<p>I finally manage to setup the ABL on my Prusa.
I run G28 to home all axes (for z I use the same probe used for ABL). Then
I run G29 E ( I use E because otherwise for some reason I have not understood yet, Z does not lift during probings) probing 4 points on the corners of the bed.</p>

<p>finally I get the map of the bed:</p>

<pre><code>Bed Level Correction Matrix:
+0.999999 +0.000000 +0.001233
-0.000005 +0.999992 +0.003905
-0.001233 -0.003905 +0.999992
</code></pre>

<p>and after that, the print starts.
The first layer looks perfect however I have not seen Z moving a single step along the printing.</p>

<p>Any hint about what to check? Is the obtained map indicating that the bed is already too leveled to act on any compensation?</p>

<p><strong>UPDATE</strong>
I printed a 180 mm diamater cylinder and the Z axis is not compensating the 1mm difference from edge to edge of the bed.</p>

I believe that the matrix is shown transposed from how it should be, but that doesn't affect the answer. The compensated Z position is derived from the original
```
(X,Y,Z)
```
position by multiplying the corresponding vector with that matrix. This means that the new Z position would be
```
```
Z' = 0.999992Z - 0.001233X - 0.003905Y
```
```
If you have a 200 x 200 printbed, a diagonal move from one corner to the other would correspond to a 1mm change in Z-height. This should be noticeable, but if perhaps you're printing something quite small you might not notice it.
Though, if your first layer is perfect, I would see no reason to mess with it.