Article: Satellite data in climate management applications
The original Danish version of this article will be published in the August 2021 issue of “Teknik og Miljø”.
Satellite data is being used in many contexts today and by a wide range of Danish authorities. This is evidenced very clearly by the recently published publication “Danish Uses of Copernicus – 50 ways that satellite Earth observations are used”.
Geopartner Inspections is also a consumer of the new satellite data and has made a business out of translating the many pieces of complex satellite data into knowledge about and mapping land surface motion. Knowledge about land uplift and subsidence is important information when working with climate adaptation and, together with data about water levels, groundwater, flood statistics and precipitation, forms the basis for our ability to design the most optimal and effective measures to counter the effects of a changing climate.
Precise elevations are crucial
An accurate DVR90 elevation measurement is a prerequisite for all building and construction work and is a parameter of particular importance in connection with climate proofing to protect people, animals and property in flood-prone areas.
The elevation is determined with reference to the Danish DVR90 vertical reference, which is a snapshot of the mean sea level for 1990. The reference or elevation system is based on nationwide precision levelling, combined with 10 water level gauges operated by DMI. The water level gauges are located in Danish ports, with a geographical distribution that is suitably representative to calculate mean sea levels for Danish waters.
DVR90 is rapidly approaching retirement age because ground motion (post-glacial land uplift and local subsidence) combined with rising mean sea levels since 1990 have meant that land and water have become between 3 and 10 cm closer to each other. The largest deviations are in Southern Jutland, the smallest deviations being present in North Jutland. The problem can be addressed by establishing a new national elevation reference, but no concrete plans have yet been announced in this regard. Neither would such a reference solve the dynamic dimension of the problem, which will, in future, be exacerbated by the fact that water levels are expected to rise at an accelerating pace and that land continues to be a state of persistent motion.
The Danish authority SDFE is the responsible for Denmark’s reference network and maintains the overall benchmarking network, which is comprised of a coarse-mesh main network.
The remaining benchmarks will not be maintained in large parts of the country, which will in all probability mean that these benchmarks will disappear over time while the elevation of those remaining will be incorrect as a result of subsidence, not least in the lowest lying areas. Furthermore, the fast GNSS systems would not be able to deliver the desired 1-2 cm elevation accuracy standard. A new geoid model has been announced that will improve accuracy, but it will not be able to eliminate GPS error at ellipsoid altitude.
” A coarse-mesh network of dynamic benchmarks will, to a large extent, be able to remedy the biggest problems associated with an obsolete local elevation benchmarking network and will allow GPS to be used as the primary surveying method, as the short distances between masts and rovers produce better accuracy ( 1 – 2 cm) partly because errors from the geoid model will be virtually eliminated.”
The dynamic benchmark
To accommodate users’ needs for accurate elevation references, Geopartner Inspections has developed “the dynamic benchmark” which has a number of application options.
The dynamic benchmark consists of a radar reflector, which provides a very strong return signal in an image recorded by a radar satellite. This means that with the help of a time sequence of radar images, it is possible to calculate the reflector’s motion over time, including its vertical motion component. This permits the assurance of an updated DVR90 elevation as well as revealing any local subsidence. Linking a water level gauge where absolute water level rise rates have been calculated allows these water level rise rates to be corrected for land uplift. This means that the rise in water level can be isolated from ground motion using the water level gauge, and that DVR90 reference errors caused by ground motion can be corrected in close to real time.
The reflector is also equipped with a benchmark, which is measured in relation to SDFE`s overall benchmark network as well as a screw thread to attach a GNNS antenna. This allows the reflector to function both as a maintained benchmark, while at the same time being used as a base station for individual GPS (RTK) stations in their local area.
Dynamic benchmarks can also be used as control points for accurate mapping when performed by drones and aircraft.
Dynamic benchmarks can therefore serve as the backbone for a wide range of municipal and commercial applications in the coastal areas, including:
- The planning, design and establishment of new coastal residential areas
- Port extensions, new quays and piers
- Climate proofing of existing real estate and other assets in flood-prone areas
- Forecasting of reduced capacity, setbacks and renovation needs for waste water pipes