TEST FIELD FOR INSAR URBAN SUBSIDENCE STUDIES Kirsi Karila, Mika Karjalainen, Juha Hyyppä, Veikko Saaranen Finnish Geodetic Institute, P.O. Box 15, 02431 Masala, Finland,
[email protected]
ABSTRACT Advanced INSAR techniques using stable targets on the ground, such as buildings, have been successfully applied to deformation studies in city areas. However, in some cases high-quality reference measurements have not been available and the validations of the INSAR results have been based on data that probably is not adequate for this purpose. In 2005 the Finnish Geodetic Institute (FGI) established an INSAR test field in the city of Turku. The FGI test field is approximately 5 km long and it is located in the main subsidence area along the river Aurajoki. It comprises roughly 100 bolts in the stone foundations of the buildings and three benchmarks in bedrock. In this paper, aspects of a good INSAR test field design are discussed, and the problems encountered during the establishment of the test field are described. In addition, precision of INSAR measurements based on stable targets is discussed. 1.
INTRODUCTION
Even though, persistent scatterer interferometry (PSI) techniques have become operational in subsidence monitoring, not many reference measurements for single buildings have been performed. Especially high quality reference measurements on several single buildings have not been available. In 2005 the FGI decided to establish a test field specially designed for studying the quality of results of the advanced INSAR measurements for urban subsidence studies.
measurement, even to millimetre level. Hence, to study the absolute accuracy of these techniques accurate reference measurements are also needed. One possibility would be to use a test field consisting of artificial persistent scatterers, e.g. corner reflectors (CR), whose controlled movements are precisely known. However, the amount of the CR’s needed would be great and they have to be distributed widely. On the other hand, to study the accuracy of PSI in cities, a test field located in a real urban environment would be preferred. Actual persistent scatterers are not ideal reflectors or scatterers and the accuracy based on them differs from the ideal case. In many earlier studies around the world the INSAR results have already been compared with ground measurements. However, authenticity of the results is based on presumptions or indirect observations. For example, the deformation is often presumed continuous, and it has been possible to interpolate the deformation from a few observations to a wider area. However, in many cases the subsidence of buildings in urban areas is non-continuous. The contiguous buildings may have different foundation or pilework or the other building might have been reconstructed. According to our knowledge non-continuous subsidence of buildings from INSAR has not been studied comprehensively enough using an accurate test field such as a high quality permanent levelling network. 2.
PLANNING OF THE TEST FIELD
2.1 Location
In recent years many advanced INSAR algorithms have become available [1][2][3][4]. Even though the algorithms are based on the same idea, the implementation varies and e.g. the selection of the stable pixels is based on different criteria. Therefore, it is essential to have a method to compare the quality of the algorithms.
The establishment of the FGI test field began with careful planning. We considered the following properties in the selection of the suitable area:
In northern areas the cities are often small and heavily vegetated, which makes the use of INSAR techniques more difficult than, for example, in large cities in Central Europe. Therefore, the lack of coherence in multitemporal images is often a problem, and the use of PSI techniques and large SAR data set is needed. PSI techniques allow high precision of deformation _____________________________________________________ Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)
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Amount of persistent scatterers
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Properties of the deformation
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Availability of SAR data
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Feasibility and convenience of field surveys
The amount or the density of persistent scatterers in the area is the most important criteria for the selection of the test field location. There should be enough persistent scatterers for the PSI algorithm used to detect the
deformation and to filter out the atmospheric effects. The amount of persistent scatterers is affected by various factors, including the size of the building, the material and structures of the roof, orientation of the building relative to radar look direction and nearby vegetation. The rate, character (type) and direction of the deformation should be considered carefully. Two aspects have to be considered here; first, the rate of the deformation should be convenient for INSAR studies. Too fast deformation corrupts the phase data, and very slow deformation cannot be detected. In addition, the spatial extension of the deformation phenomena is important. The PSI algorithms are optimised for areas, which are considerably smaller than the 100 x 100 km SAR image. There should be variation in the deformation within the study area and the resolution of the SAR data should be considered. Secondly, the area covered with field surveys cannot be very wide, since the measurements are very slow. Therefore, there should also be variation in the deformation in the area of the ground truth measurements. Seasonal changes in the deformation rate complicate the INSAR analysis and non-linear model should be used. Finally, the time series of SAR data should be dense enough to capture seasonal variations of the subsidence. Availability of SAR images around big cities is usually good, however, the seasonal effects should be considered in the selection process of the SAR data. In northern areas images acquired during winter are likely to contain snow, which reduces the amount of good data available for PSI studies.
