Null point depth calibration

1. A method, comprising:

advancing a drill string carrying a beacon to a first underground location;

emitting an electromagnetic signal from the beacon;

performing a calibration procedure comprising:

at an above ground location, detecting a first null point and a second null point with a receiving antenna;

determining a distance between the first null point and the second null point and storing the distance in a memory;

receiving a signal indicative of a pitch of the beacon;

determining a signal strength at the selected one of the first null point and the second null point;

using the pitch and the signal strength to determine a calibration constant; and

storing the calibration constant in a memory.

2. The method of claim 1 further comprising:

using the distance between the null points to determine a depth of the beacon below a selected one of the first null point and the second null point and storing the depth in the memory;

from the distance and the depth, determining a radial distance between the beacon and the selected one of the first null point and the second null point; and

using the radial distance to determine the calibration constant.

3. The method of claim 2 further comprising:

thereafter, advancing the drill string; and

using the calibration constant determined with the beacon at the first underground location, determining a location of the beacon after the drill string has been advanced.

4. The method of claim 2 further comprising:

advancing the drill string such that the beacon is at a second underground location, in which the second underground location has differing soil conditions from the first underground location; and

performing the calibration procedure at the second underground location.

5. The method of claim 4 further comprising:

thereafter, advancing the drill string; and

using the calibration constant determined with the beacon at the second underground location, determining a location of the beacon after the drill string has been advanced.

6. The method of claim 4 further comprising:

thereafter, advancing the drill string; and

using the calibration constant determined with the beacon at the first underground location, determining a location of the beacon after the drill string has been advanced.

7. The method of claim 4 in which the calibration constant determined by the calibration procedure with the beacon at the first underground location is characterized as a first calibration constant and the calibration constant determined by the calibration procedure with the beacon at the second underground location is characterized as a second calibration constant; and

in which:

each of the first calibration constant and second calibration constant are stored in the memory; and

one of the first calibration constant and second calibration constant is selected for use when the beacon is at a third underground location.

8. The method of claim 2 in which the distance is determined by a method comprising the steps of:

locating the first null point;

marking a location of the first null point;

locating the second null point;

marking a location of the second null point; and

measuring the distance between the marked locations of the first and second null points.

9. The method of claim 8 in which the location of the first null point and the second null point are physically marked; and

manually measuring the distance between the physical marks.

10. The method of claim 8 in which the steps of marking a location of a first null point and marking a location of a second null point comprise digitally marking GPS coordinates.

11. The method of claim 10 in which a processor calculates the difference between the first null point and the second null point from the GPS coordinates.

12. The method of claim 1 in which the selected one of the first null point and the second null point is located above the drill string.

13. The method of claim 1 in which the selected one of the first null point and the second null point is located in front of a projected path of the drill string.

14. The method of claim 1 further comprising determining a vertical distance between the first null point and the second null point.

15. The method of claim 14 wherein the step of determining the vertical distance comprises using an altimeter.

16. The method of claim 14 wherein the step of determining the vertical distance comprises using a laser level.

17. A method comprising:

emitting an electromagnetic field from a beacon at a below ground location;

detecting the electromagnetic field at a tracking antenna at an above ground area;

determining a first null field location on a surface of the ground;

determining a second null field location on the surface of the ground;

at a selected one of the first null field location and the second null field location, detecting a pitch of the beacon and a strength of the electromagnetic field; and

from the first null field location, the second null field location, the pitch, and the strength of the electromagnetic field, determining a calibration constant with the beacon at the below ground location.

18. The method of claim 17 in which the first null field location and the second null field location are determined by the tracking antenna.

19. The method of claim 17 further comprising

determining a distance between the first null field location and the second null field location; and

determining a depth of the beacon relative to a selected one of the first null field location and the second null field location.

20. The method of claim 17 further comprising:

with a drill string connected to a horizontal directional drilling system, advancing the beacon to the below ground location;

after the step of determining the calibration constant at the below ground location, advancing the beacon to a new below ground location;

thereafter, determining a depth of the beacon at the new below ground location using the calibration constant.

21. The method of claim 17 , wherein the below ground location is characterized as a first below ground location and further comprising:

advancing the beacon to a second below ground location; and

changing the calibration constant based upon the first null location, the second null location, and the strength of the electromagnetic field with the beacon at the second below ground location.

22. The method of claim 17 , wherein the step of detecting the pitch of the beacon at a selected one of the first null field location and the second null field location comprises:

with a below-ground sensor, detecting a pitch of the beacon;

transmitting the pitch of the beacon as an electromagnetic signal; and

detecting the electromagnetic signal at a selected one of the first null field location and the second null field location.