Sources.

1. National Library of Scotland.  

 William Roy's Military Survey of Scotland 1747-55. The originals are in the British Library: good copies, reduced versions and original protractions. It was the only detailed and comprehensive map of Scotland in existence prior to 1800.The on-line digitised version is available provided by the National Library of Scotland. The map was conceived and commissioned exclusively for a military purpose, which is important because bridges matter so much to the military. It predated the Agricultural Revolution, the Industrial Revolution and Turnpike roads. Measurements were made with a chain and simple triangulation with a gun-sighted, non-optical, early theodolite. Much of the detail was recorded by rough judgment and sketching. There is an abundance of detail and waterways are carefully recorded with every contour.  Bridges are easily identifiable. This makes it the perfect historical baseline resource. On this site, the date ascribed to finding a bridge on Roy's map is '1750'.
The Military Survey (on-line) can be found at http://maps.nls.uk/roy/index.html

 Timothy Pont's manuscripts are available, digitised and on-line, provided by the National Library of Scotland. These late 16th century documents comprise the oldest detailed maps of the mainland, but only a proportion still exist. It is probable that Pont merely walked the course, but that alone, in its day, was a remarkable accomplishment. Bridges are identified, though sometimes without much clarity, and occasionally it can be difficult to distinguish them from other detail. Many of the maps are more in the nature of sketches or drafts- often messy and over-written.  It can also be difficult to identify a Pont bridge and locate it on a modern OS: more interpretation and guesswork are required than with Roy.  Pont also provided a wealth of descriptive text and this material along with his maps, on 38 sheets, was taken in hand by Balfour Scot and Gordon to provide the basis of Blaeu's Atlas. On this site, the date ascribed to finding a bridge on Pont's map is '1600', which may be rather later than the survey date.
Timothy Pont's maps, on-line, may be found at http://maps.nls.uk/pont/

 Blaeu's Atlas Novus Vol V was published in 1654.  Much of the material was from Timothy Pont who had died around 1613.  In 1629, Pont's maps and manuscripts were bought from his family by the historian Sir James Balfour. Balfour passed on the material to atlas maker William Blaeu in Amsterdam who enlisted Sir Robert Gordon and Sir John Scot to assist in preparing the maps for engraving and publication. The existing maps were expanded with texts, other maps were consulted and some additional surveys were done.  The beautiful maps in the Blaeu Atlas have a great deal more clarity but probably no more precision than Pont provided; the geographical area covered is considerably wider and provides missing areas. However, large parts of Perthshire are sadly missing from both maps.    On this site, the date ascribed to a finding on the Blaeu Atlas is a compromise of '1640'.
The Blaeu Atlas on-line may be found at http://maps.nls.uk/atlas/blaeu/

 John Adair was an early Scottish surveyor and mapmaker, contracted by the Privy Council of Scotland to survey the shires, around 1680. Surviving maps include those of Lothians, Stirling, Fife, Kinross, Southern Perthshire, Ayrshire and Lanarkshire.
These may be found on-line at https://maps.nls.uk/mapmakers/adair.html

Other Links

1. Additional Information on William Roy, the Roy map and 18th century roads and road network may be found at https://www.roysroads.co.ik

2. Many of the linked references have pointed the reader to the resource pages of the Royal Commission on the Ancient and Historical Monuments of Scotland.  Historic Environment Scotland has now absorbed this institution.  However Canmore, the search facility, remains.
The main website URL is https://canmore.org.uk/map/about

3. Many links also take the reader to British Listed Buildings.
http://www.britishlistedbuildings.co.uk/

4. Supporting material on engineering as well as for some individual bridges from The Institute of Civil Engineers. The historical search engine is at https://www.ice.org.uk/knowledge-and-resources/historical-engineering-works

5. Old Roads of Scotland has provided many items of supporting material and cross references.  Sadly, Gerald Cummings, the author, died in 2015.
http://www.oldroadsofscotland.com

6. The Old and New Statistical Accounts of Scotland have been regularly consulted; particularly the 1791-99 Old Accounts for the parishes, in which roads and bridges are often described.  
https:// edina.ac.uk/stat-acc-scot/

7. ‘Historic Bridges of Great Britain’ is an invaluable resource. Following in the 1930s tradition of Edwyn Jervoise, the web-document reflects personal visits to some 850 bridges in the British Isles, each one older than 1700. Every data-sheet provides a description, dating, history and photographs. Additional background context and comment is also provided. Personal communication with the author, Tom Robertson, has also been invaluable.
https://historicbridgesofbritain.com

8. Walter Macfarlane’s Geographical Collections Relating to Scotland is a further important reference in a series of three volumes.  Volume 1 describes early 18th century parishes: not comprehensive geographically but a range of attributed and unattributed essays on the geography and social history of the country. Volumes II and III offer descriptions from the 16th and 17th centuries, more loosely based on parishes; this material has been largely attributed to Robert Sibbald who in turn gathered information from Timothy Pont, John and Robert Gordon and others.  The material from these volumes is older than the Roy maps and older than the statistical accounts. Volume 1 is the most searchable and  the most  useful.
https://archive.org/details/geographicalcoll01macf/page/n7/mode/2up

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