Despite this, the map is of a different caliber from its 17th century predecessors and the detail and contouring at local level certainly suggests a convincing improvement in usefulness. It is, of course, significantly less accurate than Cassini's Atlas of France, which was not completed until 1789, and cannot compare with the first edition OS maps which arrived 50 years after that. Unlike Cassini, Roy had not yet developed the process of ground-triangulation. He was using non-telescopic theodolites and simple measuring chains and the implied errors would be compounded over distance. There was no reference to latitude or longitude. The greatest shortfall appears to be absence of account of magnetic variation which alone would account for a 4 mile discrepancy over the distances covered. In practical terms the survey was completed over eight summers by a maximum complement of six survey parties: logistically improbable unless simple sketching was employed as much as careful measurement.
Whittington and Gibson have tested reliability by zoning in on a dozen small areas of the map for comparative analysis. They emphasise that much of the detail is much clearer on the protractions than in the fair copy. Some cities and town maps had been plagiarised: clearly no new survey had been done, in particular, on Aberdeen, although parts of Edinburgh are also suspect. Settlements seem more reliably depicted in location, though this is less so at house-block level and many settlements are missing. Placenames were often a guess by Scotsmen from the south, challenged by not speaking Gaelic; few of the residents to whom they would speak would be literate, so the surveyors’ spelling was imaginative. Field boundaries are little more than sketchwork and farm cultivation area is consistently exaggerated. They noted that Roy’s maps were the first to minimise the importance of great estates and fine castles.
Overall it was a military map for troop movement and very fit for purpose : more accurate on rivers, lochs and roads and less so on property boundaries, fields and habitation. Whittington judges that the level of detail was in excess of that which would be expected for military use at the time. A contemporary comment was that ’on Roy’s Map the Ground looks as difficult to traverse as it really is.’