B.C.R.A. survey grades

Introduction to cave surveys

Cave surveys are prepared in a very simplistic manner. Firstly, a team surveys the cave using little more than a tape measure, compass, clinometer, and notebook. They follow the passages, picking appropriate points along the passage, trying to remain in the centre of the passage where possible. From each point, they take measurements of the compass bearing, inclination, and distance to the next point. Vertical drops are done with a plumbline instead of compass and clinometer, as both become fairly inaccurate at steep angles.

After correcting between magnetic and grid North, the data can then be plotted as a centreline, either laboriously by hand, or much faster, using a computer program, such as Survex. Any time a loop is completed, any legs (point-to-point steps) along that loop must have their directions, inclinations, and lengths altered to compensate for any errors when closing the loop (where the two ends of the loop do not match up exactly).

For better passage dimension drawings, several more measurements are taken on the way. Firstly, at every point (called a station) along the way, the distance to each wall, and the "normal" floor and roof are measured. In big passages, the height to the roof may either be estimated, or calculated using two angled spotlights, and trigonometry.

Secondly, one member of the team will draw diagrams of the passage wall shape and roof/floor shape, as well as cross sections of the passage (cross sections are not normally done at every station). When the centreline has been plotted, the drawings are then laid over it, corrected to allow for the actual directions. This may be done by hand, or better, using a computer program. These can then be exported either as a normal image, or better, as a vector graphic, to allow them to be drawn at any size, or adjusted later to allow for any new loops that are discovered.

Grades

Grades are made up in two ways. The first is a number from 1 to 6, or the letter X, which says the quality of the centreline data. The second is a letter 'a' to 'd', which says the quality of the passage dimension drawings. Typical grades would be 1a, 1b, 2a, 3a, 3c, 5c, 5d, or 6d. The following may not be official descriptions of the grades, but they are put into normal terms to make them more easy to understand.

Centreline number

1
Centreline approximated by sketch drawings done without any accurate measurements, usually done after leaving the cave.
2
Centreline produced from approximated measurements taken while in the cave, or calculated between two points surveyed to higher grade, or a centreline that failed to meet the requirements for grade 3, but was originally supposed to be. If possible, the passage should be resurveyed to bring the survey up to grade 3.
3
A rough magnetic centreline. Horizontal & vertical angles measured to ±2.5°; distances measured to ±50 cm; station position error less than 50cm after closing loops. Compass calibration taken from official local magnetic variation.
4
An attempt to survey at grade 5 that failed to meet the requirements. If possible, the passage should be resurveyed to bring the survey up to grade 5.
5
Loop closure errors must be less than 1%, all distance measurements made to 1 cm, all compass bearing and clinometer readings made to ±1° (though typically ±0.5° is used in practice). Compass and clinometer must be calibrated locally both immediately before and immediately after the surveying trip by comparing exact measurements as given by OS to those read on the compass by the person who is going to use it. Clinometer must be calibrated by checking slope between two points from both directions (in this case an alteration must be made if these are not opposite i.e. 43° and -43°). To reduce compass errors, compass bearings must be read in 'leap-frog' style, where the station-to-station legs would be, for example: 1-2, 3-2, 3-4, 5-4 and not as: 1-2, 2-3, 3-4, 4-5. Point coordinates must be calculated in order to draw the final survey, typically using a computer, in order to properly spread any loop closure errors. Hand drawing with rulers and protractors is not sufficient.
6
More accuracy than grade 5, or an attempt to survey at grade X that failed to meet the requirements. All compass bearing and clinometer readings made to ±0.5° or less if possible.
X
Measurements taken using theodolites, or similar quality using extremely accurate compass/clinometer on fixed stands that will not be accidentally moved during surveying.

Note that loop closure errors can be fairly high for loops of only a few legs (such as 5% to 10% for a loop of 3 legs). This is not usually considered bad enough to reduce the grade from 5 to 4. On longer loops, an error of that magnitude would usually require a resurvey, or reduction of survey grade.

Passage drawing

a
Only the centreline is drawn.
b
A centreline is drawn with passage enlargements shown with a thicker line or as separate walls that approximates the passage size.
c
Passage dimensions are measured at each survey station, passage must be drawn accurately showing all major definitions.
d
Passage dimensions are measured at each survey station, passage must be drawn accurately showing all major definitions. In addition, survey stations are carefully selected to ensure they coincide with every significant change in passage size, so they can all be accurately measured.