Surface plates are considered main tools in production metrology. It acts as reference planes for different measurements of heights and depths. The flatness errors, which are the main geometrical errors of surface plates, should be determined in the right ways. There are several methods and high-precision instruments that can be used in measurements of such errors. In this study, two union-jack and xy-grid measurement methods are used to calibrate grade 1 granite surface plate. The number of measured lines differs from one Method to the other. While it is stated as eight lines for the union-jack Method, it is ranged from 10 to 12 lines for xy-grid method with line steps of 100 mm. The flatness errors of a plate are determined through straightness measurements of these lines. The number of measured lines strongly affects the determined flatness errors. Flatness errors and associated uncertainties by different methods are analyzed, evaluated and compared. The calibration by the xy-grid method gives a strong and real representation of flatness errors of the surface plate. Article Highlights: this paper studies the common strategies used in calibration of surfae plate; Union Jack and XY-Grid methods. The number of test points, lines and regions are changed according to the method used. The calibrations are carried out four times by each method. In each time; the starting corner is changed. Measurement results and associated uncertainties in these eight calibrations are compared.