With the use of astral navigation for air navigation, faster methods needed to be developed and tables of precomputed triangles were developed. When using precomputed sight reduction tables, selection of the assumed position is one of the trickier steps for the fledgling navigator to master. Sight reduction tables provide solutions for navigation triangles of integral degree values. When using precomputed sight reduction tables, such as H.O. 229, the assumed position must be selected to yield integer degree values for "LHA" (local hour angle) and latitude. West longitudes are subtracted and east longitudes are added to "GHA" to derive "LHA", so AP's must be selected accordingly. When using precomputed sight reduction tables each observation and each body will require a different assumed position.