NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: WGS 84 and BVI Charts
From: Gary LaPook
Date: 2016 Aug 16, 17:14 +0000
From: Frank Reed <NoReply_FrankReed@fer3.com>
To: garylapook@pacbell.net
Sent: Tuesday, August 16, 2016 6:43 AM
Subject: [NavList] Re: WGS 84 and BVI Charts
From: Gary LaPook
Date: 2016 Aug 16, 17:14 +0000
Cool! I've flown into Beef Island airport many times. The charts I posted are from 1978.
gl
From: Frank Reed <NoReply_FrankReed@fer3.com>
To: garylapook@pacbell.net
Sent: Tuesday, August 16, 2016 6:43 AM
Subject: [NavList] Re: WGS 84 and BVI Charts
How old are those charts? Does anyone use them?? Are the current electronic charts of the BVI that poor?
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 did not notice any large discrepancies, no offsets of a hundred meters or more, but it's quite possible that these exist only in a few limited areas, and I simply didn't bump into them. It's also possible that the Google Earth imagery has been "registered" to the vector map data, and in that case it does not represent actual coordinates. I think that's unlikely since the BVI are so close to Puerto Rico and the registration would normally be consistent over such distances. I did, however, find an interesting example of a flaw in the vector data, where a couple of peninsulas that were apparently former islands now connected to "mainland" have been accidentally excised, probably by an automated algorithm. Complete deletion of features like this is not a rare thing in vector maps. So here's a rule of thumb: if you see an island in front of you, and the map/chart says there's no island there, chances are good that the island is real and not a hallucination! To see an example, visit these coordinates in Google Maps: 18.425, -64.559. Look at the map view, and then toggle to the Earth view (satellite/aerial view). See the problem? In the map view, you can anchor there. In the satellite view, you can drive there. :)
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 did not notice any large discrepancies, no offsets of a hundred meters or more, but it's quite possible that these exist only in a few limited areas, and I simply didn't bump into them. It's also possible that the Google Earth imagery has been "registered" to the vector map data, and in that case it does not represent actual coordinates. I think that's unlikely since the BVI are so close to Puerto Rico and the registration would normally be consistent over such distances. I did, however, find an interesting example of a flaw in the vector data, where a couple of peninsulas that were apparently former islands now connected to "mainland" have been accidentally excised, probably by an automated algorithm. Complete deletion of features like this is not a rare thing in vector maps. So here's a rule of thumb: if you see an island in front of you, and the map/chart says there's no island there, chances are good that the island is real and not a hallucination! To see an example, visit these coordinates in Google Maps: 18.425, -64.559. Look at the map view, and then toggle to the Earth view (satellite/aerial view). See the problem? In the map view, you can anchor there. In the satellite view, you can drive there. :)
Frank Reed
