"Dr. Steven B. Tesser" <stesser@nycap.rr.com> wrote

A second question: the 3rd Military Mapping Survey of Austria-Hungary,
at http://lazarus.elte.hu/hun/digkonyv/topo/3felmeres.htm
the (East-West) longitudes seem to be off by about 17 degrees, 45
minutes. That is, the longitudes shown at the top of the map are
consistently 17 degrees, 45 minutes too high.

That would make sense if they started with the zero longitude running
(North-South) through the western end of cornwall, or something. Has
anyone noticed theis problem with the map?

Quite correct and that is basically the case. Prior to the early 20th
century international agreement that settled on the Greenwich Meridian as
the Prime Meridian for everyone, each country selected its own Prime
Meridian. The British Empire and the US used the Greenwich Meridian. The
French Empire used the Paris Meridian, of course. The Austrian Empire used
the Ferro Meridian, which passes through Hierro (or Ferro) Island, the
western-most of the Canary Islands. This makes sense for a European land
power, as everything in Europe would have an East Longitude. Very little
opportunity for that messy math connected with comparing points on both
sides of the Prime Meridian.

The Ferro Meridian is 17 degrees 40 minutes East of the Greenwich Meridian.
Hence the difference you noted.


--
Peter Zavon
Penfield, NY