A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2016 Aug 16, 11:11 -0700
>>Can you provide an example, maybe in the form of a screen capture? <<
Yes...this *would* be great, Frank. But at the time when I noticed this discrepancy, I was considerably less educated about horizontal datums than I am now, and it never occurred to me to get a capture. I was mostly focused on not making any boneheaded boat handling moves based on a failure to understand the navigational limitations of my chartplotter. The proverb for me to live by here is, "He who runs aground on a well-charted navigational hazard will find it difficult to ever talk his wife into another sailing trip."
Based on what I observed in Road Town Harbour, I should be able to moor just off the pretentiously-named Oceans 7 Beach Club on Peter Island and show my vessel as being positioned in the woods behind the restaurant. Knowing whart I now know, having such a screen capture would make a great wallpaper for my computer come December when the days here are at their shortest.
From an earlier post, you said:
>>At this point in time, you can basically forget about map datums.<<
According to the NGA, the statement you have made is mostly true when it comes to US waters (and entirely true of newer US charts)...
"All new NGA charts are compiled on WGS Datum...."
...but not as much so in international waters.
With GPS providing such accuracy, the mariner now needs to pay closer attention to the reliability of the chart. For example, mariners, to save steaming time, may become more trusting and rely on their GPS to pass hazards depicted on charts much closer than is prudent. However, the charted hazards may have been positioned by less accurate navigation means than GPS, and, in fact, may be significantly misplaced. In other words, the chart being used may contain unintentional errors due to limitations of the technology used at the time of data collection, which in many cases is a generation or more in the past....
...the Tokyo datum is an example of one that requires significant adjustments in both latitude and longitude to conform to GPS positions. Older Japanese and Korean charts are referenced to the Tokyo datum, for which positions must be shifted more than 700 meters to convert to WGS 84 datum.
Isolated datums, such as those used to position many islands in the Pacific Ocean, can be in error by a half mile or more (see figure). The datum shift to WGS 84 can be quite large, depending on the area of the world and the local datum in use. Remember that the chart and the navigation system used must always be referenced to the same datum.
Rather than saying, "We can basically forget about chart datums," the NGA's web site says:
Mariners should continue to give wide berths to charted hazards, and ensure that the datum used by both the chart and the positioning system are the same, or that any difference is accounted for.
>>Just now I experimented with a simple comparison between mapped data (the "vector" data from which electronic land maps, but not nautical charts, are derived) and Google Earth imagery.<<
I could not find anything on the net that contained an authoritative statement around this from somebody who actually *worked* on the Google Earth project. However, I did find this:
This is deeper cartographic water than I can swim in...but it looks to me as though Google Earth is, as nearly as can be with a flat computer monitor displaying corrdinates from a geoid, sync'ed with WGS 84.
I have looked in vain at NV-Charts' BVI products to determine the horizontal datum...though they DO indicate that the depth contours they indicate were based on a last-surveyed date of over 100 years ago.
To get back to your bold summary: >>At this point in time, you can basically forget about map datums.<<
The person who takes this at face value while navigating under way, in spite of the NGA's counsel, could find that Clint Eastwood's lines from the first "Dirty Harry" movie are relevant. "You have to ask yourself one question: Do I feel lucky?"