Sowmya Gopal
Radar Scientist
Intermap Technologies GmbH
[email protected]
Border surveillance is a major concern for security agencies around the world. Interferometric synthetic aperture radar (IFSAR) helps make data collection across border areas feasible.
Digital elevation models (DEMs) are widely used not only in the traditional mapping world but increasingly in support of new applications that are driven by consumer interests. In this new environment, the required levels of detail and accuracy of DEMs vary according to their applications, and are major considerations for the user, many of whom come from outside the GIS industry.
An additional consideration is that some applications, to be eff ective, transcend local political boundaries and require uniform datasets across regional, national and even continental scales.
Need for a Homogeneous and Accurate Countrywide DEM
Although, there are a number of terrain elevation datasets available today, many have been created over different periods of time with different data acquisition systems and hence, different resolutions. So, the resulting patchwork dataset is not practical for applications requiring DEMs that span across countries or even continents because of inconsistencies and artifacts like voids, seam lines across boundaries and borders, etc.
Figure 1 illustrates the importance of accurate and detailed elevation data for modelling applications like a flood induced by a storm surge. In this example, a Katrina-like event is applied to Miami. Note the differences between a simulation based on NEXTMap® data (left) and the prediction prepared by Federal Emergency Management Association (FEMA) based on the USGS NED elevation data (right). One area (indicated by red circles) of the FEMA prediction overestimates the inundation by 50 per cent, while it also misses an entire area that would be inundated through the channel at the bottom of the image (yellow circle).
A similar example is the integrated coastal zone management project in India, which has elevation as one of the three main requirements. Elevation information will be used along with the available tidal observation information over the past 50 and 100 years (where available) to generate a hazard line. Having an accurate and homogeneous DEM is extremely important for such a project.
Geospatial Mapping Technologies
Geospatial mapping technologies have come a long way from ground surveying to state-of-the-art airborne and spaceborne systems. Figure 2 illustrates the different technologies available today.
Photogrammetry is a passive system, dependent on sunlight and good weather. When considering active systems like light detection and ranging (LIDAR) and interferometric synthetic aperture radar (IFSAR), the former is useful for mapping small regions with high accuracies, while IFSAR is suitable for extremely large regions. IFSAR is scalable and particularly cost-effective while still providing data with an accuracy that is significantly higher than that from satellite SAR systems. IFSAR is also free of cloud cover because of its ability to see through clouds, haze, fog, etc.
When compared to spaceborne alternatives, singlepass airborne systems using Intermap Technologies®’ proprietary X-Band IFSAR technology have more flexibility as well as weather-independent system deployment, higher spatial resolution, and a lesser degree of influence from the atmosphere and temporal target changes.
IFSAR: A Cost-effective Solution for Mapping Countries and Continents
With IFSAR, height information for a scene is obtained using two antennae in single-pass mode. SAR signals, however, interact with the terrain and thus measure the distance to first-surface features. The DEM created from an IFSAR system is essentially a first-surface DEM, also known as digital surface model (DSM). The digital terrain model (DTM), representative of the earth’s terrain, is obtained by editing the DSM using a semi-automated process during which all man-made and vegetation features are removed. This process is extremely challenging and labour-intensive, and requires classification of terrain (based on land cover), a set of editing rules to handle hydrological, road features, etc., and a quality control process to ensure that the output DTM is accurate and consistent across borders and boundaries.
Intermap has mapped more than 10 million sq. km in the United States and Western Europe at 1m vertical accuracy with its NEXTMap® programme and over 2 million sq. km in Asia and S.E.Asia.
With the aid of high resolution IFSAR systems, Intermap now also has the ability to generate DEMs at 50cm vertical accuracy.
Data acquisition is made cost-effective by means of ultra long lines (ULL). The longer line lengths increase the ratio between imaging versus non-imaging time by reducing the number of aircraft turns at the end of the lines. Within these ULL blocks, parallel flight lines are planned according to the terrain. Sufficient overlap is incorporated into the plan to ensure there are no gaps in coverage between imaged swaths. Figure 3 shows two sample ULL blocks: One in Europe that covers more than half of France and another in the US that spans over four states from North Dakota down to Kansas.
