The Global Land Survey (GLS) data sets were produced to enable fine scale land change studies at regional to global scales. Currently these data sets are available for four epochs: 1975, 1990, 2000, and 2005. They are being used as a major data source for assessing land cover and changes in various research projects. Many of the quality issues of these products, however, have not been adequately assessed, and the impact of such issues on applications that rely on these data sets is largely unknown. Here we propose to look into many of the quality considerations of the GLS data sets by examining them in a systematic manner, with the goal of gaining better understanding of their quality.