Getting hold of geological maps on-line has been a hit or miss affair until recently, and those made available for free are at a variety of scales (generally less than 1:10 million) and vary in reliability and information content. Scanned versions of paper sheets rendered with JPEG compression can leave a lot to be desired. If you are able to pay, then the situation improves as there are on-line vendors of printed geological maps. But, all told, browsing the world’s geological features is a slow and generally frustrating task. The best bet might seem to be the Commission for the Geological Map of the World (http://ccgm.free.fr/) . They do, as you might expect, sell global maps, but at 1:50 million detail is sparse, although there is an alternative 3-sheet set (Old World, Americas and Polar regions) at 1:25 million, and it is possible to purchase digital versions and a variety of geophysical sheets. Maps at 1:5 million are available for Europe, Africa (6 sheets), the Middle East and South America plus various tectonic maps. However, to explore full planetary-scale geology at the modestly informative scale of 1:5 million demands visiting a lot of on-line vendors, as there is no one-stop shop for geologists
Such frustration is set to change, because in the last few years there have been moves to compile digital geology in a manner akin to Google Earth, now available at the OneGeology portal (http://portal.onegeology.org/). As soon as you enter the portal, the reason why the Commission for the Geological Map of the World is so irritating immediately becomes clear: the CGMW world map is what shows at the global scale and it doesn’t show much. Progressive zooming-in removes the 1:50 million map, to be replaced by a compilation of regional maps at scales ranging from 1:2 million to 1:12.5 million scales that does cover the entire Earth’s continental surface. A mouth-watering prospect until you start to look for legends! In fact, the associated tool box provides a means of pointing to individual stratigraphic units on the maps to get information (metadata), but whether and how it works depends on the source of the maps and the scale of viewing. For instance, the 1:10 million map of Africa gives no information, while the 1:5 million map of Europe gives quite a lot.
With a zoom to better than 1:10 million display, lots more detail appears in the form of country maps, but coverage is not comprehensive. In East Africa country maps are available for Ethiopia, Kenya, Rwanda and Tanzania – ranking with the current offerings from the USA. Moving to Europe, the range of scales improves on a country-by-country basis, generally 1:1 million to 1:250 thousand, but the UK truly grabs attention by providing digital geology at up to 1:50 thousand scale. The British Geological Survey has systematically rendered all its bedrock map data digitally to this scale, and is to be congratulated at making the ‘Full Monty’ available on the OneGeology portal. Full BGS metadata shows for all the visible stratigraphic and lithological units, together with faults and superficial deposits.
It soon becomes clear that OneGeology is a work in progress, but what a work it will be! If I have a criticism it is that geology is not linked to topography and cartographic features. The ever-present base data is the NASA Blue Marble mosaic of natural colour MODIS imagery. Unfortunately, outside of areas bare of vegetation this does not have any useful lithological connection, and is presented at such a large pixel size that only the coarsest topography shows up. At scales better than 1:2 million it is an irritating patchwork of square pixels. Far better would be shaded relief based on the ubiquitous ASTER GDEM data at up to 30 m resolution, especially as it is possible to vary the opacity of the geological maps to show the link with surface morphology. Maybe that is on its way and possibly oblique perspective 3-D viewing: one has to bear in mind that Google Earth wasn’t built in a day and geoscientific data are not yet standardised – a hugely costly endeavour, as that would involve not only digitising all maps but lengthy negotiations.
Most geologists are likely to be interested in maps that show rock units with stratigraphic age, but Jens Hartmann and Nils Moosdorf of the University of Hamburg, German have mined regional geological maps to assemble a global, purely lithological database (Hartmann, J. & Moosdorf, N. 2012. The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems, v. 13, doi:10.1029/2012GC004370) in cooperation with CGMW. Their Global Lithological Map (GLiM) consists of over 1.25 million digital polygons (ESRI shape or *.shp format), classified lithologically in three levels to give a total of 42 rock-type classes, 16 used in previous global lithological maps and two more lithologically specific sets of 12 and 14 subclasses . Though the database is said to be presentable at up 1:3.75 million scale, the version of GLiM that the reader can download is not in vector format but as a series of cells numerically coded according to class in a georeferenced grid. Since that is 360 rows x 720 columns, i.e. 0.5 degrees of latitude by 0.5 degrees of longitude, that version is useful only for rough statistics, such as the percentage of North America that is covered by evaporates, for instance. Perhaps the most useful aspect of the GLiM paper is the comprehensive referencing of the source maps. GLiM, apparently, is not an on-line resource, but no doubt the authors can provide interested parties with the *.shp files (contact email@example.com or firstname.lastname@example.org)