Google Earth & GPS For Cruisers & Boat Owners

image1Many of us are familiar with Google Earth (if you haven’t yet had a play with these satellite images then you can download Google Earth for free here). It’s good fun zooming in and out of the Grand Canyon, getting a bird’s eye view of your childhood house or playing with the built-in flight simulator. Surely there is more to it than this though? The level of data being manipulated here is so powerful government agencies are trying to get Google to blur some imagery for security reasons, so if the imagery is this great there must be some practical applications for the yottie too.

This article came about from me wanting to use Google Earth (we’ll call it GE from hereon in) to scan the Red Sea. I figured if I could plug in our GPS and track Esper we could use GE to navigate around approaching reefs, all in real-time. Good idea in theory, but can it actually be done? And if we can do this then what else can be done with GE? In this essay I’ve attempted to provide some pointers, resources and links for the yottie to consider when using GE onboard.

First, some caveats: there is a plethora of data on GE out there and this article has been put together specifically for the yottie; it is by no means exhaustive, it merely introduces some relevant applications of GE, so if you have anything else to contribute to this essay that may help other cruisers then please add your comments at the bottom of the page; this essay won’t cover HOW to get your GPS up and running on your laptop, that’s another essay for another day; please note my comments on using GE as your primary navigational aid (i.e. don’t do it).

What this article covers:

1. Introduction To GE & Some Issues
How GE works and some issues for yotties to consider.

2. Track yourself in real time
This requires some third-party software as well as a working GPS

3. Working Offline
Saving GE map data to be accessed when offline

4. Explore The Oceans
With the latest version of GE (5) you can now go underwater and check out the oceans!

5. GE Places
Loads of links to extra data pertaining to the sailor that may be overlaid GE.

6. Conclusions
Is GE actually any use to the boat owner?

1. Introduction To GE & Some Issues

When you open up GE your computer logs onto the GE server and loads up satellite images as you move around Earth. You use your mouse or keyboard to control where you move to and as you explore a new area, new images are introduced. As you zoom in, so more detailed imagery is loaded. Whilst it is doing this it is adding your already-viewed places into its memory, or ‘cache’. This means that you can then view Google Earth offline and still see the same level of detail that you did when you were online, but only for those places you have already visited. Try exploring a new area on GE whilst offline and the image will appear blurry.

This is an inherent problem that creates usability issues for yotties: you have to be online to see new places. We get round this problem by caching images, which is explained in more detail below.

Easterly aspect of Fethiye anchorage by Ece Saray Marina, clearly showing the wreck in the north of the bay. Image shows Google Earth's navigation features

Easterly aspect of Fethiye anchorage by Ece Saray Marina, clearly showing the wreck in the north of the bay. Image shows Google Earth's navigation features


There is another issue with GE. It has known differences in GPS measurements when compared to other navigational software like Maxsea and therefore makes it unreliable to use as a primary navigational aide. With this in mind anyone trying to use GE as their primary navigational aide needs their head examined. Do not do it. At the time of writing GE is showing me approx. 20m off my actual position. I don’t need to tell you how significant this error is if you were using it to navigate into a rocky anchorage. There is an article on the difference here.



There are some good reasons for having GE installed on the ship’s computer though. Indeed there is no harm in tracking yourself in GE providing you are not using it for primary navigation. My earlier example of using it to spot reefs that may not be marked on nautical charts is one useful application. You may wish to overlay weather information for your cruising area, or check buoy data across the North Sea. You can even view AIS data in real time. So without further ado:


Franson's GPSGate with built-in Google Earth GPS tracking

Franson's GPSGate with built-in Google Earth GPS tracking

2. Track Yourself In Real-Time

Plug in your GPS and show your current position on GE. You need a number of tools to do this:


  1. Windows Laptop – I’ve no idea if CrossOver Mac supports these Windows apps

  2. Google Earth – no need to upgrade to the Pro version, just download the free one

  3. GPS – either your ship’s GPS connected with a serial cable, or a more modern USB GPS

  4. Franson’s GPSGate – Tricks the computer into thinking your USB GPS is connected via a serial cable and also creates pseudo multiple connections so you can run one GPS across different applications. Worth purchasing the full version as it has a useful tracking feature.

