1 Introduction

There are two approaches to estimate the thickness of a galactic disc. One, for edge-on spirals, has been suggested by van der Kruit & Searle (1981a,b, 1982a,b), and is based on surface photometry. The other one, for non-edge-on galaxies, has been proposed by Peng (1988). It is based on the solution of Poisson's equation for a logarithmic disturbance of density. The latter is effective and simple as long as spiral arms are distinguishable on the images.

The inclination of a galaxy (i.e., the angle between the galactic plane and the tangent plane) is difficult to determine. It has been shown by Danver (1942) and Kennicutt & Hodge (1982) that the spiral arm can be represented by a logarithmic spiral form with constant pitch angle $\mu$. The inclination can be determined by comparing the fitted spiral curve with the image.

In this paper, we present the thicknesses and inclinations for 71 northern spiral galaxies which are selected from more than 600 ones whose images are taken from the Digitized Sky Survey. Most galaxies in this paper are grand design spiral ones with Arm Classification $\geq 5$ (Elmegreen & Elmegreen 1987) except for six spirals. Usually there are two arms for most galaxies, the arm closest to the center is used for measurement.

The structure of this paper is as following: In Sect. 2, we outline the steps how to measure the thickness, determine the inclination, and select the samples. In Sect. 3, we present the results and discuss errors. We discuss statistical properties in Sect. 4.