BCRA Survey Grades
As revised for inclusion in the new edition of Cave Surveying, July 2002
Table 1: BCRA gradings for a cave line survey
Grade 1
Sketch of low accuracy where no measurements have been made
Grade 2 (use only if necessary, see note 7)
May be used, if necessary, to describe a sketch that is intermediate in accuracy between Grade 1 & 3
Grade 3
A rough magnetic survey. Horizontal & vertical angles measured to ?2.5?; distances measured to ?50 cm; station position error less than 50cm.
Grade 4 (use only if necessary, see note 7)
May be used, if necessary, to describe a survey that fails to attain all the requirements of 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 10cm.
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 and is intended only as an aide memoire; 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 10cm 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 rule and protractor to obtain Grade 5.
Table 2. BCRA gradings for recording cave passage detail
Class A
All passage details based on memory.
Class B
Passage details estimated and recorded in the cave.
Class C
Measurements of detail made at survey stations only.
Class D
Measurements of detail made at survey stations and wherever else needed to show significant changes in passage dimensions.
Notes
The accuracy of the detail should be similar to the accuracy of the line.
Normally only one of the following combinations of survey grades hould be used:
1A,
3B or 3C,
5C or 5D,
6D,
XA, XB, XC or XD.
That info is exactly what I was after.
