There are many factors, such as the identification of the source, the identification or the definition of the mouth, and the scale of measurement of the river length between source and mouth, that determine the precise meaning of "river length". As a result, the length measurements of many rivers are only approximations (see also coastline paradox). In particular, there seems to exist disagreement as to whether the Nile or the Amazon is the world's longest river. The Nile has traditionally been considered longer, but in 2007 and 2008 some scientists claimed that the Amazon is longer by measuring the river plus the adjacent Pará estuary and the longest connecting tidal canal. A peer-reviewed article published 2009 in the "International Journal of Digital Earth" concludes that the Nile is longer.