An experimental method, using simultaneous vertical and slant observations, has been derived for estimating the effective shell height for electron content measurement by GPS reception. At a latitude of approximately 53¿ north, Lancashire is well placed as an observing site since GPS satellite orbits are inclined at 55¿ to the Earth's equatorial plane and, as a result, many GPS tracks pass almost directly overhead, giving true zenithal measurements. This paper focuses on the question of oblique-to-zenithal correction and related matters. In particular, plasmaspheric effective height and satellite bias corrections are determined by measuring the total electron content (TEC) from pairs of satellites to a single ground station, each pair giving simultaneous observations of oblique and zenithal TEC. Additional bias corrections are determined by extracting the TEC using pairs of satellites with the same elevation at the same time. The Chapman Production Function Model and the Sheffield University Plasmasphere and Ionosphere Model are both used to determine a theoretical value for the plasmaspheric effective height. The results indicate that the plasmaspheric effective height used in the oblique-to-zenithal thin shell conversion is considerably greater than the commonly adopted value of 350 km. It is suggested, on the basis of all the available evidence, that a value between 600 and 1200 km is preferred. Assuming a lower value could produce an error of 15 to 30% or more in the electron content.