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gvSIG-Desktop 1.1. Manual de usuario.
Georeferenciación con cartografía base y puntos de control
Introducción
The gvSIG programme allows an image to be georeferenced (.jpg, .tif, .gif, .png) using control points on a georeferenced base map. Firstly, the georeferenced base map needs to be loaded (in raster or vector format or both) to a gvSIG view.
Secondly, the image to be georeferenced must be added. You can load this image by going to the “Add layer” tool and selecting the “Georeference” tab. Use the “Load” button to access the image you wish to georeference.
If you enable the “Use image georeferencing” check box and the image is georeferenced, it will be added to the view with its current georeference. If it is not, the image will be loaded in the centre of the view. You will then find a browser button which can be used to select the map projection.
When you have finished the process, click on “Ok”. The image to be georeferenced will appear in the view and will have an asterisk “*” in the ToC to indicate that this is the layer the process is being carried out on. A new window will also appear to carry out the georeference process but first, the image to be georeferenced will have to be moved closer to the base map’s reference zone so that you can select the control points more quickly.
Aproximación de la imagen a georeferenciar
If the layer to be georeferenced is selected in the ToC, four new buttons will appear in the tool bar.
This button resizes the image to be georeferenced.
The image size can be modified and brought closer to the base map scale.
Select this tool and go to the image to be georeferenced.
A frame will be created around the image. If you place the cursor on one of the image sides, it will turn into a double arrow. This will then allow you to modify the frame size by moving the cursor.
The newly created frame boundaries will become the new reference size for the image being georeferenced.
This tool allows you to move the image to be georeferenced around the view and thus bring it closer to the reference base map.
This tool takes you back to the previous position of the image to be georeferenced in the view.
This tool is activated when the previous tool has been used and allows you to redo what you have previously undone.
Selección de puntos de control
Click on the “New point” button in the “Georeference” dialogue and go to the view.
Place the mouse pointer over the image to be georeferenced and look for a control point. This point is defined as a red circle with a cross in the middle. If you move outside the boundaries of this image, gvSIG shows an error message and you will have to select a point once again. If you click on the following button
two small windows will drop down. In the left-hand window, the location of the created control point appears over an image zoom, as the following image shows.
If you wish to specify the point further, you can zoom into the image using the zoom tools in the centre of the dialogue box.
If you then click on the point in the mini view and drag it you can move its location.
These changes will automatically be reflected in the gvSIG view.
When you have marked the control point on the image to be georeferenced, this will disappear from the view (this option is marked by default in the "Options" tab in the "Georeference" window and can be modified at any time) and will only leave the base map with the destination point marked on the view.
In this case, the point is defined by a blue square with a cross in the centre.
You can do the same with the mini view on the right as you did with the left, i.e. if you wish to move closer to the point location, you can zoom into the right-hand image and drag the point over the view to place it more precisely.
You can also specify the points in the text boxes in the georeference dialogue window. When the points have been input, press “Enter” and the point will be located in the view.
Remember that the “X” and “Y” text boxes belong to the image to be georeferenced and that “Longitude” and “Latitude” correspond to the georeferenced image. If you have made a mistake with the point and wish to delete it, click on “Remove selected point”.
If you click on this button, a confirmation message will appear.
If you click on "Yes", the point will be deleted. If you wish to remove all the points, click on the “Remove all the points” button.
The control point selection must be repeated as a minimum of three points are required for georeferencing, although at least nine are advisable.
There is another series of tools in the “Georeference” window:
This tool allows you to go to the first control point, to the previous point, search for control points using a pull-down menu, go to the following control point and go to the last pull-down point respectively.
This button allows you to select control points from the view. Firstly, select the control point of the image to be georeferenced and finally, the control point of the destination coordinate.
You can disable an input control point via the "Active” check box in the “Point” tab. This means that the point will not be used in the error and georeference calculations and the symbols will not appear in the view. If you need the point again, you can enable the check box.
If you click on the “Table” tab, a table will appear with all the points and their errors. This table can be enlarged to show more information.
Guardar y cargar puntos de control
When you have finished the georeference process, you can save your control points by using the following tools:
In a metadata file associated with the image with the .rmf extension (if data already exists in this file, gvSIG will warn you that it is going to overwrite it).
In a .csv file: In this case, a dialogue window opens to select a file to save the data (X, Y coordinates and the errors in X, Y and RMS).
However, if you wish to add points from a file, use the following tools to retrieve .rmf control points associated with the image.
To load control points from a .csv file.
N.B.: You can also save the georeference points in the table panel.
Opciones
You can use the “Options” tab in the georeference window to enable and disable some of the options according to your preferences.
Restricciones
It is currently only possible to georeference bmp, gif, jpg, png and tif formats.
