The Relink and Unlink commands are enabled when the focus is on a window for an image linked from an OGC WMS image server or from a Microsoft TerraServer image server.
· Relink - Used to reconnect a linked image to an image server. This command is also available within the linked image's context menu in the project pane. If a linked image is disconnected from an image server this command is used to reconnect it. For example, one might create a .map file that contains a linked image that is linked to an OGC WMS image server using a local URL. If the .map file is moved to another machine the link might not continue to work without adjusting the URL. In that case one can use the Relink command to reconnect the image in the project to the image server
· Unlink - Unlink a linked image to create an image in the project. All data for the image is copied from the image server or cache if caching is enabled (the default) and embedded within the project as would be the case for an ordinary, unlinked image.
Image - Download and Image - Relink / Unlink Compared
An image linked from an image server may be converted to a local image within the project either by using the Image - Unlink command or the Image - Download command. The Unlink command converts the existing linked image to a local image at whatever resolution is currently in use. The Download command may be used to automatically fetch all image tiles at any available resolution to create a local image at that resolution.
Images linked from TerraServer may also be exported to image files. In that case, the exported image will be the same as that which would be obtained by unlinking the TerraServer image.
Note that exporting or unlinking an image linked from TerraServer will only use the data already downloaded from the server and will not attempt to download any more data. It will also only use the data for the most detailed image level. It is usually a good idea to use Image - Download to check how much data has already been downloaded for the most detailed image level prior to exporting or unlinking the image.
Another way to create a local image from a linked image is to open the linked image and then to use the keyboard F6 command or the Tools - Make Image command. The Make Image dialog allows us to create a local image in several different ways from the linked image without unlinking the linked image. For example, we can use this command to create an image of the displayed window or of the entire linked image at a desired number of pixels or scale.
The Unlink command attempts to build a real image of the same pixel dimensions (width and height) of the linked image. Whatever tiles have been downloaded using the Download command will be used, and will be interpolated to cover those pixel extents. Keep that in mind if you want to unlink a huge linked image.
For example, suppose we create a linked image from a Manifold image server that covers the entire Earth. Images servers will now routinely offer to provide images at 0.3 meter resolution. If we link in such an image from an image server the linked image will be over 120 million x 120 million pixels in size. That's still easy to use within a Manifold project because zooming in or out or panning the image only shows a million or so pixels (1000 x 1000) in whatever viewport we have open, but if we unlink that image we are asking Manifold to create an image that is 120 million x 120 million pixels in size... that's over 50,000 terabytes in size at four bytes per pixel.
One might think that specifying a smaller number of tiles to download via Image - Download will do the trick to keep the data size down, since the Unlink command will use only the tiles downloaded. But suppose we download only those tiles representing, say, 100 kilometer x 100 kilometer resolution: the image we are asking Manifold to create is still 120 million x 120 million pixels in size, at 0.3 meters per pixel. What we are asking Manifold to do will be to take each of those limited number of 100 km x 100 km "pixels" and inflate them to cover the 300,000 x 300,000 pixels of 0.3 meters each within the larger blocks so that each such smaller pixel is the same color.
If what we want to do is to create an unlinked image with a smaller number of pixels, we should specify a lower resolution for the original linked image, choosing some value much greater than the 0.3 meter value for the image server we are using. For example, choosing, say, 5 kilometers results in a much smaller 7500 x 7500 pixel linked image. We can download all the tiles for that and Unlink and still have a relatively workable image.
Linked Images from OGC WMS Servers
Linked Images from TerraServer
Image - Download
Tools - Make Image