and permanent stations are needed for high accuracy measurements. When faster GPS methods are used the precision is presumably not comparable with the PSI results, especially in vertical direction. Using triangular height measurements it is possible to observe the roofs of buildings, if the surveying targets are delivered to roofs, which are likely to be the source of SAR scattering. However, the measurements are more complicated and a lot more time consuming than precise levelling. Since the movement of a foundation of a building can present the subsidence of the entire building, and precise levelling of foundations are supposed to lead to good results. 3.
ESTABLISHMENT OF THE TEST FIELD
3.1 Location The city of Turku on the south-west coast of Finland was selected as the location of the test field. It is located only 200 km from Helsinki and FGI. Turku was selected as the study area, since severe building subsidence is known to occur there. Clay and bedrock areas occur alternately in the area, and the city has partly been built on tens of metres thick layer of clay. Several older buildings in the clay areas have still wooden pile work, and as the ground water level lowers, the old pile work decomposes, and the buildings subside. The rate of subsidence within the area varies roughly from 1 mm/yr to 15 mm/yr. An example of a subsiding building is presented in Figure 1.
Existence of stable control points or areas is essential for the reference measurements as well as for the PSI. A few stable control points in bedrock should be available near the area of interest. Other reference data e.g. soil maps can be used as aid in the planning. Other factors contributing to the convenience of carrying out the field surveys are disturbances e.g. tremble from traffic and (visual) obstacles and construction sites. 2.2 Methodology Several geodetic surveying techniques are available for ground truth measurements. Selection criteria for the reference measurement method were: (1) precision of the method, (2) easiness to carry out the field surveys, (3) the time available to accomplish them, and (4) the content of the field survey measurements compared with those of the PSI measurements. Traditional precise levelling provides high accuracy, but performing measurements requires labourers and is time-consuming. When GPS is used a long time series
Figure 1. The levelling measurement of a likely subsiding building. (Photo: Veikko Saaranen) There are two wider subsidence areas in the centre of Turku. One is situated along the River Aurajoki and one around the main railway station. The subsidence map derived from a PSI analysis is presented in Figure 2. For the design of the test field and validation of the results reference data from the Real Estate Department
of Turku (REDT) was available. Aerial images, base map including boundaries of buildings, and levelling data for several buildings were useful in the process. In addition a small-scale soil map for areas surrounding the city centre was available.
The measurements in Turku have been carried out three times: in spring 2005, autumn 2005 and spring 2006. The observations were adjusted with the three bedrock points fixed. The observations are stored for each date, and so it is possible to monitor the temporal evolution of building subsidence. The accuracy of the levelling measurements can be estimated from the control point height differences and the accuracy of the levelling data is better than 1 mm. The accuracies of different levelling campaigns fulfil the accuracy requirement of precise levelling 1.0 mm / km .
4.
