Warp Data and Grids

Warping is the process of re-projecting data coordinates numerically (i.e., instead of using standard analytical methods for projecting to UTM, longitude/latitude, and other coordinate systems). With Oasis montaj warping, you define an n-sided polygonal outline (either in a file or interactively) by defining control points. Then you have the choice of warping data (creating new X and Y channels) or warping entire grids based on this shape.

Methods of Defining a Warp File

Interactively 

Interactive warping is a method that enables you to use the mouse to select a point for warping and then click to identify the new location for the point. When you are finished, the system creates a warp file (*.wrp). The warp file is then applied to the image to create a new warped image.

Semi-interactively 

Semi-interactive warping is a method that enables you to use the mouse to define up to 4 control points for warping. For each point, the system shows the current coordinates and prompts you to enter new projection coordinates. The coordinate information for these points is stored in a warp file (*.wrp). The warp file is then applied to the image to create a new warped image.

Manually

Manual warping is a method that prompts you to enter both the old and new positions in a dialog window.

Use warping when you have data in an arbitrary coordinate system and want to locate the data in a real coordinate system. For example, you may have an image with only pixel coordinates and want to import the image into a map with projected coordinates.

Do I "Apply" or "Attach" a Warp to a Grid?

Oasis montaj provides two grid warp methods; "apply a warp" which creates a new warped grid file, and "attach a warp" which warps on-the-fly when the grid is displayed. A good rule of thumb is that you should only "attach" a warp to a grid/image file if the grid is so large that you do not want to re-create it by warping it. This is most commonly the case if you have a compressed image file that is very large. Warping such an image into a Geosoft colour grid (uncompressed) can produce an impossibly large grid. If the image is scanned, and used only for displaying on a map with other elements, it is better to simply attach a warp. The attached warp information will cause the image to be warped-on-the-fly when it is displayed.

If you have warped coordinates, use the New Projected Coordinate System menu option to create a new coordinate pair that is not warped. Attach the warp file to the original X,Y coordinates and then create a new channel pair without the warp. After that, change coordinates to a new pair.