The acquired data is then processed in a semi-automated production factory with a number of quality control steps to ensure that the data meets the accuracy requirements. The throughput of the system is 400,000 sq. km per month. The whole workflow is ISO certified.
Military forces have a strong need for accurate base maps and high-resolution images in sensitive areas like Afghanistan, and IFSAR mapping has been used for these areas. Figure 4 shows examples of orthorectified radar images (ORI) of two different areas from Intermap’s mapping campaign in Afghanistan. Afghanistan has diverse topography ranging from rugged and mountainous terrain to plains. The ORI shown in figure 4 has a pixel size of 1.25m and a horizontal accuracy of 2m RMSE.
Radargrammetry
Airborne operations are not always possible in certain restricted regions, and an attractive alternative method for the production of DEMs is radargrammetry, in which elevation information is obtained by measuring the parallax between two stereo radar image pairs. With the availability of relatively high-resolution radar imagery from the TerraSAR-X and other modern satellites, the creation of DEMs meeting DTED (digital terrain elevation data) L2 accuracy is made possible. This is accomplished by using Intermap’s proprietary TopoSAR radargrammetry software which has also been tested on RADARSAT-2 as well as COSMO-SkyMed imagery.
Defence and Internal Security applications
The IFSAR DEM can be used as an accurate base map in orthorectification of imagery from multiple sources. Orthorectification is the process of accurately registering imagery to ground coordinates and geometrically correcting it to remove distortions that happen during image capture.
The accuracy of orthorectification heavily influences the usability of imagery for surveillance as well as change detection which helps detect encroachment in sensitive areas like borders or in areas with oil/gas pipelines.
Figure 5 illustrates the errors (yellow arrows) due to orthorectification of an air photo by USGS NED (left). The accuracy of NEXTMap DEM (right) does not give room to such errors.
Helicopter flight operations are particularly sensitive to terrain slope for landing possibilities; accurate slope information from DEMs can be valuable for determining those areas, and, additionally for low-level flight mission planning, particularly for special operations, in poor visibility and unfamiliar terrain. An example scenario is given in figure 6 (oveleaf ).
The difference between a paper map of the slope analysis region (top right) and an accurate landing space information (bottom right) can have a huge impact on a mission critical operation. The figure also illustrates how a DEM of lesser accuracy (bottom left), DTED L1, does not provide comprehensive information as compared to DTED L4.
Another important advantage of having accurate terrain information for mission planning is the ability to do a visibility analysis (line-of-sight or viewshed) for a particular area before going in. This will help speed up the decision-making process and improve the efficiency of mission planning. Figure 7 shows an example of viewshed (areas visible from the point of observation shown in red) overlaid on an aerial photograph for easy interpretation.
Intelligence agencies are increasingly interested in the ability of airborne sensors to map what is hidden beneath vegetation. This ability to penetrate vegetation to see the ground beneath the canopy as well as to measure the vegetation parameters is possible using Polarimetric SAR interferometry (PolInSAR). This has gained much interest in the scientific community over recent years, mainly due to its potential to derive certain vegetation parameters. From the intelligence standpoint, it has proved to be capable of extracting paths and tracks, and certain kinds of objects or structures, otherwise hidden by vegetation. In combination with long wavelengths (L- or P-band) and full polarimetric operation, the extraction of parameters such as vegetation height and ground topography has been investigated. Repeat pass operations are always hindered by temporal decorrelation. This motivated Intermap to build an experimental single-pass L-Band PolInSAR system with an azimuth resolution of 1m and a slantrange resolution of 1.1m.
Cost-effective and accurate surveillance for border security is a concern for many nations, and the nature of the sidelooking geometry of IFSAR makes data collection across sensitive country borders feasible.
