Tuesday, 16 October 2012

Using GoogleEarth Images for Marine Navigation

Hi everyone,
This post is for navigators interested in adding one more source of useful data to their armoury.  For us, GoogleEarth imagery has proven to be very useful, particularly in an area of the world in which official charts are poor.  Please note this blog states my opinion only as a cruising sailor.

The Problem:

I learned about using GoogleEarth as an aid to navigation in preparing for the Sail Indonesia 2012 rally.  Charts in many parts of the world are not as accurate as we are used to in Australia, Europe, North America etc.  Hence I was keen to find other sources of information to assist my navigation during the rally, particularly in inshore, reef-strewn waters.

Simplistically navigation is the answering of two questions.   First - where are we now?  Second - where should we go next?  It is a commonly held view that the first question has been answered by GPS, and so the modern navigator's role is to answer the second.  That's true in many, perhaps most, parts of the world, but certainly not in all.

The question of "where are we now" certainly became much simpler with the advent of GPS.  However the GPS-derived position is of less value (perhaps positively dangerous) if the chart in use has an offset from GPS.  In areas where offsets exist there are commonly charting errors or omissions in addition to the offset.  These charts are usually based on pre-GPS surveys, and sometimes from surveys more than a century old.  Hence a GPS position is of limited value without a modern chart.  In fact in inshore waters you may well be better off without the GPS position, reverting instead to traditional methods of navigation.

So the problem is how to gather additional information which could highlight chart offsets, and allow identification of useful locations (eg anchorages) and hazards (eg uncharted reefs).

The Solution:

I had heard of sailors using GoogleEarth to identify potential anchorages.  This simply allowed one to note positions while connected to the internet and using GoogleEarth, and transfer the positions to a chart (paper or electronic).

Then the chart plotting software OpenCPN, in concert with the utility program GE2KAP, provided the ability to display charts derived from GoogleEarth images.  Once saved to disk, no internet connection is required to use the KAP files in OpenCPN.  One simply clicks on a button to select a conventional chart or a KAP format chart.  KAP format charts (I'll call them KAPs hereafter) seemed like a good idea so I did some trials.

As an initial trial I made KAPs of the Darwin harbour area where we were located at the time.  These demonstrated the good alignment of GoogleEarth images with Australian charts, which we knew to be accurate.  They were very well aligned with Seafarer raster charts, and also with Garmin BlueChart, Navionics, and CM93 vector charts.

Subsequently I made KAPs of the major destinations we expected to visit during Sail Indonesia.  These were compared with Garmin, Navionics and CM93 charts for each location.  This clearly showed their value, showing various offsets and also some significant differences in port structures.  One issue with the GoogleEarth imagery was shown, which was that imagery of remote areas can be several years old, sometimes approaching 10 years - especially if one found cloud cover spoiled the latest imagery!

During Sail Indonesia the initial KAP files created proved very useful, and also proved very popular when offered to other participants.  Most critically, the alignment of GoogleEarth imagery with GPS locations was demonstrated to be extremely accurate, without exception in our experience.

Our experience was that entering an anchorage with an offset-free chart on the plotter was certainly an aid to navigation, where displaying a chart with an offset was a hazard, even if known to be present.  Clearly, entering reef-strewn anchorages was a job involving the Mark 1 Eyeball, depth sounder etc, but the accurate KAP helped.

In locations such as anchorages/ports and their approaches we identified apparent reefs on the KAPs which were not charted.  In almost all cases the Mark 1 Eyeball confirmed their presence.  This is clearly a critical advantage of KAPs.  However caution is necessary.  It appears that Google "greys-out" their imagery away from the shoreline, so reefs you'd otherwise expect to see are not present.  This is probably to reduce the size of downloads intended for shore-side use.  Therefore it is still prudent to use the Mark 1 Eyeball, depth sounder and charts in addition to any use of KAPs.

Later in the Sail Indonesia rally a "library" of KAPs circulated around the participants.  These proved extremely popular and were certainly an aid to our navigation.


Here are the benefits and cautions I've identified when using GoogleEarth imagery "charts" for navigation:

  • Well aligned to WGS-84 GPS positions
  • Identifies chart offsets
  • Identifies potential anchorages
  • Identifies some hazards (eg shallow reefs) close inshore
  • Presents helpful views of features ashore (e.g. mountains, towns, rivers etc)
  • Obscures offshore hazards
  • Clouds can obscure parts of images
  • Imagery may be several years old
  • Not an official Google product as such
  • Unknown whether Google provides any assurance regarding WGS-84 alignment
Used with caution, GoogleEarth images can be very helpful to inshore navigation in poorly charted areas.  They provide a different perspective which may well be useful in well-charted areas.

1 comment:

  1. An excellent intro to KAP charts Mike. One small thing that very occasionally can help with the cloud snag: enable "Historical Imagery" in "View" on Google Earth and sometimes an older image still has a good coastlne but without the clouds.