Compressors pump gas for a wide variety of applications each of which has its own flow resistance which the compressor has to meet to keep the gas flowing. A map shows the pumping characteristics for the complete range of flows and pressure requirements for its application. The map may be produced by driving the compressor with an electric motor with the flow resistance selected artificially using a variable area throttle valve. The compressor may also be mapped if it is part of a gas generator with a valve at the turbine exit. Campbell shows a General Electric J79 compressor mapped in this way.