About the mapping service
The mapping service has various functionalities. The map can be displayed in both 2D and 3D, using either topographic maps or orthophotos as background. There is a functionality that zooms to your own position in the upper right menu.
Clicking a point brings up the time series of the point. Time series for additional points can be added by holding the shift key while clicking the points. The average time series for a number of points can be generated by use of a polygon tool in the upper right menu. It is possible to download the values of up to 50 000 points from the time series window that is brought up using the polygon tool. A description of other functionalities can be found by clicking the i-button.
Detailed information about a particular point or the average of more points can be seen by clicking “View Data” in the plot window. A description of the various parameters can be found here.
The Sentinel-1 satellites move in polar orbits around the Earth. Data from the satellites are divided into two – ascending and descending – depending on whether the satellite move towards north of south. The interpretation of the data depends on whether the data are ascending or descending, as described below.
The polar satellite tracks approach each other near the poles, resulting in a large overlap in Norway. Therefore, two datasets with full coverage of Norway can be made for both ascending and descending data. This means that there are four datasets to choose between in the upper left menu (ascending 1, ascending 2, descending 1 descending 2).
The fact that there are two full datasets of both ascending and descending provides a possibility to check the quality of a signal in the data. If a certain movement is seen in both ascending 1 and 2 or both descending 1 and 2, then it is a good indication that the movement is real and not just noise.
Some areas will have overlap of satellite tracks within each dataset (where there is overlap between two green or two red tracks in the figure above). Using the polygon tool in such area with overlap will bring up two time series in the plot window.
Ascending and descending data
As described above, the dataset is divided into two – ascending and descending – for north and south going satellites, respectively. The radar looks towards right in the flight direction, therefore, the azimuth angle is different for ascending and descending data. In each pass, the satellite scans a 250 km wide swath of the Earth’s surface. The incidence angle (the angle between the satellite’s look direction and vertical) varies from about 30° along the edge of the swath that is closest to the satellite to 45° along the distal edge.
Information about the satellite geometry of a point or a number of points chosen with the polygon tool is displayed in the upper part of the plot window with the time series.
Because the radar measures motion in line-of-sight from ground to satellite, the same horizontal motion on the ground will result in different results depending on the satellite geometry Eastward motion appears as negative value in ascending and positive value in descending data. In contrast with this, westward motion appears as positive values in ascending and negative values in descending data.
This point can be illustrated with an example from two unstable rock slopes on each side of Sørfjorden in Troms county. The unstable slope in the left side of the picture moves towards south-east and shows negative values (red points) in the ascending and positive values (blue points) in the descending data. The unstable slope in the right side of the picture moves towards north-west and shows the opposite, positive values in ascending and negative values in descending.
Downward motion will give negative values in both ascending and descending data. The InSAR method is not very sensitive to motion in north-south direction.
InSAR is a relative technique, and our data is not yet put into a reference system. As of now, all the data are processed in approximately 5 by 5 km tiles, each with a local reference. Before putting the tiles together for the national map, the tiles are adjusted so that the peak of the velocity histogram is 0 mm/yr. This means that the map so far only shows relative and local deformation. In future, we plan to integrate the InSAR data with data from the GNSS stations of the national mapping authority to get everything in a common, absolute reference frame.
Starting point for the time series
The time of the first Sentinel-1 observations varies from area to area. If the first observation had been chosen as the starting point for the time series, then it would have been difficult to compare time series for different points. Instead, an arbitrary date during the summer 2017 has been chosen as the reference point for all time series.
Known issues with the data set
InSAR Norway will be in development phase until 2020. Therefore, the published dataset is a preliminary version with certain problems, that are described closer here. The problems will be removed in future versions of the data set.
Due to noise in the data, some points will have wrong values. Single points with deviating values may be right, however, they should be interpreted with caution.
Alignment of neighbouring processed tiles
The national data set is put together from a number of individually processed tiles. When data from different times are used to process neighbouring tiles, this may cause a linear shift in the values where the tiles are stitched together. These block effects will be mitigated as more data are acquired and included in the processing.
Atmospheric noise in deep valleys
Deep valleys may show erroneous positive values (light blue colours). In the processing, atmospheric noise is filtered out, but the filtering may be incomplete due to layered stratosphere in deep valleys.
Snow cover effects
To avoid most problems with snow, the national data set is processed using only data from June to October. In high mountains, there may be snow cover during these months also, which causes problems for the processing. More sophisticated techniques to select which scenes to be processed for different areas are being developed.
Annual variations in wetlands
Wetlands typically show annual variations due o changes in water content. Because only data from half the year is used, such variations may cause problems for the processing. These effects are seen as areas with either large pointwise variation (a mixture of red and blue points) or areas with even positive values (blue points).
About the dataset
The data are acquired by the two satellites Sentinel-1A and Sentinel-1B from the EU Copernicus program for earth observation. The processing is done using software developed at Norut in Tromsø.
The dataset covers all mainland Norway with islands along the coast. Data are acquired during June 2014 to October 2018. In the processing of the national data set only data from months with little snow cover are used, that is, June to October, to minimise problems with the measurements due to snow. The dataset will be updated once every year, during the autumn, when data from the summer have been acquired. In a future version of the dataset, data from all year will be used in urban areas.
The ground resolution is approximately 5 x 20 meters (5 meters in the east-west and 20 meters in the north-south direction).