  Coronal holes are regions on the Sun's surface that map the foot-prints of
open magnetic field lines. We have developed an automated routine to detect and
track boundaries of long-lived coronal holes using full-disk EUV images
obtained by SoHO:EIT, SDO:AIA, and STEREO:EUVI. We measure coronal hole areas
and magnetic flux in these holes, and compare the measurements with
calculations by the PFSS model. It is shown that, from 1996 through 2010, the
total area of coronal holes measured with EIT images varies between 5$\%$ and
17$\%$ of the total solar surface area, and the total unsigned open flux varies
between $2-5 \times 10^{22}$ Mx. The solar cycle dependence of these
measurements are similar to the PFSS results, but the model yields larger hole
areas and greater open flux than observed by EIT. The AIA/EUVI measurements
from 2010-2013 show coronal hole area coverage of 5-10$\%$ of the total surface
area, with significant contribution from low latitudes, which is
under-represented by EIT. AIA/EUVI have measured much enhanced open magnetic
flux in the range of $2-4 \times 10^{22}$ Mx, which is about twice the flux
measured by EIT, and matches with the PFSS calculated open flux, with
discrepancies in the location and strength of coronal holes. A detailed
comparison between the three measurements (by EIT, AIA-EUVI, and PFSS)
indicates that coronal holes in low latitudes contribute significantly to the
total open magnetic flux. These low-latitude coronal holes are not well
measured with either the He I 10830 line in previous studies, or EIT EUV
images; neither are they well captured by the static PFSS model. The enhanced
observations from AIA/EUVI allow a more accurate measure of these low latitude
coronal holes, and their contribution to open magnetic flux.
