Navigating with Google Earth


Moving south from Uligan, we used Google Earth in conjunction with our primary chart plotter (OpenCPN) to help navigate to our second anchorage in the Maldives. Google Earth a great tool for cruisers, helping when chart accuracy is in question, or to benefit from the different kind of information provided by satellite images. The big picture route was set up shown above.

Some places don’t have dependable nautical charts. We don’t assume ours are correct and always look go compare data from multiple sources- not least of which is our eyeballs, especially around these atolls and reefs. It’s not a question of one or the other set of digi charts we’re using being “better” than the other, but which one happens to be more accurate for the place we find ourselves- it’s not consistent.

Our first destination south from Uligan was a reef that promised some stunning snorkeling. Here’s how the spot looked on our CM93 charts in OpenCPN:

with charts

The Navionics chart wasn’t all that different.


The charts aren’t really all that bad. If you were aiming to avoid this spot, you’re covered. Of course, we’re not looking to go around this reef- we want to snuggle up close and make friends with it!

Layering a Google Earth image in OpenCPN, we get a more accurate sense of the spot. It’s not about charted depths, of course, but the visual cue to both how accurate our charts are the profile of reefs and shallow areas. This gives us some really valuable information about the depths and profile that the charts are lacking. It shows that the channel runs all the way through. We can see from the color there’s depth for the full length of the channel (no soundings for that part of the charts). And, it looks to be bang on with regard to our position.

with kap file

Although it seems like there’s an internet signal just about everywhere in Maldives, this is all offline. These GE images are saved as KAP files to layer in OpenCPN- there’s no reliance on an internet connection. Just how we like it!

Getting the KAP files is a data intensive, time consuming process. We lucked out and were able to get extensive files for the Indian Ocean (everything we’re likely to need) from another cruiser, saving a bucketload of work (and internet expense)- thank you SY Geramar!

If you want to try it yourself, there are a few tools to convert GE to KAP files you can layer in OpenCPN. You can get conversion software (Jamie played with GE2KAP), instructions, and tips via forums and blogs from a quick search online; this kind of thing changes, but there are write-ups of various vintages from the boats Soggy Paws, Valhalla, and Ocelot. For a myriad of additional ways to use GE as a sailor, check out Tahina’s site.

It seems likely KAP files be passed around on memory sticks like cruiser candy. Some helpful soul will surely soon build a library for all (like the awesome track library on Bluesipp). I’ll put ours out on Dropbox where anyone can reach them at some point, when it’s practical to upload a half gig of data.

Navigatin’ fools know we love it when you read this on the Sailfeed website– thanks for kicking some change in our cruising kitty!

19 Responses

  1. Thanks, I always wanted to see how the data could be blended together. I am glad to hear sharing of knowledge is prevalent caveat YMMV.

    Very much enjoy following your blog

    1. Thanks for pointing that out – the YMMV is definitely an important caveat! And what works in one situation for us- as this does- doesn’t make sense in others. Thanks for commenting Mike.

  2. Even without a large Google Earth cache a few GE screen shots of likely entrances and anchorages made prior to a passage when Internet was available has proved useful for me to help my visualisation of unfamiliar destinations.

    However having done a lot of sailing using out-of-date paper charts I love that I can now get regularly updated iNavX electronic charts, especially as my Cm93 charts for OpenCPN, gratefully obtained from s/v Kirsten Jane, are several years old.

    1. Caching an entrance snapshot is great too. Rona, you will appreciate that our pretty-much-new digital charts for the place we currently find ourselves are primarily based on surveys from the late 1800s! Ten years ago we did all our Salish sea boating with paper charts. We usually do keep a backup small scale paper chart for our nav area but it’s usually converted to wrapping paper, unused.

    1. My pleasure Drew, glad you liked it. It IS incredible! It’s a time when we have so many tools to help, but still sail in company with boats that were limited to sextant and paper, maybe an RDF. Amazing.

  3. Hi Behan,

    Because getting the KAP files is so data intensive, is there a reason you don’t use an easy offline sat image downloader/nav program (free) like SASPlanet?

    We are also using GPS HD for the ipad similarly. Similarly to SAS there is autodownload and caching of satellite imagery, basic navigation, all available offline and you can choose your provider (not just google) for the sat imagery. This app cost a couple of bucks but very worth it to us.

    1. I have bought the app and don’t see how to cache satellite images. There is a note that suggests it might be possible for Bing Aerial views:-

      Bing: Displays a Bing Aerial map. Supports Direction Up, Course Up and North Up.

      Apple: Displays an Apple Satellite map. Due to licensing policies, map caching is not supported for Apple maps. Supports North Up only.

      Google: Displays a Google Satellite map. Due to licensing policies, map caching is not supported for Google maps. Supports Direction Up, Course Up, and North Up map orientations.

      1. We used it just today. We had both google and bing images that we had viewed online, and they were there when we sailed today (offline). Settings maybe?

          1. I havent heard of OvitalMap, thanks for the info. I will look and see! If you are being given KAP files and dont use them that often SASPlanet doesnt really make sense. We use it to find kiting spots, to pick out sandy patches in anchorages before we arrive, and for off chart pre-routing which we did a lot of in French Poly and now in Fiji are doing again. It is a good dreaming tool for us before we arrive if we have internet.

          2. I will keep wishing for a visit from the Fast Unlimited Data Internet Fairy so I can try ALL these- SASPlanet, Ovitalmap, everything! My wishes exceed practicalities at the moment though. And yeah, we basically use GE the way you’re using SASPlanet, it sounds like… cache in advance, then scope it out at leisure. Imagine being able to drop in on Miles & Beryl or Hal & Margaret with a tablet and sat images…what would they say?!

    2. Hey Livia, thanks for reminding me of your awesome post about using sat imagery! SAS is a great option, although I kind of prefer the overlay to the side-by-side. Ultimately, though, we’d only done a few KAP files ourselves when a bundle covering our route across the Indian Ocean dropped in our lap, courtesy of another cruiser. At that point, there was really no reason for us to consider any other method, since the hard yards were done!

      I’ll check out GPS HD – any idea how it compares with OvitalMap, other than (I think) data source? I like the route alignment aspect of Ovital, woul that be a relative benefit over GPS HD? Definitely don’t mind paying a few $$ for something like this. Although, “easy” is just using the GE app for those times that we need it.

      Love the thread with you and Rona!

  4. Hey, just stumbled across this post. I am a cruiser who is also a developer. I wanted an easier way to generate KAPs so I developed the website There are several features on this site but the one related to this post is KAP generation. On the KAP generation page you just move/zoom the map to where you want, choose either Google, Bing or Nokia satellite images and click a button. The KAP is built on the server and then returned to download. No software to install and works on both Windows and Mac. The KAP engine is also directly integrated with SEAiq (iPad nav app) to request KAPs.

    1. Oh, very cool! Thanks for reaching out. When we’ve got better internet access I’m really looking forward to checking your site (and KAP generator) out!

  5. If you have any questions or run into any issues just let me know, contact info is on the site. Currently I also allow depths to be recorded using the VDR plugin in OpenCPN. Once recorded you can upload the VDR output file. VentureFarther takes these recorded depths and makes them available online. In the KAP generation page these depths can be included in the KAP generation so in remote areas you not only get satellite images but also depth recordings in your KAPs. This is a pretty new development so not many recordings at this point but hopefully that will pick up. As a side note, VF just passed 75,000 KAPs generated!

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