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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 ±2.5º; 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
±1º; 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 ±0.5º; 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.
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