Repositioning Layers

We will often want to move layers up, down, left or right to reposition them relative to each other within a map. Moving a layer within Manifold means repositioning the component that makes up that layer. If the component shows a drawing or image at some geographic location, moving the layer means moving all the objects or pixels in the layer to a different geographic location. This changes the georegistration of the component.


In ordinary graphics editors (like PhotoShop) that work with layers, moving layers is a simple matter of using a move tool to move the layer as desired. Because Manifold always works within a geographic context (even for "non-geographic" images or CAD drawings), moving layers about within Manifold involves some nuances that may require additional thought. There are two approaches to moving a layer within Manifold:


·      Repositioning the component using Georegistration or the Register dialog. These dialogs may be used at any time to reposition one component relative to another. Although normally used within geographic maps of various kinds, the georegistration dialog may also be used to align images or CAD drawings in non-geographic work.

·      Interactive repositioning using mouse and keyboard moves. These methods may be used when the layer to be repositioned has the same native projection as the projection in use in the map. Since all non-geographic image and CAD work is done in Manifold using an Orthographic projection by default, such methods are available for casual repositioning when using Manifold as a CAD system or as an image editor.


Georegistration and the register dialog are covered in their own topics. This topic discusses interactive methods for repositioning layers.


Requirements for Interactive Repositioning


To move a layer using interactive methods the layer must have the same native projection as is used for the map. The projection in use for a map may be seen in the status bar. The projection used by a particular layer may be seen by right clicking on the layer's tab and choosing Properties from the context menu. To re-project the map window so it uses the native projection of a layer, right click on that layer tab and choose Use Projection from the context menu.


This requirement is automatically met when using Manifold as a CAD editor or as a graphics arts editor. That is because blank images and drawings are created by default in Orthographic projection unless some specific geographic context is specified for them. Images and drawings imported from non-geographic formats (such as those used for graphics arts or photographic work with images or CAD formats such as .dxf) will likewise be imported using Orthographic projection.


The requirement is also met in much geographic work that combines both images and drawings. Images are usually overhead aerial or satellite photos that are best saved in Orthographic projection. It's a good idea to re-project all drawings, images and the maps that show them into the same Orthographic projection centered on the same central latitude and longitude point. This will assure fastest possible performance and will allow use of interactive repositioning methods.


Interactive Repositioning Methods


images\btn_grabber.gif If the mouse is not occupied in a command mode, we may interactively reposition layers in maps using the following CTRL-Grabber and keyboard commands.



Pressing the CTRL key while clicking and dragging with the Grabber tool will reposition the layer. Using the Grabber tool without CTRL will pan the viewport of the window. Using CTRL grabber will move the actual layer.

CTRL <arrow>

Pressing the CTRL key while pressing one of the keyboard arrow keys will move the layer in that direction.

SHIFT - CTRL <arrow>

Pressing both the SHIFT key and the CTRL key while pressing one of the keyboard arrow keys will move the layer in that direction in greater jumps.


To use these commands, the layer being repositioned must use the same projection as the map. Use the Project to Map choice in the layer tab's context menu to re-project the layer's component to match the projection currently used in the map. This is a permanent alteration in the data.




The illustration shows a map that includes a partially transparent drawing layer above an image layer. The image layer is the SanFran image that has been georegistered to a position close to, but not quite exactly, the desired position. Both the drawing and the SanFran image are in Orthographic projection, as is the map.




We can see that the SanFran image should be moved to the right.




We accomplish this by pressing SHIFT- CTRL right arrow a few times and then pressing SHIFT right arrow to make the match as exact as possible.


[The illustration shows the city of San Francisco as seen from Landsat 7. The prominent dark rectangular resion is Golden Gate Park. The dark region under the triangular point at the top of the San Francisco peninsula is the Presidio, with the point itself being the launching spot of the famous Golden Gate bridge. The bridge itself is just barely discernable between the point and the opposite shoreline.]


See Also



Orthographic Projection


Manual Georegistration