An environmental radiation survey using a gamma-ray spectrometer is used to rapidly detect radioactive contamination over a wide area of ground that was released from nuclear events. For the successful application of a gamma-ray spectrometer to the calculation of the radioactivity concentration in the ground and the dose rate at 1 m above the ground, it is necessary to build a calibration procedure to obtain the counting efficiency at the in situ measurement, which means in situ calibration factor to report the calculation results from the measured net count rate according to the diverse detection geometries. This study is focused on the development of a program to calculate the in situ calibration factor and report the survey results in the environmental radiation surveys using three kinds of gamma-ray spectrometers, which have been widely used in the field of in situ measurements: a coaxial HPGe detector, cylindrical NaI(Tl), and rectangular NaI(Tl). The program is based on the results of diverse theoretical calculations of the unscattered photon fluence at the detector height, detector responses of three detectors, and their angular corrections. The developed program was successfully applied to the estimation of the radioactivity concentration of nuclides in the ground and the dose rate at 1 m height above the ground induced from them.