This paper describes the structure and the implementation of an extended parametric representation of compressor characteristics for a modern object oriented gas turbine performance simulation software (PROOSIS). The proposed methodology is the map fitting tool (MFT) methodology. The proposed MFT methodology for modeling the off design performance of gas turbine turbomachinery components (fans, compressors, and turbines) is based on a concept conceived and developed collaboratively by General Electric (GE) and NASA. This paper provides a short description of both BETA and MFT compressor maps, as well as the development of compressor component models in PROOSIS capable of using both types of maps for off design compressor performance prediction. The work presented in this paper is the outcome of a collaborative effort between Snecma Moteurs and Cranfield University as part of the European Cycle Program of the EU FP6 collaborative project—VIVACE. A detailed description of the MFT map methodology is provided with a “step-by-step” calculation procedure. Synergies between compressor MFT and compressor BETA calculations are also highlighted and a description of how these two components have been integrated into an object oriented simulation software with component hierarchy is also presented. Advanced parametric representations of fan and turbine characteristics have also been developed within PROOSIS. However, a description of these methodologies is beyond the scope of this publication. Additionally, a comparison between the advantages and disadvantages between BETA and MFT maps is an interesting debate. However, this is also beyond the scope of this paper.