  Coronal holes (CH) are regions of open magnetic field lines in the solar
corona and the source of fast solar wind. Understanding the evolution of
coronal holes is critical for solar magnetism as well as for accurate space
weather forecasts. We study here the extreme ultraviolet (EUV) synoptic maps at
three wavelengths (195A/193A, 171A and 304A) measured by Solar and Heliospheric
Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and Solar Dynamics
Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two
datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT
level by means of histogram equalization. We then develop a novel automated
method to identify CHs from these homogenized maps by determining the intensity
threshold of CH regions separately for each synoptic map. This is done by
identifying the best location and size of an image segment, which optimally
contains portions of coronal holes and the surrounding quiet Sun allowing us to
detect the momentary intensity threshold. Our method is thus able to adjust
itself to the changing scale size of coronal holes and to temporally varying
intensities. To make full use of the information in the three wavelengths we
construct, a composite CH distribution, which is more robust than distributions
based on one wavelength. Using the composite CH dataset we discuss the temporal
evolution of CHs during the solar cycles 23 and 24.
