Photon-Dominated Regions (PDRs) are ubiquitous both in the Milky
 Way and external galaxies, and exhibit large luminosities, much
 of it emitted in the form of FIR atomic and molecular lines. As
 such, their astrophysical importance is large and many ISO GT
 programs are aimed at their study. However, few if any of these
 programs seek to MAP PDRs, in spite of the fact that mapping is
 crucial to the interpretation of PDR observations given the rapid
 spatial variation of their physical properties. Among the lines
 emitted by dense PDRs, high-J CO lines (J>~10) have been shown to
 be among their most effective diagnostics, because of their
 brightness and sensitivity to high excitation conditions. In addition,
 high-J isotopic CO lines arise in cooler layers than their more
 abundant counterparts, and thus provide an indicator which allows
 to differentiate between the surfaces and cores of (possibly
 clumpy) PDRs. We here propose to map a prototypical dense PDR (either
 Orion or M17, depending on the launch date) in a well-chosen
 pair of high-J CO lines and their isotopic counterparts, namely
 the J=15->14 and J=20->19 lines of CO and (13)CO. These maps are
 practically impossible to obtain from an airplane-based platform
 such as the KAO and thus should be done using ISO.