Veel gestelde vragen over InSAR

Wat is InSAR?

InSAR staat voor Interferometric Synthetic Aperture Radar en is een bewezen techniek voor het meten van bewegingen op de grond. Satellieten nemen beelden op van de aarde. Deze beelden worden gecombineerd om bewegingen van het grondoppervlak te berekenen.

Hoe nauwkeurig kun je deformatie meten?

De precisie van elke individuele meting is in de orde van millimeter. Dit hangt af van het type satelliet dat we gebruiken. Door meerdere metingen over de tijd te combineren, kunnen we de vervormingssnelheid met een precisie van ongeveer 1 mm / jaar afleiden.

De metingen zijn afkomstig van harde voorwerpen zoals rotsen, gebouwen, bruggen of andere infrastructuur. Hoe betrouwbaar zijn deze punten?

Zeer betrouwbaar. Wij bieden alleen deformatiemetingen uit reflecties die niet veranderen tijdens het waargenomen tijdsinterval. Grote veranderingen in het oppervlak – zoals onderhoud – leidt tot een verandering van het radarsignaal en is er geen betrouwbare schatting mogelijk. De reflectiepunten worden niet handmatig geselecteerd door harde objecten te identificeren maar door het criterium van een constante reflectie.

Wat is de geldigheid van een individuele meting?

InSAR is gebaseerd op radar verstrooiing om de veranderingen te berekenen. De basisprincipes en de technologie zijn in honderden wetenschappelijke papers en in duizenden gevallen gevalideerd. Toch benadrukken we niet te veel waarde te hechten aan individuele meetpunten.

Kunnen de locaties van de meetpunten worden beïnvloed?

Er worden geen sensoren geplaatst dus de exacte locaties van de metingen kunnen we niet beïnvloeden.

Wat is de typische omvang van een interessegebied?

Wij bieden metingen aan van gebouwen, bruggen, mijnen, straten, complete steden en gebieden tot 10.000 km2.

Wat is de meetpuntdichtheid voor mijn interessegebied?

We kunnen tot meer dan 100.000 meetpunten per km2 genereren. Het aantal meetpunten hangt af van veel factoren, waaronder de hoeveelheid harde objecten zoals gebouwen en stoepen.

Hoe vaak kan mijn gebied worden gemeten?

We kunnen om de dag meten, afhankelijk van uw wensen.

Hoe verschilt InSAR van conventionele meetmethoden?

InSAR is gebaseerd op radar signaal wat vanuit de satelliet wordt verstuurd en reflecteert op infrastructuur. De metingen zijn niet gerelateerd aan vooraf bepaalde fysieke punten op de grond. In tegenstelling tot de traditionele meetmethoden die allemaal gebaseerd zijn op fysieke markers zoals meetbouten in de infrastructuur.

Hoe groot is de vervorming die u kunt meten?

Het hangt af van welke satelliet wordt gebruikt maar het is mogelijk om met een grote precisie veranderingen van meerdere meters te meten.

Jullie beweren terug te kunnen kijken in de tijd. Geldt dat voor alle gebieden en hoe ver terug in de tijd?

Radar satellieten begonnen in 1992 met het opnemen van beelden. Sindsdien is het aantal satellieten en de resolutie van de beelden aanzienlijk toegenomen. We hebben momenteel een archief met historische beelden die 90% van Nederland bedekken. Deze gegevens gebruiken we om historische bewegingen te berekenen en te visualiseren.

Can the measurements be made ‘absolute’ to link them to a National Height Reference System (NHRS)?

This is possible through the use of radar reflectors, where the precise location of the radar reflection point is known. Two variables must be distinguished: (i) the instantaneous absolute height of the point, for example in the NHRS, and (ii) the instantaneous absolute displacement rate (ie, absolute altitude change of the point relative to the NHRS).

The value of a link to the NHRS should be critically considered. Just like GPS measurements are made in a reference system (‘datum’) compared to leveling measurements, the radar measurements are referenced – only in a different system. In most cases our measurements are about changes of the points over time.

In what direction does the satellite measure displacement?

We measure motion in the line of sight from the satellite; in an East-West vertical plane, looking down in a 45-75 degree angle. If we do an analysis from different satellite viewing directions, multiple projections are measured. Based on this the absolute direction of the displacement vector can be determined, and we can distinguish displacement from e.g. shear along a slope or horizontal displacement.

Can you measure horizontal motions?

Yes, we can measure horizontal motions. SAR measures the projection of ground motion onto the Line of Sight (LOS) direction. The horizontal motions also contributes to the projection and can thus be measured by InSAR. However, if we want to calculate the contribution of horizontal motions based on the LOS measurement from either one satellite or the decomposition of two satellites, there has to be certain assumptions. In general, it is true to say that the SAR is less sensitive to north-south movements; and if we neglect the contribution of ground motion from north-south direction, we can estimated the vertical & east-west motion with two satellites given the above assumption.

