A stable procedure is presented to attain most probable and unbiased estimate of principal strain, rotation, and dilatation for 2-dimensional geodetic data on triangular network. The proper network size should be chosen carefully, because the errors of these estimates of strain tensor and other associated observables grow inversely proportional to the area of station triangle. As a case study, the deformation observables for the GPS-monitored co-seismic displacement in Korea due to the 2011 Tohoku-Oki earthquake were attained accordingly.