  The century-long (1907-2007) Ca II K spectroheliograms from Kodaikanal Solar
Observatory (KSO) are calibrated, processed and analysed in the present study
to follow the evolution of bright on disc structures called plages, the
possible representatives of magnetic activity on the Sun. This has been the
longest dataset studied in Ca II K till date covering about 9.5 cycles of 11
year periods. Plages were segmented with area $\geq 1\:\textrm {arcmin}^2$
using global thresholds for individual full disc images and subsequent
application of morphological closing operation. Plage index was calculated and
seen to have close positive correlation with fractional disc area covered by
plages. The newly generated plage area cycle (from KSO) was compared with the
same from Mount Wilson observatory (Correlation~$95.6\%$) for the overlapping
years i.e. 1915-2000. Study illustrated time-latitude distribution of plage
centroids rendering butterfly diagram (as observed for sunspots). The 3D
visualisation of the diagram showed one to one mapping between plage location,
time and area. This work further delineated positional correlation between
magnetic patches and plage regions through comparison of synoptic maps derived
from both Kodaikanal Ca II K images and space based full disc LOS (line of
sight) magnetograms. Regular synoptic magnetograms from ground based
observatories are available only after 1970s. Thus the long term Ca II K data
from KSO can be used as a proxy for estimating magnetic activity locations and
their strengths at earlier times.
