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Performing increasingly sensitive surveys is a fundamental endeavour of astronomy. Over the past 60 years, the depth, fidelity, and resolution of radio surveys has continuously improved. However, new, upgraded and planned instruments are capable of revolutionising this area of research. The International Low-Frequency Array (LOFAR) is one such instrument. LOFAR offers a transformational increase in radio survey speed compared to existing radio telescopes. It also opens up a poorly explored low-frequency region of the electromagnetic spectrum. An important goal that has driven the development of LOFAR since its inception is to conduct wide and deep surveys in order to advance our understanding of the formation and evolution of galaxies, clusters, and active galactic nuclei (AGN). The LOFAR Surveys Key Science Project is conducting a survey with three tiers of observations: Tier-1 is the widest tier and includes low-band antenna (LBA) and high-band antenna (HBA) observations across the whole Northern sky; deeper Tier-2 and Tier-3 observations are focussing on smaller areas with high quality multi-wavelength datasets.
The ongoing LOFAR HBA 120-168MHz Tier-1 survey is hereafter referred to as the LOFAR Two-metre Sky Survey (LoTSS) and is described in Shimwell et al. 2016. This is the second northern hemisphere survey is being conducted with the LOFAR HBA and is significantly deeper and higher resolution than the first, the Multifrequency Snapshot Sky Survey (MSSS; Heald et al. 2015).