Static survey methods are the most precise form of GNSS surveys. These methods can be used for baselines of any length. However, rapid static survey methods can be used for shorter baselines typically found in surveys. The maximum length of a baseline for a rapid static survey is dependent on ionospheric conditions. Typically, the maximum length is between 10 km to 20 km. Beyond these distances, a static survey should always be performed. No matter the type of survey used, session lengths should be long enough to ensure good results. While the appropriate length of a session is dependent on several factors including number of visible satellites, ionospheric conditions, type of receiver, processing software, and so on, Table 17-1 recommends session lengths as a guide for both dual- and single-frequency receivers using both static and rapid static methods. Due to ionospheric refraction, single frequency receivers should only be used on baselines that are less than 20 km in length. The following example demonstrates the use of this table to determine minimum recommended session lengths for various baseline lengths.