_______________________________

                        |                                               |

              Elev. 1,200 - 540ft                             |

                    = 660ft                                         |

            ______|_____ QNH 993hPa         1,200ft indicated

                        |                                               |

              20hPa x 27ft = 540ft                          |

            ______|_______ 1013hPa________|_____