Normally the only data correction applied to ground magnetic data is a diurnal (a.k.a base station) correction. This type of correction simply subtracts the static base station reading from the moving rover magnetometer reading of the same time.
Cs vapour sensors also require a geophysics heading correction to account for the changes in magnetic field readings that occur when the sensor is oriented in different directions – even at the same location! In airborne surveys where Cs sensors are common, the heading error is usually removed by a heading correction, or at the very least with the compensation system. However compensation systems are complex and expensive and thus are not used for ground magnetic surveys. Given the heading error can be as large as +/- 0.5nT, data acquired with Cs sensors on the ground can show considerable heading errors. These are most commonly seen as along line striping in the first vertical derivative (1VD) data. If a set of orthogonal survey lines are acquired, these “tie lines” can be used to correct for the heading errors, albeit in a less direct and crude manner.
In comparison the K-vapour sensors as used by Modern Mag are the most sensitive commercially available and have a negligible heading error of 0.0025nT. We do not need to apply a heading correction.
So if your contractor is using a Cs sensor for your survey, be sure to check the final data for heading errors using a first vertical derivative (1VD) and at least request a heading correction on the data provided to you.
Alternatively, you could just entrust the collection of your precious data to a contractor with a k – vapour sensor. That way you get the best quality data without the need for unnecessary corrections that modify your data.