Grade 5 but is more accurate than a Grade 3 survey. Grade 5 A Magnetic survey. Horizontal and vertical angles measured to ±1 °; distances should be observed and recorded to the nearest centimetre and station positions identified to less than 10 cm. Grade 6 A magnetic survey that is more accurate than grade 5, (see note 5). Grade X A survey that is based primarily on the use of a theodolite or total station instead of a compass, (see notes 6 and 10 below). Notes The above table is a summary, omitting some technical details and definitions; the definitions of the survey grades given above must be read in conjunction with these notes. In all cases it is necessary to follow the spirit of the definition and not just the letter. To attain Grade 3 it is necessary to use a clinometer in passages having appreciable slope. To attain Grade 5 it is essential for instruments to be properly calibrated, and all measurements must be taken from a point within a 10 cm diameter sphere centred on the survey station. A Grade 6 survey requires the compass to be used at the limit of possible accuracy, i.e. accurate to ±0.5 °; clinometer readings must be to the same accuracy. Station position error must be less than ±2.5 cm, which will require the use of tripods at all stations or other fixed station markers ('roofhooks'). A Grade X survey must include on the drawing notes descriptions of the instruments and techniques used, together with an estimate of the probable accuracy of the survey compared with Grade 3, 5 or 6 surveys. Grades 2 and 4 are for use only when, at some stage of the survey, physical conditions have prevented the survey from attaining all the requirements for the next higher grade and it is not practical to re-survey. Caving organisations, etc., are encouraged to reproduce Table 1 and Table 2 in their own publications; permission is not required from BCRA to do so, but the tables must not be reprinted without these notes. Grade X is only potentially more accurate than Grade 6. It should never be forgotten that the theodolite/Total Station is a complex precision instrument that requires considerable training and regular practice if serious errors are not to be made through its use! In drawing up, the survey co-ordinates must be calculated and not hand-drawn with scale