Distance based position sensing

1. An apparatus comprising:

first and second targets each comprising a retro reflector arrays;

a laser distance meter (LDM) configured to emit a laser beam and to determine first and second distances between the LDM and the first and second targets, respectively, such that the targets reflect the laser beam back to the LDM, wherein the first and second targets have known positions in three dimensions with respect to a reference plane relative to which the LDM is located;

a trilateration unit configured to compute a two dimensional position of the LDM relative to the targets based, at least in part, on the first and second distances; and

where the first and second targets are configured to intermittently shield the retro reflector arrays from the laser beam.

2. The apparatus of claim 1 where the first and second targets comprise barcodes that encode identifying information for the targets.

3. The apparatus of claim 1 where the LDM determines a third distance between the LDM and a third target having a known position with respect to the reference plane and further where the trilateration unit computes a three dimensional position of the LDM relative to the targets based, at least in part, on the first, second, and third distances.

4. The apparatus of claim 1 where the LDM is configured to emit a wide divergent laser beam.

5. The apparatus of claim 1 where the LDM is configured to emit a thin scanning laser beam.

6. The apparatus of claim 1 further comprising a laser plane emitter configured to emit the reference plane.

7. A method comprising:

emitting, with a laser plane emitter, a reference plane;

positioning an LDM proximate to a feature of interest and at a known position with respect to the reference plane;

determining first and second distances between the LDM and first and second targets, respectively, the first and second targets having known positions in three dimensions with respect to the reference plane; and

computing a two dimensional position of the LDM relative to the first and second targets based, at least in part, on the first and second distances.

8. The method of claim 7 comprising computing the position by:

computing x axis position component and a y axis position component using the following formulas:

x i =( d 1 2 +L 2 −d 0 2 )2 L

y i =( d 1 2 −( H−h ) 2 −x i 2 ) 1/2

where:

d 0 =the first distance;

d 1 =the second distance;

L=distance between the targets;

h=height of the LDM relative to the reference plane; and

H=the height of the first and second target relative to the reference plane.

9. The method of claim 7 comprising emitting a divergent beam with the LDM and determining the first and second distances based on a property of light reflected back from the first and second targets to the LDM.

10. The method of claim 9 comprising filtering light reflected back to the LDM with a polarizing filter to determine if the light was reflected from a target.

11. The method of claim 9 comprising emitting the divergent beam in a scanning pattern until light is reflected back from a target.

12. The method of claim 7 comprising mounting the first and second targets and determining their position with respect to one another and the reference plane.

13. The method of claim 7 comprising determining a third distance between the LDM and the reference plane.

14. The method of claim 7 comprising determining a third distance between the LDM and a third target having a known position with respect to the reference plane and computing a three dimensional position of the LDM relative to the targets based, at least in part, on the first, second, and third distances.

15. A position measuring system comprising:

first and second reflective targets configured to reflect light from a laser beam; and

a position calculation unit configured to be positioned proximate a feature of interest, the position calculation unit comprising:

a sight through which the feature of interest is visible when the position calculation unit is aligned with the feature of interest;

an LDM configured to emit a laser beam and to determine first and second distances between the first and second targets based on a property of light reflected back from the first and second targets, wherein the first and second targets have known positions in three dimensions with respect to the reference plane;

a trilateration unit configured to compute a two dimensional position of the LDM relative to the targets based, at least in part, on the first and second distances; and

a graphical user interface unit configured to display the two dimensional position of the LDM as the survey unit is moved on the reference plane.

16. The position measuring system of claim 15 , wherein the position calculation unit further comprises laser transmitter configured to emit the reference plane and an associated laser detector configured to determine a height of the LDM with respect to the reference plane.

17. An apparatus comprising:

first and second reflective targets;

a laser distance meter (LDM) configured to emit a laser beam and to determine first and second distances between the LDM and the first and second reflective targets, respectively, such that the targets reflect the laser beam back to the LDM, wherein the first and second targets have known positions in three dimensions with respect to a reference plane relative to which the LDM is located;

a trilateration unit configured to compute a two dimensional position of the LDM relative to the targets based, at least in part, on the first and second distances; and

where the LDM comprises a circular polarizer that acts on the laser beam to produce circular polarized light having a selected hand orientation and where the first and second targets comprise a quarter wave retarder film that acts on a reflected laser beam emitted by the targets to produce circular polarized light having the selected hand orientation.

18. The apparatus of claim 17 where the LDM determines a third distance between the LDM and a third target having a known position with respect to the reference plane and further where the trilateration unit computes a three dimensional position of the LDM relative to the targets based, at least in part, on the first, second, and third distances.

19. The apparatus of claim 17 where the LDM is configured to emit a wide divergent laser beam.

20. The apparatus of claim 17 where the LDM is configured to emit a thin scanning laser beam.

21. The apparatus of claim 17 further comprising a laser plane emitter configured to emit the reference plane.