Lines 106 to 125 of tz/europe mention the old meridian before Greenwich. The estimated position is 51° 28' 30" N, 0° 18' 45" W, which would put it north of the river in the grounds of Syon House. The meridian markers are in fact all south of the river, and can be seen by walking the Thames Path on the south bank. The meridian passes through the King's Observatory, with obelisks to the north and south. The map on the information boards shows a total of four obelisks marking the meridian. The website for the King's Observatory quotes the following: "This task was later transferred to Greenwich Observatory but not before three obelisks had been erected in the Old Deer Park in June 1778 to assist in the exact alignment of instruments in the Observatory. The northern one and the western one of the southern pair are on the true north-south meridian line passing through the west room of the Observatory which housed a tracking telescope. The southern obelisk is due south from the east room that at the time housed its great mural quadrant." The position of the King's Observatory is given on Wikipedia as 51°28′08″N 0°18′53″W, which would give STDOFF = -0:01:15.53 I don't suppose anyone is particularly concerned about this, but I thought I would point it out.