First time post to these forums! I have been lurking/posting in the reprap.org forums but this seems a much better place for marlin questions. I will be digging through the enhanced G29 thread as soon as I have posted this.

I have been having bed leveling issue since updating to Marlin 1.0.2 (and they were even worse when I tried the 1.1 release candidate). The height of the first print step varies a great deal. I'll have the z probe offset tuned in, and then the next print it will be printing too far above the glass or grinding into the plate.

My start code has in it:

G28
G29

and I'm printing with a SmartRap Core (corexy) on plate glass with aluminum foil tape underneath. I'm using an inductive sensor that triggers almost 2cm above the glass/foil bed.

I ran a series of G29s (after a G28) to check on consistency and found that it varies quite a bit, certainly enough to explain the differences I'm seeing:

Bed x: 40.00 y: 40.00 z: 3.09
Bed x: 115.00 y: 40.00 z: 3.18
Bed x: 190.00 y: 40.00 z: 3.53
Bed x: 190.00 y: 115.00 z: 3.59
Bed x: 115.00 y: 115.00 z: 3.98
Bed x: 40.00 y: 115.00 z: 3.98
Bed x: 40.00 y: 190.00 z: 4.03
Bed x: 115.00 y: 190.00 z: 3.91
Bed x: 190.00 y: 190.00 z: 4.04
Eqn coefficients: a: 0.00 b: 0.00 d: 3.13
planeNormal x: -0.00 y: -0.00 z: 1.00
echo:endstops hit: Z:4.05

Bed x: 40.00 y: 40.00 z: 2.83
Bed x: 115.00 y: 40.00 z: 2.97
Bed x: 190.00 y: 40.00 z: 3.32
Bed x: 190.00 y: 115.00 z: 3.38
Bed x: 115.00 y: 115.00 z: 3.78
Bed x: 40.00 y: 115.00 z: 3.77
Bed x: 40.00 y: 190.00 z: 3.81
Bed x: 115.00 y: 190.00 z: 3.72
Bed x: 190.00 y: 190.00 z: 3.82
Eqn coefficients: a: 0.00 b: 0.00 d: 2.89
planeNormal x: -0.00 y: -0.00 z: 1.00
echo:endstops hit: Z:3.83

Bed x: 40.00 y: 40.00 z: 3.15
Bed x: 115.00 y: 40.00 z: 3.24
Bed x: 190.00 y: 40.00 z: 3.60
Bed x: 190.00 y: 115.00 z: 3.65
Bed x: 115.00 y: 115.00 z: 4.03
Bed x: 40.00 y: 115.00 z: 4.05
Bed x: 40.00 y: 190.00 z: 4.10
Bed x: 115.00 y: 190.00 z: 3.98
Bed x: 190.00 y: 190.00 z: 4.09
Eqn coefficients: a: 0.00 b: 0.00 d: 3.20
planeNormal x: -0.00 y: -0.00 z: 1.00
echo:endstops hit: Z:4.10

This suggests quite a bit of slop in the sensor reading. The probe itself is held quite rigidly . So I ran a series of M48 Z-probe repeatability tests at various points around the bed and they were very consistent. Measurements made in the same spot were very consistent. For example:

M48 Z-Probe Repeatability test. Version 2.00
Full support at: http://3dprintboard.com/forum.php
Positioning probe for the test.
1 of 10 z: 4.045954 mean: 4.045954 sigma: 0.000000
2 of 10 z: 4.039710 mean: 4.042832 sigma: 0.003122
3 of 10 z: 4.033466 mean: 4.039710 sigma: 0.005098
4 of 10 z: 4.033466 mean: 4.038149 sigma: 0.005177
5 of 10 z: 4.033466 mean: 4.037212 sigma: 0.004995
6 of 10 z: 4.039710 mean: 4.037629 sigma: 0.004654
7 of 10 z: 4.039710 mean: 4.037926 sigma: 0.004370
8 of 10 z: 4.033466 mean: 4.037369 sigma: 0.004345
9 of 10 z: 4.033466 mean: 4.036935 sigma: 0.004277
10 of 10 z: 4.039710 mean: 4.037213 sigma: 0.004142
Mean: 4.037213
Standard Deviation: 0.004142
echo:endstops hit: Z:4.05
M48 Z-Probe Repeatability test. Version 2.00
Full support at: http://3dprintboard.com/forum.php
Positioning probe for the test.
1 of 10 z: 4.027223 mean: 4.027223 sigma: 0.000000
2 of 10 z: 4.033466 mean: 4.030344 sigma: 0.003122
3 of 10 z: 4.033466 mean: 4.031385 sigma: 0.002943
4 of 10 z: 4.033466 mean: 4.031905 sigma: 0.002704
5 of 10 z: 4.027223 mean: 4.030969 sigma: 0.003059
6 of 10 z: 4.033466 mean: 4.031385 sigma: 0.002943
7 of 10 z: 4.027223 mean: 4.030790 sigma: 0.003090
8 of 10 z: 4.027223 mean: 4.030344 sigma: 0.003122
9 of 10 z: 4.033466 mean: 4.030691 sigma: 0.003103
10 of 10 z: 4.033466 mean: 4.030969 sigma: 0.003059
Mean: 4.030969
Standard Deviation: 0.003059
echo:endstops hit: Z:4.04

The repeatability tests are showing much less variance than I'm seeing with G29. Any ideas why?