NavList:

Walter Timothy Ingham you wrote:

Can anyone clarify what changes are required to be applied to the originally quoted latitude and longitude of a point which was based on when “Greenwich Meridian” was the zero Meridian and now, when Lat and Long are quoted in terms of WGS 84 and the zero meridian is about 5.3” east of the “Greenwich Meridian”.

The query is prompted by a check on some data quoted in Lecky’s Wrinkles in navigation (1884)  which claimed 2 locations, St. Paul’s rocks and Cape Agulhas Lighthouse to be “Both of these are very accurately observed positions”. The other locations are taken from his List of Time Signals. 

Also, I appreciate the need for caution when quoting “Mr Wikipedia”, but what information is there on the accuracy of Google Earth’s Lat and Long positions?

Walter

It takes a while to accept, about 60 years in my case, but a place can have as many lat & longs as there are geographic datums.  Look deeper into set-up mode in an old-style, hand-held GPS receiver e.g., Garmin GPS12.  You’ll find there must be ten’s if not verging on 100 different datums you can select.  It helps if you know what a datum is; that only took me about the subsequent five years to get to grips with.  Basically, it all stems from the Earth not being a perfect sphere, or even a perfect ellipsoid.  Think of it more as an ugly grapefruit.  Now if spherical geometry is hard, and ellipsoidal geometry is like bagpipe music to most of us, using the absolute exact shape of the Earth for mapping would be impossible, so cartographers over the years have tried to define there own geographic datums, usually one which best fitted the shape of the Earth around the area they were mapping.  The crunch came with the arrival of world-wide navigation systems, such as GPS.  Then we needed a datum which best fitted the shape of the whole Earth.  The one chosen was World Geographic Survey 1984 (WGS84).  For most places, the difference between its WGS84 lat & long and its position using the previous local favourite datum is only a few metres, so most of us needn’t worry, but there are some places on Earth where the difference can be up to 1km (someone correct me if I’m exaggerating), so it’s worth setting the datum on your GPS receiver the same datum as the local map or chart.  I only noticed this once at Linkoping on Lake Roxen, Sweden when plotting my GPS pos’n on the local hiking map (the datum of which wasn't printed) put TIKI upon dry land way outside the marina 58.437650 1N 015.622365E (Google Maps).  Lars might be able to explain why.  Hence the requirement several years ago to convert all new marine navigation charts to WGS84 datum.  For many users the difference is negligible, but if you’re piloting a super-tanker or similar up a narrow, dredged channel already scraping the soft mud at bottom of the channel, a few metres inaccuracy in position can be critical.  The fact that you can select which datum your GPS receiver is going to give you your position in shows that conversion algorithms are readily available an included in the receiver’s ‘firmware’.  Phew, I think I need to get out more! DaveP