The technology to determine the narrowest point in northern England is as nothing compared to that necessary for supplying Roman towns with running water and baths, as with the Nimes Aqueduct in Southern France, shown here at the Pont du Gard crossing of Gardon River.
The Fontaine d'Eure, at 76 m (249 ft) above sea level, is only 17 m (56 ft) higher than the repartition basin in Nîmes, but this provided a sufficient gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city. The aqueduct's average gradient is only 1 in 3,000. It varies widely along its course, but is as little as 1 in 20,000 in some sections.
Those sections where the gradient is only 1:20,000, or 1m in 20km, are deliberate not accidental, designed to allow the Pont du Gard section to be considerably lower, and easier to build, than would have been required by an even gradient.
Further note that while the isthmus of Hadrian's Wall is only 118km long, contemporaneously Roman Engineers were undertaking the building of a 170km tunnel aqueduct, the Gadra Aqueduct, to supply the city of Gadra in Jordan with water.
In summary, the Romans were excellent engineers and knew well how to design and survey large tracts and structures. It is faulty reasoning to assume that the absence of modern technology made such feats impossible.
Note also, from this outline of ancient measuring devices:
It is evident from his description that the dioptra differs from the modern theodolite in only two important respects. It lacks the added convenience of two inventions not available to Hero - the compass and the telescope.
It's worth noting that a drop of 1:20,000 equates to only 50 cm over 10 km. In that same 10 km stretch the earth curves by about 10m, or 20 times as much.
d = (10 km / 10,000 km) * 10 km = 100 km / 10,000 = 100,000 m / 10,000 = 10 m
One can only engineer such a slight grade with a very accurate value for the Earth's radius - and they performed all those calculations in Roman numerals!