What is the horizontal resolution of InSAR?

This varies per satellite and depends on the look angle towards the surface. ERS and Envisat have a ground resolution of approximately 5 to 23 meters, and a sampling of approximately 4 to 20 meters. TerraSAR-X has a variable resolution, usually we use 3 by 3 meters. Radar images resolution should be interpreted differently compared to optical images. Within a radar resolution cell of 4 x 20 meters, an object with dimensions of less than 1 meter can dominate the reflection. For example, a lamp post covers a much smaller area than 4 by 20 meters, but can provide a useful reflection.

Do you use ESA’s Sentinel-1 satellite?

Sentinel-1 is a valuable source for SkyGeo but the satellite cannot be controlled for a specific monitoring task.

How are measurement points distributed in time?

We rely on the revisit frequency of the satellite.

Do you use ESA’s Sentinel-1 satellite?

Sentinel-1 is a valuable source for SkyGeo but the satellite cannot be controlled for a specific monitoring task.

What is the absolute geolocalization?

The absolute location has a standard deviation on the order of 1-2 meter, our algorithm is set to make sure we are measuring the same object every time and measures the motion of that object with millimeter precision over time.

Platform FAQ

What is SkyGeo Maps?

SkyGeo Maps is a secure, online platform with your personalized maps and displacement data. SkyGeo Maps has a variety of tools to help you detect patterns in dynamic displacement. View the time series to understand the displacement dynamics over different distance scales: from 10m2 all the way up to 10.000 of km2.  Contact us for a quick demo of SkyGeo Maps.

What are these colored dots in the displacement map?

The colored points are radar reflections that are consistent over time. The point density depends on the characteristics of the terrain. A very ‘smooth’ terrain (flat relative to the radar wavelength) provides little or no reflection to the satellite. Another area may be “rough” and reflects in all directions. It is important that the characteristics of an object do not change significantly between consecutive satellite recordings. We call this type of scatterers coherent. If a reflection remains consistent over time, we call it a persistent scatterer.

What does the color of the measurement points mean?

The color quantifies the extent of motion of the measurement point over a given interval – it can be either the total displacement or the slope of a fit over time, typically in mm/year.

What is the significance of the size of the dots?

The size of the dot is uniform for the map. The size is only adjusted for the sake of visualizing and pattern detection.

The measurements are ‘relative’. What does a strain rate of 0 mm / year mean?

All measurements must indeed be interpreted relative to one another, as from space is not possible to determine if any point can be considered stable. The interpretation is always “how does point A move relative to point B.”

However, the results need to be shown numerically in the displacement map. For this reason, a reference point is chosen. At this reference point, the strain rate is by definition 0 mm/year. The movement of all other points within the analysis is relative to the reference point.

How is the reference point chosen, and what is the physical interpretation of this point?

The exact location of the reference point is not relevant for displacement rate.

For the physical interpretation, however, this choice is relevant. All measurements are calculated on the assumption that the reference point is stable. Without prior knowledge of the physical characteristics of  the area of interest, it is impossible to select an absolute stable reference point. This is a very important factor when interpreting the data, as it easily could lead to misunderstanding.

Note that this assumption is identical to leveling, where stable reference points are assumed. Although in case of leveling usually more physical information about the reference point is available, all estimates are based on this assumption.

Why do you separate high and low points? What is the threshold for high/low division of points?

You can filter measurements that originate at street level. We can adjust the cutoff height for the included vs. excluded measurements in light of the standard deviation of the X,Y,Z.

Can I use my own aerial photos as a background?

Yes, if you supply your own aerial photos, preferably in .tiff format, we can put them in the viewer and overlay these photos with the displacement data.

Can I supply my own map-layers, such as building outlines, location of pipes, etc.?

Yes you can add your map layers into SkyGeo Maps and then query the data based on their position relative to the objects in this layer.

How precise is the location and height of the measurements?

The standard deviation of location and height of the measurement points is 1 meter for high resolution date and 2.5 meter for standard resolution data. So InSAR is particularly useful for monitoring dynamic position changes.

Why is there a measurement point on this building/street/tree here, but not on a similar building/street/tree next to it?

Which objects we measure depends on:

  • The way the objects reflect the radar signal to the satellite. The reflections from buildings and other solid infrastructure are generally good.
  • The consistency of the reflections over time. We measure with mm-precision, so to be sure we are measuring the same object every time the reflections have to be consistent in time. Vegetated surfaces are therefore hard to measure; a tree moving because of the wind is not constant enough.
  • The satellite we use.

Can I use SkyGeo data for my own GIS system?

Yes, you can download the data for your own system.

Ontvang gratis een InSAR voorbeeld van uw asset.