Screen grab of Franson's GPSGate GPS tracker, displaying Esper's location at her anchorage by Yacht Plaza Hotel

Screen grab of Franson's GPSGate GPS tracker, displaying Esper's location at her anchorage by Yacht Plaza Hotel

There’s another application that may be used: GooPs. This links your GPS with Google Earth. It also allows you to control GE by tilting angle etc. You’ll need to buy the pro version as the freebie only allows 5 minutes of tracking. If you purchase any version of GPSGate, however, you get an automatic Google Earth tracking system so using Goops is almost negated. For more information on setting up the GE plug-in in GPSGate, see this link.

Both GooPs and GPSGate websites have comprehensive guides on getting your GPS to talk to GE.



It is also possible to overlay official nautical charts. I haven’t tried this because the only charts readily available are EarthNC, which covers North America only (if anyone is aware of charts for the Med or Red Sea, please let me know!). When you see this in action you start to understand how close GE could be to becoming your primary navigational tool, especially when you tilt the angle of view to 45 degrees and overlay 3D polygons of mountain ranges and submarine layouts. There is a great article by someone who went sailing with this whole set up and he was impressed.




This screen-grab, taken from the above-mentioned blogger, demonstrates digital nautical chart data overlaid on top of Google Earth. It clearly illustrates depth and obstruction information.

This screen-grab, taken from the above-mentioned blogger, demonstrates digital nautical chart data overlaid on top of Google Earth. It clearly illustrates depth and obstruction information.

Now that you have your GPS showing your position on GE you can go off for a sail and have GE maintain your boat in the centre of the display, just like Maxsea, whilst exploring third-party information and pictures of possible anchorages/marinas/locations. Oh, wait a minute. There’s just one problem…



3. Working Offline

There are some issues associated with GE that immediately puts-paid to the above-mentioned fun. It works by loading images from its server and the images used are high definition. This means you need to be online for it to work and a fairly big hard drive to handle it all.


Fear not. Google Earth has a built-in cache feature (cache is another name for memory). This means you can save your already-viewed images to your hard drive so that next time you open GE the images don’t need to be downloaded from the GE server. Instead they are loaded from your laptop, which is far quicker and an internet connection is not required. The cache limit is 2Gb, which I would like to put in context for you but at the moment I am still running some tests to see exactly how much surface area this actually covers. It varies greatly depending upon the complexity of downloaded images. Because half of the images we’re downloading to cache have some sea in them, they are probably not as large as city-based images. This is because they contain less colour and detail (i.e. some of your images will just contain shades of blue, which will help reduce the size of each image being downloaded). At a rough guess the rectangle that covers Skopia Limani, Gocek and Fethiye (where the top left corner is 36.75N and 28.83E) will take around 400mb. This covers land as well as sea as it is everything that falls within that rectangle, approx 25 square miles. It is possible to reduce this significantly by only downloading the specific area required. My example included all the moutain ranges around the three sides of my rectangle.


Providing you have the disk space, check that GE is set to the maximum cache limit. To do this go to Tools/Options and select the Cache tab. Change the Disk Cache Size, in MB, to 2,000.


It’s worth bearing in mind that the cache doesn’t stop saving when it reaches 2Gb. All that happens is that the oldest viewed images are scrubbed and replaced with the more recent views, so if you are saving a whole area to cache, keep an eye on the file size:


  1. Firstly locate your cache file.Your Google Earth cache is located under the files dbCache.dat and dbCache.dat.index in the following folder:

    Windows XP –
    C:Documents and Settings{username}Local SettingsApplication DataGoogleGoogleEarth

    Windows Vista –
    C:Users{username}Local SettingsApplication DataGoogleGoogleEarth


  2. Keep an eye on dbCache.dat. This is the file that will reach 2Gb.


One way of getting round the 2Gb cache limit is to save different cache files in a separate folder to the active one. You can then load them back into your GE cache folder as and when you need them.



