According to the position in the data flowing graph in GIS, geographic data are classified into the following kinds:first data, database data, middle data and final data. An index for measuring data quality is added to the data "atom" in GIS. The geographic data quality model at different step in GIS is proposed. The analysis of the models indicates that the data quality of first data plays the most important role in GIS products. The factors effecting the data quality in GIS and three quality control ways are generalized.