8. Conclusions 



It seems clear that the most significant con- important as obtaining larger samples. The 



elusion which can be drawn from this study effects of such an analysis on the results given 



(pertaining to future requirements) is that in this atlas would probably be slight in the case 



careful selection of data by local time of obser- of the N(z) maps, somewhat larger in the case 



vation, and the segregation of these data into of the AN maps, and might well have a profound 



types, e.g., nighttime and midafternoon, prior effect on the ground-based gradient maps of 



to analysis or mapping, is probably equally as appendix C. 



25 



