In 1992, Rasmussen took the position of assistant to S. Dillon Ripley, the former secretary of the Smithsonian, who was planning to produce a definitive guide to the birds of South Asia. When he became ill shortly after beginning the project, Rasmussen took over the project, and with artist John C. Anderton, produced Birds of South Asia: The Ripley Guide, a two-volume bird guide for the Indian subcontinent which was the first field guide for the area to include sonograms. Volume 1 contains the field guide with over 3400 illustrations in 180 plates, and more than 1450 colour maps. Volume 2 (Attributes and Status) gives specimen measurements, data about identification, status, distribution and habits. Vocalizations are described from recordings, and there are over 1000 sonagrams.1508 species that have occurred in India, Bangladesh, Pakistan, Nepal, Bhutan, Maldives, the Chagos archipelago and Afghanistan are covered, including 85 hypothetical and 67 'possible' species, which are given only short accounts. Notable aspects of Birds of South Asia are its distribution evidence-base – the book's authors based their distributional information almost completely on museum specimens – and its taxonomic approach, involving a large number of species-level splits. Its geographical range was also greater than that of older works, notably in the inclusion of Afghanistan.Many allopatric forms previously regarded as conspecific are treated by Rasmussen and Anderton as full species. Most of these had previously been proposed elsewhere, but the book introduced a number of innovations of its own. Experts on Asian birds, Nigel Collar and John Pilgrim, in 2008 analysed Rasmussen and Anderton's proposed changes, indicating which had previously been proposed by other authors, and which were novel, and required further justification.Although reviews in the birding and ornithological press have often been favourable, there have been criticisms. Peter Kennerley, author and Asian bird expert, considered that some of the illustrations are small and garish or technically inaccurate. He also believes that the over-reliance on sometimes very old museum specimens and dismissal of the wealth of observational data filed by amateur travelling birders is a mistake, and states that many of the taxonomic decisions appear to be random choices, unsupported by published research.Apart from the Meinertzhagen fraud, which is discussed in the next section, and the death of S. Dillon Ripley, other problems in the production of Birds of South Asia included the loss of the main map database during a trip to Burma, and poorly prepared specimen skins. There were also difficulties reconciling sources, delays in producing illustrations and maps, and in obtaining reliable data for "difficult" areas like Assam, Arunachal Pradesh, Bangladesh and Afghanistan. The Andaman and Nicobar Islands also presented serious challenges with regard to the status and taxonomy of their avifaunas.Rasmussen considered in a 2005 paper whether the revised taxonomy of the book, with its many species splits, had significant conservation implications, but felt that the effect on species richness in South Asia was limited, and would have only a moderate conservation impact, increasing the number of potentially threatened species in the region from 6% of the total avifauna to about 7%.

Question: What geographical areas provide obstacles in obtaining reliable data?
Answer: Assam, Arunachal Pradesh, Bangladesh and Afghanistan