Friday, 10 April 2015

Google Earth Charts - A New Aid To Navigation

The Red Icon Shows Our Position, Safely Anchored In The Lagoon At Dhanghethi Island
 We have multiple electronic charting systems aboard Crystal Blues, with multiple sources of navigation data.  But they are all inaccurate in many parts of the world.

In some cases smaller nations simply can't afford to fund extensive maritime surveys.  Further, the electronic charting companies often simply won't pay for the latest data, so we rely in many cases on data and soundings that were collected by master mariners (sometimes) centuries ago.

This means that many islands and reefs are not exactly where the charts say they are, with the errors being both substantial and dangerous.

The image above is captured from our Open CPN navigation software, using charts created from Google Earth images.  It doesn't have depths and soundings, but in these tropical areas we can use the water color to plan our route and we certainly know exactly where we are at all times.

Contrast that with this image of the exact same area, extracted from our (expensive) ECDIS based charting package.

This puts our position as outside the lagoon !  In fact it doesn't show the lagoon clearly at all.  I estimate the charting error places the island at least 500 meters further east than it actually is.  Not surprising, when you think it was probably positioned by a 19th century chronometer and sextant.

So charts made from Google Earth images are now a valuable part of our navigation tool set.

You can produce your own, it's easy and fast so long as you have a decent internet connection.  Here is how it's done :

Satellite Image Chart Production

First, download and install a shareware program called GE2KAP.  Like the names says, it creates precise charts using Google Earth (GE) data, and publishes them in KAP format.  Download GE2KAP hereInstall it using all the default settings.

The KAP format images can be read by the excellent shareware navigation program, Open CPN.  Download Open CPN here.

To create a chart, load GE2KAP - it will then automatically open Google Earth.

Then pan and zoom in the Google Earth environment to define your chart area, then select "create chart" in the GE2KAP control window (at right).  It is very simple to use.

Advanced users will be able to combine chart data with satellite imagery to create composite charts that have a wealth of navigation information.

In the image at right you'll see how the GE charts will "tile" or overlay automatically on the Open CPN plotter screen (click the image at right to enlarge).

You can also create a route (with Open CPN) and export it as a GPX data file.

Load that GPX file into the GE2KAP application and it will automatically create a series of overlapping satellite charts that follow your selected route.  Powerful stuff indeed.

Our friend Dana Fowlkes on the Villa G has used this function to create accurate charts covering all his recent travels through Indonesia.

Satellite Image Charts - Useful, But Not Perfect

In the image at right you can some of the issues that mean these new charts are useful, but cannot always be trusted as your primary navigation tool.

On the left you can see clouds obscuring part of the lagoon, which means any reef areas are hidden from view.

Top right, the satellite image for that area is extremely low resolution - of little use to us.

At bottom right, the image quality is astounding, but wave action offshore can mask some of the bottom detail.

So we use these new charts concurrently with all our other aids to navigation, plus a very intense application of our Mk.II eyeballs, to stay safely afloat.

Now We Can All Share The Data

These .KAP files are readily stored, transmitted and shared.  Ours are now available for download - check the tabs at the top of this page or use the link here to access them.  Simply download and install in Open CPN to view.


  1. Ley, is your method affected by Google's propensity to change its systems as happened to Bluesipp's Track Library?

  2. Daria, once the satellite chart has been downloaded and saved then you have it forever - it is not dependent on anything that Google may (or may not) do with their networks.

    Changes in Google's systems could effect the operation of the GE2KAP software, however the developer has been on this case for some time and I believe they are aligned with Google's new "business model". Right now its working well and has been for some years.

  3. Daria, I just took a quick look at the Bluesipp site. Shame all that data cannot be shared effectively now, however it was essentially a very good planning tool, requiring Google Earth availability at all times to view the tracks. That doesn't happen out here. This newer system provides real time GPS positioning over calibrated and stored Google Earth images. Unfortunately it doesn't save tracks for sharing.

  4. That's good to know. Thanks. I will be giving your system a try as the charts for the west coast of Ireland are about as inaccurate as you can find anywhere in the world. Admiralty soundings dating back to the 1800s as you say. GPS anomalies, too. The works! Thanks for this.


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