Help to analyze the heightmap

adavidm

Mean is a simple average of the deviation from a theoretical perfectly flat, level bed at each probe point.

RMS is Root Mean Squared. As I understand it (I am not a statistician), you square each of the deviations, take the mean of those squared numbers and then square root that mean. The most important effect, in our case, is that it eliminates negative values which might otherwise give a misleading impression of how level the bed is.

Here is a better explanation:
http://www.analytictech.com/mb313/rootmean.htm

The square/root process also exaggerates higher deviations, so it's not quite the same as just stripping the negative sign off and taking the mean.

As to which is more important, I'm not sure there is a simple answer to that as there are so many other considerations ( where the deviations are on the bed, if the errors are randomly distributed or in a clear pattern, etc).

If you have min and max deviations that are a long way away from the RMS then I'd suggest looking for the causes of that. Your Max deviation is 0.176, which is on the same order as the layer heights commonly used in 3D printing. That might not cause issues, especially if you don't print in that area of the bed and/or use mesh compensation, but it's something to look into, IMHO, as the plastic flow will be impeded to some degree at that point.

Your bed looks like it's reasonably level but it's noticeably higher at the four corners. What is the bed made out of and how is it attached, out of interest? It's fairly large, around 300mm per side, so quite likely sagging in the middle under it's own weight to some degree. I don't have a heated bed on my printer but I believe this makes a big difference too.

hope this helps

adavidm

pro3d

I just think this all should be simplified somehow. Even though I have a masters degree in engineering design it is somewhat confusing understanding the rms results

I am not sure I got much smarter reading your reply but I will try analyzing it when I have some time for this stuff and thanks for your effort

Phaedrux

I'm not sure I really understand it either but I can tell from the picture that your bed is quite flat and that mesh grid compensation would have no trouble ensuring a perfect first layer with a quick taper off.

T3P3Tony

@pro3d fair enough. They way I think about it is the mean error is all the errors added up and then averaged. So if you have a positive mean error the bed is mostly a bit higher than the 0 point. if you have a negative mean error its mostly a bit lower. The RMS error is a way of thinking about the spread of the points measured away from the 0 point. The maximum deviations show the worst case of how far the highest and lowest point are from each other, which can either be a small issue if the highest and lowest points are far away from each other in XY (allowing for a gentle compensation) or a larger issue if they are right next to each other in XY (resulting in a sharp compensation).

In your case you have a pretty flat bed overall. might be slightly raised up on YMax (the top of the picture as you have it displayed now). I have seen (and printed on) worse beds!

pro3d

Yes I get the min/max deviation - I just think the result map should have some values that we all can relate to like those

It should btw read the min/max difference so I dont have to calculate this - It is something of interest

Maybe rms error is a common thing but for me and maybe more like me it is just a confusing factor and maybe it could read something else - What I dont know

(still confused about its meaning and value)

JoergS5

@pro3d
https://en.wikipedia.org/wiki/Root_mean_square
is directly related to the statistical sigma calculation, the calculation of the standard curve for probability calculations. It is also used to find correlations of variables in regression tests (the question: how are variables dependent?).

pro3d

@joergs5 The point is joergs5. I would like something useful there not something from another planet or wikipedia but something that average joe could understand.

I dont have time to read up on this

JoergS5

@pro3d ok, I tried to explain. No Problem

Phaedrux

@pro3d said in Help to analyze the heightmap:

@joergs5 The point is joergs5. I would like something useful there not something from another planet or wikipedia but something that average joe could understand.

I dont have time to read up on this

Well I guess a picture says a 1000 words. Green means good.

JoergS5

@phaedrux I thought he wanted to understand what RMS means.

For the heatbed, green is ok and it's fine. But if I would have this picture, I would want to know why the four corners are not even, because 0.2 mm is a lot. Maybe extension due to heat and fixing it too tight or the bed too thin. If it's due to fixing and you heat the bed up now, it will not stay green. 100 °C heat up makes 1 promille extension, 0.3 mm, this will bend the bed a bit, but uneven, depending on the forces.

pro3d

I dont actually want to understand what RMS error means. I just want easy to understand information on the probing results as I do analyze everything I deal with when designing printers

I am perfectly able to read the picture, points and min/max then the average (mean error)

It seams like no one is able to explain rms error in a way that average joe can understand so I wonder why it is there as it only confuses me (and possibly other users) so we use a lot of unnecessary time and energy to figure it out

I do miss the difference between min/max but of course I know how to calculate this but yes I am lazy as well as the next guy

I wonder if anyone uses the rms error for anything when analyzing these results and if so in what way?

JoergS5

Ok, now I understand what you mean. I thought you didn't understand rms. I agree that the usefulness is limited.

In the middle of your bed your values are very good, you will get good prints.

pro3d

@joergs5

I might come out as a noob that dont understand the map. This is not the case as I am very able to see what is low, high, average and flat..

But when there is some information on the results map I dont understand I would like to know why it is there and how/what I can use it for..

Since it is there I am now going to take my time to read up on the use of rms so that this thread can die out as it is no longer progressing and adavidm long above here answered most of this question if I just read thoroughly

/end

JoergS5

@pro3d in fact yesterday I was annoyed spending time to research rms and then getting an unfriendly answer. But it's ok now, we have misunderstood each other.

I wish you luck solving the other problems, and if BLTouch turns out to be unreliable, I have a plan B to use a digital dial gauge with data transmission. I will tell about how I connected it to Duet in this forum.

pro3d

@joergs5

Thanks -all good! The BL-Touch has been my friend for years - one that I trusted so I dont see why it should not work in the duet/rrf environment..??

I just want a plug and play sensor that works on glass and dont see any other option as I dont want a piezo solution

T3P3Tony

@pro3d alot of people have had success with the BL touch - and actually your bed map is pretty good. as has been previously mentioned it is a little high on one side (Y Max) but not so much that mesh bed levelling wont work.

pro3d

@t3p3tony

Thanks and I understand the tl;dr issue here for sure.

I am NOT concerned about the map posted but trying really badly... to figure out what I can do with the rms error in my analyzes - in any map good or bad

I am done with this now as I understand I need to figure it out by reading some theory, and that is ok

I am not going to use the Duet controller for this time any way so I will get back to this when and if I manage to cope with RRF again