinates is sometimes used loosely to apply to reference coordinates and rectified coordinates. Reference coordinates - the coordinates of the map or reference image to which the source image is being registered. Source coordinates - usually data file coordinates in the image being rectified. GCPs consist of two X,Y pairs of coordinates: GCPs are specific pixels in an image for which the output map coordinates (or other output coordinates) are known. A GCP determines the relationship between the raw image and the ground by associating the pixel (P) and line (L) image coordinates to the x, y and z coordinates on the ground. Ground coordinates can come from a variety of sources such as the Global Positioning System (GPS), ground surveys, Geo coded images, vectors, geographic information systems (GIS), topographic maps, chip databases, or by using photogrammetric pro- cesses to extend the number of GCPs in your images. Ground Control PointsĪ ground control point (GCP) is feature that you can clearly identify in the raw image for which you have a known ground coordinate. Often the process of rectification includes georeferencing, because one can both shift the pixels to remove distortion and assign coordinates to those pixels at the same time. The process of shifting pixel locations to remove distortion is known as rectification. The process of assigning geographic coordinates to an image is known as georeferencing.
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