Figure 2. Subsidence map of Turku derived using a PSI algorithm. Blue indicates subsidence and yellow corresponds to stable areas.(Aerial image © Turku city) 3.2 Measurements Precise levelling was selected as the surveying method due to its simplicity and incomparable accuracy. FGI’s expertise on precise levelling includes the three precise levellings of Finland. The test field in Turku follows the river Aurajoki. In addition, it is possible to extent the test field close to the railway area in the future. The test field is a levelling network tied to three stable control points in bedrock and they are optimally situated; in the ends of the network and one in the middle. It comprises circa 100 bolts in the stone foundations of the buildings. On each building 1-3 bolts were measured. The network is 4.9 kilometres long and it has a few loops in the middle section for the reliability purposes. The FGI test field is presented in Figure 3. It is possible to carry out the measurement of the entire network in a time period of one week. The benchmarks were carefully documented. GPS coordinates, digital photographs and distance measurements between benchmarks and height from the street level were collected. Examples of different kinds of bolts are shown in Figure 5.
RESULTS
The results were converted to annual subsidence rates for each benchmark. Vertical movement and constant rate were assumed. The time period of one year is relatively short and for good generalisation this should be done again after a longer observation period. The locations of the bolts were digitised on a digital map data containing the outlines of buildings. For each location name of the point and height change in different levelling campaigns were attached. Thus, the possible seasonal changes were also preserved. A time series of 35 ERS SAR and 7 ENVISAT ASAR images was used to accomplish the PSI analysis. The analysis was carried out using the Coherent Target Monitoring (CTM) module of EV-INSAR software released by Microsoft Canada. In previous studies the PSI results from ERS data have been compared with additional levelling data provided by REDT [5]. An example of comparison between ERS-ENVISAT time series and REDT levelling data is presented in Figure 4. Preliminary analysis of FGI levelling results against PSI results has been carried out. The building in examined Figure 4 was observed during FGI levelling campaign, and the detected subsidence rates for two bolts were 4.5 and 5.3 mm/yr. In general the results are not as good as the results when REDT levelling data was utilized, likely due to the temporally very short levelling time series available. Otherwise, the FGI data set is larger, more covering and more versatile. In the preliminary analysis the RMSE of the PSI results with respect to FGI levelling was 2.5-3 mm/yr. The final results will be published later, after more levelling data has been acquired.
Figure 3. FGI precise levelling network (map data © Turku city)
Figure 4. Levelling vs. insar Envisat acquisitions are displayed in orange color.
included in the network for future studies. The FGI will continue to survey the test field on a yearly basis. Continuation of the measurements is essential to get more information on the variation in the subsidence rate and more accurate average subsidence rate. Unfortunately, some benchmarks are likely to be lost due to subsidence and reconstruction works. 6.
ACKNOWLEDGEMENTS
Authors would like to thank the Real Estate Department of Turku for the reference data and maps, and Microsoft Canada/Vexcel Canada for the EV-INSAR CTM software used in the processing of the SAR data. The SAR images used in the study were acquired under ESA CAT-1 project 1422. 7. Figure 5. Examples of bolts in the foundations. 5.
DISCUSSION AND CONCLUSIONS
Problems encountered during the establishment and repeated surveys of the test field dealt with the quality of bolts, sand or dust on bolts, tremble, perishing of the bolts and reconstruction work. Many different kinds of bolts were used and damaged bolts may cause an error of one millimetre quite easily due to bending or asymmetry of the bolts or wearing of the coating. Tremble from the traffic and construction sites slows down the measurements and to get acceptable observation takes time. The bolts may be perished due to the subsidence or reconstruction of buildings. The reconstruction of the foundations or pilework may also change the deformation rates. Up-to-date information on construction work is needed in the interpretation of the results since construction work may cause radical changes in the subsidence rates. The location accuracy of the persistent scatterers seems to be rather good; in most cases it is possible to attach the PSI observation to a building in the centre area where buildings are relatively large. The levelling data from the Real Estate Department covers the time period of SAR data, however, there is a temporal difference of the FGI levelling and SAR time series. Therefore, more satellite and ground truth data should be acquired in the following years. Suitability of the test field to different SAR configurations was considered during the design phase; ascending and descending passes, different tracks, satellites, resolutions, have been considered and buildings not visible in the current SAR dataset are also
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