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Objectiv

To obtain a BCRA Grade 5 survey – a magnetic survey. Horizontal and vertical angles accurate to ±1°; Distances accurate to ±10cm. Station position error less than ±10cm.

Equipment Required

• Chalk – used to mark survey station

• Tape – measure in meters e.g. 1.18m. Always include two digits after the decimal point e.g. 2.50m.

• Compass (Suunto) – hold horizontally and measure in degrees e.g. 316°. Always use bottom (larger) scale this is calibrated in degrees. Always include two digits before the first digit e.g. 008°.

•  Clinometer (Suunto) - hold vertically and measure in degrees e.g. 83°. Always use left hand scale this is calibrated in degrees. Always include one digit before the first digit e.g. 06°.

Reading the Compass and Clinometer

Look through the optical window keeping both eyes open. An optical illusion extends the cross hair. Place the extended cross hair on the station point to be measured, allow the instrument to stabilise and take the measurement.

Method

The forward method will be used for this technique which involves the following steps –

Set up instruments at first station and sight them towards the second station. When readings have been taken move forward to second station and repeat for third station etc.

On data sheet complete the following Cave Name, Date of Survey and GPS Reading (where possible)

Calibrate Clinometer – Near the cave entrance set up two stations a and b one higher than the other. Measure from a to b then measure from b to a. Subtract one from the other and this is the clinometer error.

 Calibrate Compass – Only applicable when accurate maps are available. Choose two points which can be accurately located on an OS map, one close to the cave and the other some distance away. Take a reading from this point, then move 20m away from this point and take another reading and then move 20m away from this point and take a final reading. If all bearings agree this confirms that there are no spurious magnetic fields present. On the OS map draw a line from the point close to the cave to the distant point and measure the angle with a protractor. Compare this measurement with the reading obtained and this gives the declination from grid north.

 Nominate one person as the surveyor and the other as the recorder.

1.      Taking the end of the tape the recorder moves to station 2 and holds the tape end at that point. The surveyor then shouts out the reading from the end of the tape e.g. distance between Station 1 and 2, the recorder shouts the reading back and if the figures are in agreement writes them on the data sheet in the column marked Tape. Shouting the readings back confirms that the recorder has not misheard the reading. The tape should be kept taught.

2.      The Surveyor takes a Clinometer reading from station 1 to station 2. The surveyor then shouts out the reading and the recorder shouts the reading back and if the figures are in agreement writes them on the data sheet in the column marked Clinometer.

3.      The Surveyor takes a Compass reading from station 1 to station 2. The surveyor then shouts out the reading and the recorder shouts the reading back and if the figures are in agreement writes them on the data sheet in the column marked Compass.

4.      Using the Tape the Surveyor measures Roof Height and Station Height at station 1. The surveyor then shouts out these reading and the recorder shouts the readings back and if the figures are in agreement writes them on the data sheet in the columns marked Roof Height and Station Height.

5.      Using the Tape the Surveyor measures Wall Distance Left and Right at station 1. The surveyor then shouts out these reading and the recorder shouts the readings back and if the figures are in agreement writes them on the data sheet in the columns marked Wall Distance Left and Right.

The surveyor then moves to Station 2 and the recorder moves to station 3 and steps 1 to 5 are repeated until the survey is completed.