All-sky radio surveys

One of the essential data bases in radio astronomy are the all-sky surveys. These have been traditionally made by combining some northern survey with a southern one, often made with different instruments at some nearby frequency. The combinations led to many calibration problems since beams and observing methods were often not identical. For successful all-sky surveys a well organised cooperation between a northern and southern observatory, before the observations start, is desirable. Observing methods, calibration procedures and data reduction should be made jointly for best results. In this talk the development of this field of research and the present status of several all-sky survey ventures will be discussed.