Originally Posted by LH2
Wouldn't that be "perpendicular to the local vertical" or equivalently, "perpendicular to the local gradient of the gravity field"? Is the parenthesised phrase part of the quote from the book, or is that yours?
Nope, perpendicular to the geodetic latitude. The "local gravity field" is known as the astronomic latitude, apparently, and it is not necessarily the same. Indeed, it is explicitly distinguished in Figure 2.2 of the reference I gave. (The third possibility is "perpendicular to the geocentric latitude", referenced to the "center of the ellipsoid and mass center of Earth", which is different again.)
No, it's not a quote from the book, it is my words saying what Figure 2.4 shows as a vector diagram (triangle with arrows and labels).
PBL