Idiomatic phrasing of "to the [cardinal direction] of [something]"

Question

I am currently writing a small geography of the Roman Empire at its greatest extent (in the year 117 AD, under Emperor Trajan) in an effort to practice my composition skills. So far everything has been going well, however, via a discussion in CONLOQVIVM, a very specific question came up concerning cardinal directions.

In my writing, I am currently things similar to the following phrase a lot (in Latin of course; this is simply an example):

(written) Gaul is found to the north of Italy.
(or, more explicitly phrased) Gaul is found in the region north of Italy.

In Latin, I would usually write the above as follows, using ad plus a cardinal direction in the accusative and a genitive or in plus a cardinal direction in the ablative and a genitive, like so:

Gallia ad septentriōnem Ītaliae invenitur.
Gallia in septentriōne Ītaliae invenitur.

To me, when writing out my Latin, without trying to think too much in English about meaning or phrasing, this is what felt natural to me. However, I can't help but feel (as a result of it being pointed out by @JoonasIlmavirta and @Cerberus in CONLOQVIVM) that this isn't quite right and that the Romans probably would have had a more idiomatic way of phrasing this.

In short, what would be the appropriate or idiomatic way of phrasing the English concept of to the [cardinal direction] of [something]?

Answer 1

This passage from Tacitus 'Judaea' partly supports your phrasing;

Terra finesque qua ad Orientem vergunt Arabia terminantur, a meridie Aegyptus obiacet, ab occasu Phoenices et mare, septentrionem e latere Syriae longe prospectant.
The land and borders which turn to the east are bound by Arabia; Egypt lies on the south; the Phoenicians and the sea on the west; and the land and borders lie on the north for a long way on the side of Syria.
and this variant from Caesar B G 5.13
'towards the setting sun.'
...alter angulus ad occidentem solem;

But consider also the following worldviews:

Classical: Rome is the centre of the world; locations defined as ‘this side of' cis, 'far side of' ultra.

Sunt enim ultra Gorgadas sitae sub Athlanteum litus in intimos maris sinus; Isidore 14 .6.9
citra Caledonia (Cicero)

Classical: thinking in terms of flat Earth and central Rome, direction is given according to the Seneca Windrose. Although the Isidore examples are late, he is a traditionalist who describes old-fashioned knowledge in classical terms.

Haec ab Africo in Boream porrigitur. Isidore 14 .6.6

Taprobane insula Indiae subiacens ad Eurum, ex qua Oceanus Indicus incipit, patens in longitudine octingentis septuaginta quinque milibus passuum, in latitudine sescenta viginti quinque milia stadiorum. Isidore 14 .6.9

...tertium est contra septentriones, cui parte nulla est objecta terra; Caesar B G 5.13

Post Classical Adelardus of Bath (Quaestiones Naturales. 1120?) Spherical Earth, does not use Arctic and East as absolutes, but as descriptive of areas, shores, or regions.

NEPOS: Aiunt enim bracchia......fluentia ab orientali et occidentali plaga ...in Arcticam et Antarcticam regionem...refundere.

For they (Macrobius/ Ptolemy) say that the arms [of the tidal flood]... flowing from eastern and western shore... pour back...into the Arctic and Antarctic region.