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GSA TODAY | JULY 2016 England and Wales), which gave the best fit on the Smith County b1–b75 or possibly to b100 (Series IV); and a late unnumbered
maps. One additional piece of indirect evidence is that projection series (Series V), which is divided into Va maps, similar to late
tests on the small-scale General Index to Cary’s 1794 New Map of Series IV and Vb, possibly part of a second 1830s series (Sharpe,
England and Wales, which does contain internal coordinate lines, 2007, and personal commun.). The early and late unnumbered
give RMS errors of 1448 m (4750 ft) for Cassini against 1637 m maps are not signed by Smith, while the numbered issues are
(5370 ft) for Bonne. The difference in error is too small for any usually signed. The map is dated 1st August 1815; however, two
definitive statement to be made as to which projection was used, early Series I unnumbered maps were issued before this date, with
but on balance it is more likely that a Cassini projection (on a a further 20 in September–October 1815. However, most maps
sphere) was used for both the Smith 1815 and County maps. were not issued until after 2 November 1815, when Smith began
signing and numbering his maps. Countermarks on the very late
TRANSFORMATIONS Z map (which hangs in The Geological Society of London’s
Burlington House) date it as 1836 or later. Smith’s memoir lists
Having decided that Cassini was overall the best-fit projection, 410 subscribers for 414 copies of his 1815 map. Not all subscribers
the maps were transformed to that projection. Graticules on the took their copies, and some maps went to non-subscribers; the
1815 map provided some limited information, which was supple- total production run was probably around 350, of which 120–130
mented by John Cary’s locations of towns and villages. For the may still survive (Tom Sharpe, 2014, personal commun.).
county maps, some 1,800 graticule longitude and latitude points
were transformed using town and village locations and RMS In order to demonstrate the evolution of Smith’s geological
errors obtained. In general, transformation errors were lower in mapping, extracts from various maps around the city of Oxford
southeast England and tended to increase to the north and west. have been produced from the GIS and are shown in Figure 2. All
three Smith maps use the same basic scheme of colors and tinting
THE 1ST PRINCIPAL TRIANGULATION technique, although there are some significant changes in geolog-
ical content. All the maps show a succession of strata, oldest
The 1st Principal Triangulation of the UK and Ireland was (Great Oolite) in the northwest and getting progressively younger
conducted over six decades and was a remarkable achievement to the southeast (Chalk). The Smith A map at the top of the panel
that significantly improved the accuracy of nineteenth-century is a very early unsigned copy and was probably the Geological
maps of the country. The Principal Triangulation commenced in Society subscription copy. Tinting on the A map tends to be light,
1783 under the direction of General William Roy with the objec- almost tentative, often grading to no color. Smith himself says, “In
tive of connecting the observatories of Greenwich and Paris and many cases the edge of one stratum terminating on another is so
determining the difference in longitude between them. After gradual, as not to admit of any distinction or definite line”
Roy’s death in 1790, the survey was extended across the UK and (Smith, 1815, p. 8). Where there is no color, the formation is
Ireland under the direction of Colonel Williams, General Mudge, assumed to be intermediate between the strata above and below.
General Colby, Colonel Hall, and finally Colonel Henry James. However, on early issue maps (like the A map) the Greensand is
Apart from improving map accuracy, the triangulation allowed purposely shown colorless. Also, in early maps the Coral Rag is
Alexander Clarke to compute a spheroid for Earth and make not shown as a separate formation, only sandstone between the
density calculations. The first stage in the triangulation was the Clunch Clay and the Oaktree Clay. The later issue P map (Series
measurement of an accurate baseline between Kings Arbour (now Va, possibly 1816, Tom Sharpe, 2014, personal commun.) is more
the site of the Heathrow Airport car park) and the Poor House at fully and confidently colored. On this map, the Coral Rag appears
Hampton. This was first achieved by iron bars and deal rods and and the Oaktree Clay is extended to the base of the Chalk. Smith’s
then remeasured using glass tubes. Corrections were made for Oaktree Clay is somewhat problematic; on the 1815 map legend,
temperature and humidity and the line accurately leveled; the he shows “Blue Marl or Oak Tree Soil” beneath the Greensand and
final accuracy is estimated at an amazing one inch in 27,400 ft. above the Purbeck stone. However, on his later cross sections,
After completion of the line, a number of triangulations were geological tables, and the county maps, the Oaktree Clay is clearly
made in a southeasterly direction toward the Kent coast. The below the Portland stone and above the Clunch/Coral Rag,
triangulations were made using a specially commissioned theo making it the equivalent of the modern Kimmeridge Clay. The
dolite constructed by Jesse Ramsden. As the triangulation changes in geological coloring are accompanied by new engraved
proceeded, it become obvious that there were large inaccuracies in geological lines. The third map is a composite of parts of Smith’s
existing maps of the country. In 1799, Mudge and Dalby made a geological maps of Oxfordshire and Berkshire published between
number of severe criticisms of the “erroneous state of our maps.” 1819 and 1820. The topographic and cultural information shown
Cary’s maps were not specifically mentioned by Mudge and Dalby is more detailed because the county sheets are at a larger scale; the
and while at the local level Cary’s maps are accurate, errors geological boundaries are also more detailed. For general compar-
become amplified in country-wide maps. ison, the last map illustrates a simplified version of the modern
geology published by the British Geological Survey.
EVOLUTION OF SMITH’S GEOLOGICAL MAPPING
GEOLOGICAL SIGNIFICANCE OF SMITH’S MAPS
Smith’s 1815 map exists in at least six main variants in five
series as recognized by Eyles and Eyles (1938), based on changes to William Smith did not make the first geological map. Jean-
the geology, coloring, and topographic detail. These include an Étienne Guettard (1715–1786) and Philippe Buache (1700–1773)
early unnumbered issue (Series I); an early issue second series published a map in 1746 containing a large amount of regional
numbered 1–100 (Series II); another, mostly second issue third lithological information that significantly shows an attempt at
series, numbered a1 to a100 (Series III); a third issue, numbered the distribution of the chalk in France and England. Later in the
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