Language survey
A language survey is conducted around the world for a variety of reasons. Measuring people's ability to speak and understand another language (usually community based, not school based) (multilingualism)
studying people's attitudes about different languages (Rensch 1993)
evaluating the differences and similarities in speech of communities that speak related speech forms, noting comprehension or collecting details of linguistic form (dialectology)
assessing the vitality of languages that may be disappearing (language death)
doing initial descriptions of languages in areas that are linguistically undescribed

Methods 
Methods used in language surveys depend on the questions that the survey is trying to answer. Methods used include collecting word lists, playing recorded texts to assess comprehension, sentence repetition tests, questionnaires, group and individual interviews, retelling of stories, direct observation, pointing to pictures after listening to instructions, and even internet surveys

There is a growing trend to involve communities more in language survey, using a variety of methods. As with any form of research, the methods used depend on the questions that the researchers are trying to answer. Also, the reliability of the results varies according to the method and the rigor with which it is applied, proper sampling technique, etc. Applications 
The results of language surveys are use for a variety of purposes. One of the most common is in making decisions for implementing educational programs. The results have also been used for making decision for language development work (Holbrook, 2001). And of course, academics are always interested in the results of any language survey. Agencies 
Surveys have also been conducted by ethnic associations (Saskatchewan 1991), government agencies (Statistics Canada 1993),  NGO's (Toba, et al. 2002), foundations (Pew Hispanic Center 2004), etc. Often such groups work together (Clifton 2002). Some large and notable surveys include the Linguistic Survey of India which was begun by George Abraham Grierson  late in the 19th century (Sociolinguistics research in India) and the Survey of Language Use and Language Teaching in East Africa, sponsored by the Ford Foundation from the 1960s. Both resulted in a number of volumes describing locations of languages, patterns of multilingualism, language classification, and also included descriptions of languages, such as Language in Ethiopia (Bender, Bowen, Cooper, and Ferguson 1976). The single agency conducting the most language surveys around the world is SIL International (Summer Institute of Linguistics). Language survey work is also done by academics, such as graduate students doing dissertation or thesis work or faculty members doing research. Sign languages 
Surveys have usually been conducted among spoken languages. However, surveys have also been done among users of sign languages (Vasishta, Woodward, and Wilson 1978,  Woodward 1991, 1993, 1996, Parkhurst & Parkhurst 1998, Al-Fityani & Padden 2008). As with surveys among spoken languages, surveys among sign languages have studied multilingualism, attitudes about various languages both spoken and signed (Ciupek-Reed 2012), differences and similarities between signed varieties (Aldersson and McEntee-Atalianis 2007, Bickford 1991, 2005, Parks 2011), and assessing the vitality of signed languages, and initial descriptions of undocumented sign languages. Adopting and adapting the concept of "extensibility" from spoken languages, Jason Hopkins wrote about how this could be applied to surveying sign languages. 