The simple interface of Google Earth Voyager (GEV)

The simple interface of Google Earth Voyager (GEV)

Normally to cache your images you go to your desired location, zoom right in, wait for the image to download, scroll the mouse over to the next part of your location, wait for that image to download, and so on. You’ll quickly realise that trying to manually update your cache for a large area becomes a bit of a pain. Fortunately there are tools out there that do this for you. The one that worked best for me is called Google Earth Voyager, which can be downloaded here.



Essentially GEV does what you have manually been doing: it flies over a pre-determined area of Google Earth, caching the images as it goes. It works by setting the co-ordinates of certain points and then travels between those points, pausing at each point allowing time for the image to load. It is therefore important to remember that this must be done whilst online, so some preparation is required. It’s also the sort of thing you’ll need to set up and let run over night. And don’t get too ambitious: if you want crystal clear, sea-level images to be saved to cache it takes a while to get them all. The faster your connection to the internet, the quicker you can cache.



4. Explore The Oceans

This is one of those fun applications of GE that has huge potential for the future. It could work like a forward-pointing sonar, giving you real-time imagery of the sea bed and warn you of approaching shallow patches. There are many projects out there striving towards this and other submarine data applications, but as a useful cruiser’s application it’s pretty useless currently. In the meantime just point your cursor downwards and splash into the ocean and have a play around underwater. One to watch for the future.


places5. GE Places

One of the great strengths of GE is the ability to add third-party content like weather information and physically overlay that data on top of GE’s satellite images. In GE these are known as ‘Places’ and are made up of .kmz or .kml files. These relatively small files are loaded into GE and go online to load up the latest data onto your GE. As a user you may select any number of ‘Places’, add them to your library and turn them on and off when you need them. The one drawback is that you need to be online to update many of them so whilst these are less relevant to offshore navigation, they are quite useful for passage planning.



There are literally hundreds of Places for you to add to GE. Hundreds. Here are a few that the yottie might find useful, followed by a very comprehensive list of other Places. They include a link to .kmz or .kml file. Click on it and it should automatically open in GE and add a link to your Places section.




Esper’s Location

An easy one to get you started! This is the same file that is displayed in a Google map on our home page and is updated as and when we move around. Whilst it shows you where we are in a little embedded map on our website you can also upload the file to GE. Try it by clicking on this link, which should ask you if you want to open it up in GE.


Click here to view in GE[urldisplaymode=nomap]



Nautical Charts from NOAA

If you are cruising north America then the whole country is pretty much represented by nautical charts provided by the NOAA and they can be overlaid onto GE. They work pretty well though we should of course re-emphasise the point that they should not be used for navigation, especialy as these are 2D, flat charts overlaid onto GE’s curved surface. Even if you’re not cruising this area it’s well worth downloading at least one just to see the potential.

Click here to open an example in GE[urldisplaymode=nomap]

Tide Predictions

Tidal data covering 7,500 harbours, based on Xtide.


Click here to open in GE[urldisplaymode=nomap].



Sea Surface Temperature

Data coming from the Space Science and Engineering Center.


Click here to open in GE[urldisplaymode=nomap].



Sea surface temperatures across the Indian Ocean

Sea surface temperatures across the Indian Ocean

Worldwide Weather Stations
This is data supplied from (mainly American) buoys and stations NDBC, US National Buoy Data Center (NOAA). Information includes wind speed, air pressure and other sensors data, latest satellite wind map (QuickSCAT) and SST/Wave Height from Weatherunderground.

Click here to open in GE[urldisplaymode=nomap].

