This report describes the calibration of one of the Swedish Armed Forces' detectors for field measurements of gamma emitting radionuclides. Gamma spectrometry measurements on site, or in situ, is an important method in the event of fallout or other dispersion of radionuclides in the environment. It is a method that allows for, not only an identification of radionuclides, but also a determination of activity levels. In addition, in situ gamma spectrometry can, in contrast to various sampling procedures, deliver a result directly after a conducted measurement. In order to facilitate an estimation of activity levels, the detection efficiency must first be determined for the relevant photon energy and measurement situation. This will, for in situ measurements, be a more complex task than for laboratory measurements. This is due to the fact that one can not, as in calibrations for laboratory measurements, use sources of known activity. For this reason, the calibration is carried out in a sequence of different steps. First, the intrinsic detector efficiency is determined for a set of photon energies (122-1408 keV) and angles of incidence (0-90 degrees). To these data, a function is fitted that describes the intrinsic detector efficiency in the energy and angular intervals. Subsequently, the absolute detection efficiency for in situ measurements, which will depend on both detector efficiency, source geometry and measurement setup, is calculated. In this work, two different models have been used for description of the source: an emergency preparedness model, in which all activity is assumed to be homogeneously distributed in a 2 cm thick soil layer of density 500 kg/m3, and a perfect plane. When the absolute detection efficiency is determined for a number of photon energies, and for the two source models, these data can be entered into calibration files for the GammaVision software. This enables the operator to obtain an estimate of ground deposition activity levels (in Bq/m2) directly after performed measurements, which could be of great importance after a radiological or nuclear event. Great care has also been taken to estimate the combined measurement uncertainty of in situ gamma spectrometry measurements. The uncertainty is an important measure of the quality of the measurement and the merit of the results as basis for decision-making.