System and method for using a magnetometer in a gyro-while-drilling survey tool

Various implementations directed to a system and method for using a magnetometer in a gyro-while-drilling (GWD) survey tool are provided. In one implementation, a method may include acquiring gyroscopic data using gyroscopic sensors of a GWD survey tool while the tool is disposed at a first position within a wellbore. The method may also include acquiring first magnetic data using a magnetometer of the GWD survey tool while the tool is disposed at the first position. The method may further include determining an offset value for the magnetometer based on the gyroscopic data and the first magnetic data. The method may additionally include acquiring second magnetic data using the magnetometer while the tool is disposed at second positions within the wellbore. The method may also include determining magnetic azimuth values for the tool disposed at the second positions based on the second magnetic data and the offset value.