The hourly or daily values of global radiation are commonly used in environmental modelling and its accuracy of estimation can impact on the assessment or the net radiation, reference evapotranspiration etc. Evaluation or the measurement method impact on absolute and relative errors committed during the estimation or hourly or daily global radiation values is the object or this paper. The global radiation flux time variability can be relatively high during the cloudy days. This can impact on the precision of hourly and daily values estimation. The measurements were carried out using the CM I1 Kipp & Zonen pyranometer installed 7 m above the ground in Poznań city for a 217 days period ranging from March 1^st through October 2^nd 2001. The single measurement was taken every second. Two kind methods were assessed during this study. The continuous methods where single measurements were carried on with certain time interval (TI). The periodic methods – the measurements were made in certain periods (TP) within 1-hour lime interval. The absolute (AE) and relative (RE) errors were calculated each method. The conclusion of the study is that the increasing of TI (for continuous method) and decreasing of TP (for periodic methods) cause the increase of both maximum and average of absolute and relative error values. The formulas that describe the relationships between measurement methods and both absolute and relative average errors were created for practical reasons.