Abstract: Methods and apparatus for precisely tracking a borehole using electronically controlled DC current switching in a source loop in the vicinity of the borehole together with measurements of the apparent Earth's magnetic field are disclosed. A field averaging method optimally gives the location of points in the borehole in the presence of intense magnetic noise. The expected uncertainty of these locations can be found from a method of generating an ensemble of simulated surveys using a noise characterization parameter found from apparent Earth field measurements. Time synchronization between apparent Earth field measurements and the instants of switching to positive current flow into the loop is found using a correlation method in conjunction with different time durations for positive and negative current flow in the source loop.