Managing Control Points

Use the Control Points pane to add new control points to a drawing, image or map. Control points are used to georegister an image or drawing to the same geographic location and projection used by a "known good" image or drawing.




The Control Points pane always shows a list of control points defined for the active image or drawing together with their coordinates. When we click on a different window the Control Points pane will switch context to show any control points defined for the active window. See the Georegistration topic for basic use of control points.


For more sophisticated use and management of control points we can use four controls in the Control Points pane:



Load Points - Load control points from points that exist in the drawing (enabled for drawings only).


Save Points - Save control points as points in the drawing (enabled for drawings only).


Load Points


Drawings often contain named points that can be used as control points. This saves us the extra work of adding control points for those locations.




Suppose we have a scanned image of a paper map that we would like to georegister using control points.




Suppose we also have a drawing of the same geographic region that contains points. In this example we will assume that the points represent waypoints we have acquired with a GPS device.




If we look at the table for the points we can see that each has a Name plus additional fields, such as the name of the street on which each waypoint is located.




We will pick out four of the waypoints to use as control points. We could use all of the points in the drawing, but for the purposes of this example we will choose only four so the screen shots do not get too cluttered with labels.




We've picked out the points named WP 043, WP 017, WP 006, and WP 001, as can be seen by looking at the table.




The Control Points pane so far is empty for this drawing.


images\btn_load_points.gif We click on the Load Points button to load points from the drawing. This opens the Load Control Points dialog.




The Load Control Points dialog asks us which field to use as the name field for the control points. The list box will be loaded with all text fields available in tables associated with that drawing. We choose the field called Name since that is where the names of our waypoints are stored. We click OK to load the drawing's points as control points.




The drawing's points will be imported into the Control Points pane as control points, using the designated name field value for their Name.




The control points will appear in the drawing in the usual way, with labels using the imported names for each point.




To use these control points, we click on the image to be georegistered and add control points in the image at the same locations and with the same names as the control points that were added to the drawing with the Load Points button. The image is shown slightly desaturated and lightened so that the control point labels stand out better.


Save Points


After adding control points to a drawing using the Control Points pane we may wish to add them to the drawing as point objects as well.




Suppose we have added many control points to a drawing of the United Kingdom.


images\btn_save_points.gif We can add these locations to the drawing as point objects by pressing the Save Points button in the Control Points pane.




The result is the creation of a new point object at every location of a control point. We can see the new points by turning off display of control points by unclicking the Preview button in the Control Points pane. We have formatted points in the drawing to be largish, green squares so they are more easily visible.




By opening the table associated with this drawing we can see that the Save Points command created new point objects and also saved the name of each control point into a text field that we specified (in this example, a field that is called "Name").


The Save Control Points dialog allows us to save the names of the control points into any text field in the database table associated with the drawing. The Name list box will display a list of all available text fields in the table that we can use for saving the name. There is also a choice available to not save the name text, in which case the control points will be created as point objects without their names being saved to any text field.




It does no harm to load lots of control points if there are many points in a drawing. If desired, we could have loaded dozens of control points into the drawing's Control Points pane and then specified only four control points in the image. All of the extra control points in the drawings whose names didn't match those in the image would have been ignored. Why then, did we use only four control points in the drawing in this example?


There are two main reasons: first, in real life usage it is easier to see where to place control points in an image if there are only a few control points in the drawing that need to be matched. Second, since Help file illustrations must be small in size there would have been too much visual clutter in the screen shots if we had loaded dozens of control points.


One of the illustrations above shows only four points being used in a drawing out of many original points. To create this illustration, we selected the four points desired, used a Select Box with Invert selection mode to select all the other points except these four and then deleted all the other points. In real life we would probably have copied the four selected points to a new layer instead of deleting all the unnecessary points.


The Ignore blank values and Ignore duplicate values checkboxes are disabled in the Load Control Points dialog screen shot because no point records in the example table had blank values or duplicate values.


Why is there an Ignore duplicate values checkbox in the Load Control Points dialog? Why would we ever want to load more than one control point with the same name? If more than one point in the drawing has the same name, we might wish to see all of the duplicates as control points. We can then easily decide which duplicates should be deleted from the control points list.


images\btn_native_coords.gif The Native Coordinates button was not used to toggle to regular latitude / longitude values in the screenshot of the Control Points dialog, so the X / Latitude and Y / Longitude fields were shown in the control points list using the "native" internal coordinates of the projected map.


Very important: Points in drawings that we might wish to use as control points will often have data fields associated with each point in which latitude or longitude values occur. When acquiring points from a GPS, for example, we will often ask the GPS console to save the latitude and longitude for each point in a data field in addition to creating a point at that location. It is very important to understand that the location of the control point is taken directly from the actual geometric location of the point within the drawing and is not taken from any fields with latitude or longitude values that happen to exist within the data table. See the Editing Data in Tables topic for a discussion of this issue.


For brevity, this topic and other georegistration topics use images as examples. However, the same procedures apply when georegistering drawings or surfaces .


The font used for control point labels may be changed in Tools - Options .


Tech Tips


When we have lists of control points measured from a paper map or taken from a GPS device it is often convenient to enter many control points by creating a table with latitude, longitude (type latitude and longitude, of course) and name (type text) fields. Each control point can then be entered into the table.


When editing fields in tables, Manifold will automatically make sense of many different ways of entering latitude and longitude data into latitude and longitude type fields. This is extremely useful when entering coordinates for control points read off from paper maps that are marked using degrees, minutes and seconds notation. See the Create a Table and Add Records example for a step by step example of how one could add control points to a table using degrees, minutes and seconds format.


Uncheck the Autoscroll window on edit or selection operations option in Tools - Options when working with control points. Because using the New Control Point tool activates the mouse as soon as it leaves the Control Points pane, if this option is not deactivated the target window will begin to autoscroll as the mouse enters it.


See Also


·      The File - Print dialog used to print a component includes a Control Points option that may be used to print the control points in a component.

·      See the Control Points pane topic for a reference guide to the pane used to assign and manage control points.

·      The Georegister a Scanned Paper Map example topic shows a common georegistration task in step-by-step detail.