# Positive and negative signes in displacement map

Dear all,
When we convert phase (fringes) to displacement by SNAP. We have positive and negative amounts. What is the meaning with positive and negative amounts?
Note: I know positive and negative amounts in fringes are going toward or away from the satellite but I do not know what is the meaning with positive and negative amounts in displacement map?
Cheers,
Marjan

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it means movement of the ground along the line of sight between your first and second image.

Yes I know but I do not know what is difference between positive and negative signs? Satellite is descending in our case. I got displacement map but it is from -5cm to 27cm and I do not know what is the meaning with negative and positive signs? for example what is difference between -5cm and +5cm in displacement map?

This is amount of displacement from converting by SNAP (Phase to displacement).

0 means surfaces were equal in both images.
Negative means that this location is farer away in the second image, thus indicating subsidence.
Positive means that this location is now closer to the sensor, indicating uplift.

EDIT: This always depends on what is your master and slave images. The values always are relative to the master image.

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Thanks. Do you mean there is not any difference between ascending and descending in displacement map?
I mean Negative means that this location is farer away in the second image and Positive means that this location is closer to the sensor in both (ascending and descending in displacement map)?
NOTE: I think this is true for ONLY âdisplacement mapâ because as I saw in below link, there is different interpretation for ascending and descending in interferogram image.
Am I right?
http://vldb.gsi.go.jp/sokuchi/sar/qanda/qanda-e.html#q2-9

It depends of the scale of the deformation and on the angle of the deformation towards the sensor. If it is just subsidence, it makes no difference. But for uplift, it does make a difference if it just an uplift or - as suggested in the given example - the uplift is additionally moving a surface closer to the sensor along the LOS.

Dear friend,
Yes but I mean in DISPLACEMENT MAP (NOT PHASES AND FRINGES), negative means away from satellite in LOS and positive means close to satellite in LOS. Is it OK?

I havenât tried it but I guess so, yes.
Does it make a difference if you use ascending or descending data?

I do not have ascending data in this case. Actually I found that link that explain about positive and negative signs in âPHASE and INTERFEROGRAM IMAGEâ but I do not know meaning by negative and positive signs in âDISPLACEMENT MAPâ in SNAP.

with âdisplacement mapâ you mean the raster that is created by âPhase to displacementâ? Or is this some special mode in SNAP which I didnât know so far?

As it no longer refers to the phase values it can be assumed that these values are uplift (positive) and subsidence (negative). Or, in case of lateral movement: Away (negative) or towards (positive) the satellite:

https://senbox.atlassian.net/browse/SITBX-390

Yes I mean âPhase to displacementâ. OK thanks I got it now.

Dear friend,

The sign of the displacement should be the inverse of what is currently done.
Decrease in phase should be positive
Increase in phase should be negative
But still I am not sure about meaning positive and negative signs in displacement map.

Maybe one of the developers can clarify this. According to the mlodule info @lveci has written it.

Yes. Thanks. I should ask him because for my topic this one is important.

OoohâŚI did not know. That is good:slight_smile:.

Hi @falahfakhri

Thanks @falahfakhriâŚSee you tomorrow