Rask opted for a phonemic orthographic system. To accompany the recording there are three transcriptions&colon; orthographic, phonemic and allophonic. A core theme of Icelandic language ideologies is grammatical, orthographic and lexical purism for Icelandic. Multiview is a type of orthographic projection. In others, the phonetic and orthographic form has changed considerably. Nautical charts using stereographic and orthographic projections date back to the second century BC. German orthographic (or rather typographic) rules require that the mandatory blackletter ligatures are retained. This produces a number of orthographic alternations in verbs whose pronunciation is entirely regular. The formulas for the spherical orthographic projection are derived using trigonometry. One source of orthographic variation is in the representation of vowels. See References for an ellipsoidal version of the orthographic map projection. There were no doubled orthographic vowels in Mesokemic. The perspective projection requires a more involved definition as compared to orthographic projections. The use of orthographic projection in cartography dates back to antiquity. Some circular fisheyes were available in orthographic projection models for scientific applications.