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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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