  High spatial resolution observations of CO isotopologue line emission in
protoplanetary disks at mid-inclinations (${\approx}$30-75{\deg}) allow us to
characterize the gas structure in detail, including radial and vertical
substructures, emission surface heights and their dependencies on source
characteristics, and disk temperature profiles. By combining observations of a
suite of CO isotopologues, we can map the 2D (r, z) disk structure from the
disk upper atmosphere, as traced by CO, to near the midplane, as probed by less
abundant isotopologues. Here, we present high angular resolution
(${\lesssim}$0."1 to ${\approx}$0."2; ${\approx}$15-30 au) observations of CO,
$^{13}$CO, and C$^{18}$O in either or both J=2-1 and J=3-2 lines in the
transition disks around DM Tau, Sz 91, LkCa 15, and HD 34282. We derived line
emission surfaces in CO for all disks and in $^{13}$CO for the DM Tau and LkCa
15 disks. With these observations, we do not resolve the vertical structure of
C$^{18}$O in any disk, which is instead consistent with C$^{18}$O emission
originating from the midplane. Both the J=2-1 and J=3-2 lines show similar
heights. Using the derived emission surfaces, we computed radial and vertical
gas temperature distributions for each disk, including empirical temperature
models for the DM Tau and LkCa 15 disks. After combining our sample with
literature sources, we find that $^{13}$CO line emitting heights are also
tentatively linked with source characteristics, e.g., stellar host mass, gas
temperature, disk size, and show steeper trends than seen in CO emission
surfaces.