Clouds Overlay
Worldwide clouds overlay : data from xplanet (global cloud map updated every 3 hours using GOES, METEOSAT and GMS satellite imagery downloaded from Geostationary Satellite Imagery page at Dundee University) and compiled by Hari Nair.

Click here to open in GE[urldisplaymode=nomap].

MTSAT and GOES Satellite Pictures

MTSAT and GOES satellite pictures : worldwide daily updated data from Cooperative Institute for Meteorological Satellite Studies (color and B&W Infrared / water vapor).

Click on the following regions to open them up in GE:

Google Earth still has a way to go before it can be used efficiently and with safety as a primary aid to navigation. Because of the differences in the way it reads lon/lat position it is less precise than a ship’s GPS repeated onto a dedicated chart-plotter. The fact it has to be online for it to work is not practical for sailors, at least until satellite phone internet browsing becomes affordable. It is, however, possible to cache data to be used whilst at sea and this alone is a very useful aid to navigation in small areas and when the NOAA charts are overlaid in America you can see how close it’s getting. Add to that the hundreds of possible data overlay ‘Places’ (see the list below) and suddenly Google Earth has the potential to be a very powerful passage planning tool. It won’t be long before that submarine data becomes powerful enough to navigate your boat into a tricky anchorage, whilst providing aerial images and restaurant data, all at the same time. Get to grips with GE now and the desired technology for cruisers won’t be far behind. Best of all, everything we’ve looked at in this article is FREE!

If you enjoyed playing with the .kmz files above then prepare to amuse yourself for hours with this very comprehensive list of GE Places, provided by Below is an extract from their list.

Marine-based kml/kmz links :

  • Weather :

Go to for loads more .kmz and .kml files, including live AIS and shipwreck data.

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7 Comments on “Google Earth & GPS For Cruisers & Boat Owners”

  1. A very comprehensive article Mr Furlong.

    I recently downloaded GE again after uninstalling it some time back for being too resource heavy when in use.

    This time it was the new Google Underwater feature that brought me back as a potential tool for scoping out new sea fishing spots.
    Like you say though it’s very limited at the moment and purely a fun application but knowing Google this will be constantly developed.

    I just hope that in the long term this does not open up the ocean to further exploitation by creating familiarity.

  2. Absolutely excellent, especially all of the links – many thanks – this page is now bookmarked for posterity!

  3. I’m a pro navigator, I deliver sailboats now, and find the new ‘Street View’ feature surprisingly useful. Y’see, it’s difficult getting people to create in their own minds a ‘feature-rich image’ of a scene – a harbour entrance or the layout of a tiny anchorage – from conventional 2D paper/electronic charts.

    Here’s an current example proving my point. I’m planning to take a sailboat from England down to St. Malo, in northern France. I’ve been there a score of times, and am quite happy navigating in along the several miles of reef-strewn aproach channel. Not so the boat’s owner.

    He wants me to leave the boat securely moored in through the Rance tidal barrage. I’ve never been in there, so don’t know which of the many tiny yacht harbours might be suitable. My wife is bringing a car over on a car ferry, but we need to identify a rendezvous point both she and I will recognise. Enabling ‘Street View’ in the Layers menu gives me a vast choice of panoramic views, I can rotate 360 degrees, pick out a couple of likely spots, identify that there are similar boats already there ( with similar depth needs ) and agree a solution with the owner. Using the same panoramic feature allows me to show my wife exactly where the ferry-port exit slipway enters the public road system at a roundabout,spot a legal parking place with a cafe nearby, and agree a rendezvous she can find.

    What would have involved lots of mobile phone time can be reduced to a few previewed and stored images. Hell, some can even be loaded onto a little USB pen-drive and uploaded to a netbook on board. That’ll help confidence when cold and tired, and in the rain, on the final approach to the anchorage at St. Servan, ‘cos we can’t yet get across the tidal sill to the big marina with the showers…..

    Try it yourself. St. Malo. North France ( N48.38.05 W02.01.49 ) Have a look around….

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