Calibration

Calibration points specify the mapping between the page and coordinates. The Map | Plot | Calibrate command specifies a coordinate system for new boundaries digitized within MapViewer or changes the coordinate system on an existing map. Calibration is based on two "control points." Usually the calibration points are set when a boundary file is imported and need not be changed.

The Map | Plot | Calibrate command can be used in several different situations.

Establishing a map coordinate system on an image, such as a bitmap. If the bitmap you are importing does not contain any georeferencing information, the coordinate system is based on raster coordinates. The Map | Plot | Calibrate command allows you to replace the raster coordinate system.
Establishing the projection type for an imported boundary file that does not contain projection information. When you import boundaries files that do not contain projection information, the projection type is set to Unknown by default.
Allowing for specification of a coordinate system for new boundaries drawn within MapViewer. When you draw boundaries in a new map window, MapViewer uses a coordinate system based on 1,000,000 boundary units per inch. The origin of the internal MapViewer coordinate system is the center of the page.
Reestablishing coordinates for a map after they inadvertently are changed. If you move your map on the page, you can reset the coordinates so your boundaries are again in the correct position.

Using the Calibration Command to Recalibrate a Map

To establish a coordinate system on a map, you must have two control points defined for which you know the XY coordinates. These two coordinates give MapViewer enough information to extend the coordinate system over the entire extent of the map.

To change the calibration of a map:

1. Click the Map | Plot | Calibrate command .Notice that for unprojected maps, the Units in the Calibrate and Digitize Calibration Limits are Lat/Lon. For projected maps, the Units are converted to Meters.

2. In the Calibrate dialog, click the Recalibrate... button on the Calibrate page. The Digitize Calibration Limits dialog appears with a red arrow highlighting the Upper right line. The pointer changes to a cross hair and the upper right calibration point is indicated on the map. You do not need to use the calibration point indicated on the map.

3. Move the cursor to a point in the upper right region of the map with known coordinates and click. The Source X and Source Y fields change to the location on which you have clicked. Enter the Destination X and Destination Y coordinates of the point. If your coordinates are in latitude/longitude, enter the longitude for the X coordinate and latitude for the Y coordinate.

4. Click the Lower left line and follow the same procedure for the lower left calibration point.

5. Click the OK button in the Digitize Calibration Limits dialog to return to the Calibrate dialog.

6. The Assign Coordinate System page is used to specify the current map projection, if it is known.

7. Click the OK button in the Calibrate dialog to change the map coordinates.

You can press ESC at any time to exit without establishing the new coordinate system.

Zooming In and Out During Calibration

If necessary, you can zoom in and out while using the Calibration command by rolling the mouse wheel forward and backward. Pan the window by clicking and dragging with the mouse wheel button. The higher the zoom level you use during calibration, the more precise you can be when clicking the calibration points.

If your mouse does not have a wheel, before you begin the calibration zoom in on the upper right calibration point for higher precision when clicking the point. Then use the scroll bars at the bottom and right of the plot window to locate and click the second calibration point.

Setting the Projection Type During Calibration

The current projection is displayed on the Assign Coordinate System page. Use this page to tell MapViewer what the projection should be. If you do not know the projection type, you can select the Unknown option. A Cartesian coordinate system is used when the Unknown option is selected. Setting the projection requires specific knowledge of both the old and new projections.

Note: Use Map | Plot | Calibrate to set the existing projection of the map, not to change the projection of the map. If you would like to change the projection from one known projection to another projection, change the Coordinate system in the Coordinate System page of the plot properties.