Catalogs of HII regions of spiral galaxies can be used to obtain a great deal of information on their underlying galaxies. For instance, we can study the radial profile of the distribution of HII regions and compare it with the radial distribution of the light in different wavelengths. We can study also the number distribution of HII regions according to their flux and use this information to study the star formation rate in galaxies. Yet more information can be obtained by Fourier analyzing the spatial distribution of the HII regions. The distribution can then be decomposed into components of a given angular periodicity, whose pitch angles and relative amplitude can be calculated. Using such results for a large sample of galaxies should provide useful information on the properties of spiral structure in external galaxies. With this aim, we started in 1991 a study of all catalogs published until that date (García-Gómez & Athanassoula 1991, hereafter GGA; García-Gómez & Athanassoula 1993; Athanassoula, García-Gómez & Bosma 1993). Since then, however, a large number of new catalogs has been published, nearly doubling our initial sample. This warrants a reanalysis of our previous results, particularly since the new published catalogs extend the initial sample specially towards barred and ringed galaxies, which were sparse in the first sample. The new sample contains also a large number of active galaxies.
In this paper we will concentrate on the fundamental step of the deprojection of the HII region positions from the plane of the sky onto the plane of the galaxy, determining the position angle (hereafter PA) and the inclination angle (hereafter IA) of each galaxy using several methods of deprojection. In future papers we will study the spiral structure of the galaxies which have a sufficient number of HII regions and use the whole sample to study the global properties of the spiral structure in external galaxies. This paper is structured as follows: In Section 2 we present the new sample and in Section 3 we discuss the different methods of deprojection used to obtain the values of PA and IA. In Section 4 we give the PA and IA values adopted for the galaxies in our sample and finally in Section 5 we compare the values of the PA and IA obtained by using the different methods of deprojection.