How to create offsets in cm93 charts      Back to Valhalla's Mooring Page          Back to NAVIGATION

First go here to read from the OpenCPN help file.

My procedure uses Google Earth to find a 'ground truth' point of known position, create a mark(s) at that point in the GE image, then by trial and error adjust the X and Y values in the cm93 Offset Dialog to bring the mark(s) in alignment with the cm93 cells.  Here's an example:

In OpenCPN display the area that is known/suspected be require an offset. 

Launch GE2KAP and enter the information as shown.  In Chart Location/Name I have previously made a folder C:\ChartsGE2KAP\  (need to be done a priori) and now enter a name for this location (Kuop Atoll) followed by the suffix '.kap'  The 'Overlay Chart/..' window has 'opencpn' entered and the Depth Units (which I have selected on my OpenCPN vector charts) is entered a 'Feet'.  When this is entered click on 'Create Chart'.

The GE2KAP program runs until the following is displayed.  BE SURE YOU DO NOT TOUCH ANYTHING UNTIL YOU SEE THIS SCREEN.

Using the Transparency slider, check the alignment between the cm93 chart and the GE image.  As shown below there is a discrepancy that can be corrected by changing the offsets in the OpenCPN display of the cm93 chart cells.

Move the slider all the way to the left to 'Clear'.  This will create a .kap chart of only the GE image.  Click on 'OK'.

This screen appears when the chart has been created.

In the OpenCPN program go to Toolbox/Charts and select 'Force Full Database Reload'.  Note that the Active Chart Directories includes the path to where GE2KAP has stored the image just created.

In OpenCPN, display the newly created chart by clicking on the blue bar at the bottom.  Now find a point or two that can be recognized on both this chart and the cm93 chart.  Right click on the place to drop the mark and select 'Drop Mark Here'.

Here I've done the same for another mark.

Display the cm93 chart (click on the yellow bar at the bottom).  Note the location of the marks created previously. 

Check to make sure you have Chart Quilting selected in OpenCPN.

Press F9 to change the quilting.  Right-click on the screen and select 'CM93 Offset Dialog'.

A window with the cells near to where you right-clicked appears. Selecting a Cell shows that cell outlined in yellow.

Now through trial and error enter values for X and Y to make the marks coincide with the locations on the cm93 display.

Positive values of X move the chart to the right and negative values of Y move the chart up.

Note that a nearby cell is also listed.  It's a good chance it can use the same offsets ... which it does. Now click on 'OK' to return to the normal display and press F9 to change back the chart quilting.

It is now also a good time to delete the marks used for calibration.  Use Route Manager for this.

When you are finished with the offsets you want to make, you can find them in the OpenCPN configuration file.  In Windows XP the file is at C:\Documents and Settings\All Users\Application Data\opencpn  In Windows7 it is at C:\ProgramData\opencpn\    A subdirectory, 'cm93' contains the offset data.  This folder can be installed on other computers and will give the offsets for any version of cm93 that is installed

Mine are available for download here.