ClueTrust KB

How can I georeference maps in Cartographica?

Question: I do not understand something about importing a map. If I have a image file (TIFF, JPEG, etc) I can import the image, but the image will not be rectified. How do I georeference the map image so imported GPS positions will be properly placed on the image? Answer:

Cartographica 1.2 has the ability to manually, or through the use of Ground Control Points or georeference matrixes, georeference raster data.

If you have an ungeoreferenced image, you will need to georeference it before it will show up correctly on the Map.   Georeferencing involves telling Cartographica the location and size of the raster image relative to some known Coordinate Reference System (CRS), so that it can be displayed correctly along with other data.

The process is as follows:

1. Make sure you have a reference layer loaded, unless you are manually setting the matrix or ground control points.   A vector map of the area or a live map are often good choices for this.
2. Load your Raster layer by using drag-and-drop or File > Import Raster Data...
3. Click on the new Layer in the Layer Stack to select it
4. Choose Layer > Set Layer's Projection to choose the CRS in which the layer will be referenced (usually the same as the current map or the reference layer).
5. Zoom and pan the reference layer until you can see a location that is basically the same area as the area of the raster layer.   Ignore for the time being the raster layer, since it likely will be very far away from its actual location to start with.
6. Choose Edit > Georeference Image... to begin the georeferencing
   At this time, the Georeference panel should come up, providing control over the georeference process.   By default, it starts in Ground Control Point mode, which allows you to drag the dots in order to match points in the reference layer.
7. Click Fit Display
   At this time, the Raster image should be visible on the screen (although probably not in exactly the right place). 
8. If your raster layer appears upside down, click Flip Vertical to correct that.
9. On a machine with a trackpad, you can adjust the image's size and rotation using pinch and zoom and rotate.
   Arrow keys can be used to "nudge" the position of the raster image
   The mouse can be used to move the existing four corner GCPs so that they align with data in the reference layer
   If you hold the Option (Alt) key down and click, you can create new GCP anywhere within the image.   Option-click on the image to create the reference and drag to the point in the reference layer that represents that location.   With more than 4 locations, a minimum of 7 GCPs is suggested for smooth representation.
10. When you are done with the georeferencing, select Georeference Image again from the Edit menu to end the edit 

Versions of Cartographica between 1.2 and 1.2.3 used Edit Selected Feature and required the selection of the actual image (not just the layer) in the Data View before editing could commense.

Versions of Cartographica before 1.2 require the use of already georeferenced material.

Related Articles

No related articles were found.

Attachments

No attachments were found.

Visitor Comments

No visitor comments posted. Post a comment