[paul]

A question about choosing reference point:

The image below is the study (urban) area (central Florida) which features with buildings. My question is about choosing reference point. I studied the online material which says integrated residual height and velocity are supposed to be peak at 0 value. However, there might be that majority of PS are on the roof/structure. In this case, does the rule to choose the reference point (velocity peak at 0 and residual height peak at 0) still works?

I also attached the histograms according to the reference point I have chosen.

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[periz]

first of all,
velocity and residual height are two different parameters belonging to two different physical characteristics, very different from each other and usually not correlated.
then, even if the majority of radar reflections would be higher than the ground level, still, reflections would be at different height values. Unless you are in an are where all constructions have the same shape and orientation and all of them are equipped with the same reflecting features at the same height. Pretty unlikely.
So, I’d say that in 99% of cases, if there is a clear peak in the height distribution, that peak is at ground level. in your case the peak is visible and it is at about -5m, showing that the reference point is on a roof, at about 5m.
best

[paul]

Thank you so much for the suggestion.

About the reference point choosing, i still have a question. When i read the SARPROZ tutorial ‘Demo: APS Estimation Multitemporal analysis with Sarproz’, it says that best reference point should reflects that both velocity and residual height distribution peak at 0. However, in the Site Processing – APS module, i can see 25 auto reference candidate points. I checked every single one of them, and not any of them can do both – 0 peak at velocity ad residual height.

From the manual, it is very clear that i should choose a point on the ground and stable, which corresponding to residual height and velocity peak at 0. My questions is that when there is no such a point, which rule is preferrable. If choose a stable point, it could shows a bias in residual height distribution, like what i showed in the previous images.

Thank you again!!
Paul

[periz]

By looking at your results again, I notice that the deviation of the estimated height is particularly high: is this Sentinel? If so, the estimated residual height is not very accurate, because the normal baselines are very small. Do not get crazy with the ref point selection.
The suggested ref points are chosen also considering their phase stability. So, they should be fairly equivalent.
If you want, at the end of your analysis you can still correct for height biases.
best

[paul]

Thank you for the information.

It is sentinel-1A data. One interesting thing is that in Site Processing – APS estimation, i generated the graph, and it gives me 25 auto point, and i chose a ‘best’ point with good distribution. Then, i moved to Site Processing -Sparse point processing. When i checked the distribution of the parameters, there is a obvious bias in the parameters distribution (esp. for residual height and velocity). I think the distribution calculated here is from all the sparse points, instead of all the pixels like APS does. Do you have any advice on this?

So from your suggestion, i guess that velocity is kind of more important than residual height when choosing the ref point.

Regards,
Paul

[fakhrulfadzil]

Hello, I am confused about the reference points selection. I am processing sentinel data. First, I used the auto reference point. The distribution of parameters is great (peak at zero). However, based on Google Earth, the selected reference point is located on top of the mountain range. As I understand, the reference needs to be on the ground. The other auto reference points are also on top of the mountain. Does the auto reference point is legit to be used? Should I find another reference point manually? Please advice. TY.

[periz]

Your comment does not match this discussion, which is around an urban area.
The distribution of the residual height should be at zero, which means, the reference point is close to the external DEM you used, and if you are in an urban area (and the DEM is in fact a DTM), this implies it’s on the ground (not on buildings).
But you are on the mountains and this does not apply. In fact, the distribution of your height values (not residual height!) should not be zero.
Why we should not take a reference point on a building? because buildings are affected by phenomena like thermal expansion, vibration and so on which would bias the whole analysis.
Can it be a problem to select a point on the top of a mountain? It can if the atmosphere is different from the rest of the area, or if the top is affected by other phenomena like snow or else, not affecting other areas.
In your case? we don’t know, you have to judge.
Keep that point and if you have problems try an other one.

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