  Most of our knowledge about the Sun's activity cycle arises from sunspot
observations over the last centuries since telescopes have been used for
astronomy. The German astronomer Gustav Sp\"orer observed almost daily the Sun
from 1861 until the beginning of 1894 and assembled a 33-year collection of
sunspot data covering a total of 445 solar rotation periods. These sunspot
drawings were carefully placed on an equidistant grid of heliographic longitude
and latitude for each rotation period, which were then copied to copper plates
for a lithographic reproduction of the drawings in astronomical journals. In
this article, we describe in detail the process of capturing these data as
digital images, correcting for various effects of the aging print materials,
and preparing the data for contemporary scientific analysis based on advanced
image processing techniques. With the processed data we create a butterfly
diagram aggregating sunspot areas, and we present methods to measure the size
of sunspots (umbra and penumbra) and to determine tilt angles of active
regions. A probability density function of the sunspot area is computed, which
conforms to contemporary data after rescaling.
