In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the large scale structure of the universe. The Great Wall, a vast conglomeration of galaxies over 500 million light-years wide, provides a dramatic example of a large-scale structure that redshift surveys can detect. The first redshift survey was the CfA Redshift Survey, started in 1977 with the initial data collection completed in 1982. The most notable, recent and low-redshift surveys are the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. At high redshift there exist the DEEP2 Redshift Survey and the VIMOS-VLT Deep Survey (VVDS). Because of the demands on observing time required to obtain spectroscopic redshifts (i.e., true redshifts measured by velocity), it has become commonplace to use photometric redshifts, [1] based on brightness. Such "redshifts" can be used in surveys and to find the space distribution of galaxies, provided the galaxy types are well known, but, obviously they cannot be used to define points on the Hubble curve, because of circular reasoning.
