  This work has aimed to analyze the 1998 to 2000 campaign of solar diameter
surveying. The employed instrument was a Danjon astrolabe, at the
Observat\'orio Nacional campus, and specially modified for the solar
observations. During the time lapse, 10807 independent measurements of the
solar diameter were made, Eastwards and Westwards from the local meridian and
evenly distributed. An image treatment has been devised to account for the
camera dark current and flat field, using IRAF routines. A study of the
observational conditions upon the final outcome was made. The mean temperature
at the observation is shown as the most influential parameter upon the final
result. Next to it also the temperature variation, the Fried's factor, and the
standard deviation of the reflected parabola presented a minor and complex
degree of influence. The derived corrections are of the order of hundredths of
arc seconds, thus being tenfold smaller than the typical error of one
observation. The mean semidiameter for the time lapse (1998-2000) is found as
959".107\pm0".006. Through the use of a CLEAN algorithm the periodic terms of
the semi diameter were obtained. The largest amplitude has a period of 515
days. By using a second type of algorithm, namely the DCDFT, the found periods
were confirmed. A dependency of the semidiameter on the observed heliolatitude
is verified. The difference between the equatorial and polar radii was
calculated as \Delta r=0".013\pm0".004, and the solar quadrupole moment was
inferred as |J_2|=(3.61\pm2.90)*10^(-6).
