Transverse phase-space measurements are an essential aspect for FEL facilities. After acceleration in the injector the energy is sufficiently high to bring the beam out of the space-charge dominated regime, thus optics based techniques are favored. The beam moments at a given point in the machine are fitted to beam size values downstream with different phase advances between the reconstruction and the measurement point. Two key methods are possible. Beam sizes can be measured at different positions in the beam line keeping the lattice unchanged. The other possibility is to actively change quadrupoles and use only one screen. These two techniques are compared in this paper including Monte-Carlo studies on systematic errors using the SwissFEL Injector Test Facility as an example. Beam size measurements, for instance done with OTR screens as profile monitors at the SwissFEL Injector Test Facility, are critical for such measurements. The analysis of these images can be a challenge, especially if the signal-to-noise is compromised for example by low bunch charges. This study on the phase-space measurement techniques will be completed by a discussion of the image post-processing procedures.