SUBSIDENCE DETERMINATION BY DIFFERENT SENSOR TYPES USING STANDARDIZED TRANSFORMATION PARAMETERS Volker Spreckels Deutsche Steinkohle AG (DSK), Dept. DIG 2.2 Photogrammetry/Remote Sensing Tel.: ++49 2361–308 450, Fax: ++ 49 2361–308 411, e-mail:
[email protected] Commission VI, WG VI/5
KEY WORDS: Monitoring, Surface Movements, Geodetic Reference System, GPS, DEM, Laser, Radar, dInSAR, Transformation.
ABSTRACT: The mining activities of the German coal mining company Deutsche Steinkohle AG (DSK) lead to subsidence-induced large area influences e.g. to the ground water level or to watercourses. Aerial photogrammetry, and thus Digital Elevation Models (DEM) and stereo plotting of mine heaps and industrial sites, is based on the ‘TP network’ of the Ordnance Survey of North Rhine-Westphalia (LVA-NRW). The terrestrial survey, and thus line-levellings or site plans, generally base on the local cadastral ‘AP network’. These networks contain differences of about 1 m to each other. Aerial flight campaigns with modern radar or laser sensors are based on GPS/INS data. Spaceborne satellite data will often be geocoded by using the standard DREF transformation parameters set up for Germany that lead to differences of several meters to local coordinate systems. For the combination and comparison of different DEM and other available data like terrestrial measurements, the current ground reference systems and transformation parameters have to be known. According to the recommendations of the AdV [ADV 1995] and [JAHN 2001] a common homogeneous coordinate system with standardized transformation parameters from the “European Terrestrial Reference System 1989” (ETRS89) to the “Gauß-Krüger” (GK) coordinate system (Deutsches Hauptdreiecksnetz 1990, DHDN90) was defined for the mining areas of the DSK.
1. INTRODUCTION 1.1 Introduction to the System DSK-Ruhr-23 This paper bases on the analysis and results of research and development (R&D) projects set up by DSK in cooperation with the Institute for Photogrammetry and GeoInformation (IPI), University of Hannover. The projects dealt with the potentials and limitations of modern satellite- and airborne systems to derive DEM for the large area monitoring of surface movements in the German Ruhrgebiet [IPI 2000, 2001, 2003; SPRECKELS 1999, 2000, 2001, 2002, 2003]. The comparison of airborne radar- and laser DEM to analytical photogrammetric DEM showed systematic differences in height even for areas that were not influenced by coal mining subsidence. Further on, differences in position of an amount of some meters could be detected. The origin of these differences is primarily due to the datum of different geodetic reference systems for the diverse DEM produced for or by DSK, like the “System Preußische Landesaufnahme” (PrLA) from 1895 or the “System Netz 1977” (Netz77). But further on each data set seems to contain interior instabilities, maybe due to improper tie point measurements or the loss of L1 or L2 GPS phase during a GPS-based flight campaign. For this reason it is not definitely possible to estimate the interior accuracy and aptitude of each sensor for the aforementioned monitoring tasks. At this point a reliable common reference system would be helpful to detect the inaccuracies in every DEM. To avoid the relation on different more or less accurate GK systems, a new reference frame network was set up based on the transformation parameters from ETRS89 to DHDN90 by 23 specific official NWREF points of the LVA-NRW, enclosing the regions of DSK. Table 1 presents the transformation
parameters of the system DSK-Ruhr-23, the German DREF parameters and the “Germany National 7PAR” parameters used for the georeferencing of ERS satellite data: Germany National DREF DSK-Ruhr 7PAR 23 Translation X -580,00 -582 -564,544 [m] Y -80,90 -105 -100,981 Z -395,30 -414 -389,993 Rotation X 0,35 -1,04 -0,8993894 [``] Y -0,10 -0,35 -0,2348494 Z 3,58 3,08 3,2528431 Scale m 0,9999888 0,99999170 0,9999866 Table 1: Transformation Parameters: ETRS89 => DHDN90. The system DSK-Ruhr-23 owes an inner accuracy of less than ±10 cm in position and less than ±1 cm in height for a region of about 150 km to 80 km (see figure 1). For the reconstruction of the network geometry of every DEM region used by DSK, anew GPS measurements were performed on still findable points. These points were used for the transformation of GPSmeasurements to the GK coordinate system. The ETRS89 coordinates were transformed to the new system DSK-Ruhr-23 using the program TRABBI-3D [LVA-NRW 2002]. With the program TRAN3D of the BLUH software [BLUH 2003] the ground control points, single points as well as photogrammetric measurements and the different above mentioned DEM were transformed to DSK-Ruhr-23. The comparison with the GK systems showed differences of ±10 cm to ±20 cm to the Netz77, ±20 cm to ±40 cm to the PrLA and of more than ±80 cm to local cadastral AP-networks.
x,y: 5 cm z: 5 cm
Max. Deviation: 57,5 cm
x,y: 5 cm z: 5 cm
x,y: 5 cm z: 5 cm
Figure 2: Differences between system PrLA and DSK-Ruhr-23. Upper image: DEM 1996. Lower left: Deviations between System DSK-Ruhr-23 and GPS coordinates 2000, system PrLA. Lower right: Deviations between PrLA coordinates 2000 transformed to system DSK-Ruhr-23 and flight campaign 2003 in system DSK-Ruhr-23.
2. COMPARISION OF AIRBORNE DIGITAL ELEVATION MODELS 2.1 Max. Deviation: 9,78 cm
Figure 1: Accuracy of the Northrhine-Westfalian reference system. Upper image: deviations for all 135 NWREF points. Lower image: deviations for the selected 23 NREWF points defining the system “DSK-Ruhr-23”.
With a sufficient amount of identical points for each sensorDEM, all data, like elder DEM or other measurements, can be homogeneously transformed via ETRS’89 coordinates to or from the system DSK-Ruhr-23 with an accuracy of about ± 5 cm [SPRECKELS 2003].
Aerial photogrammetric DEM
The analytically measured photogrammetric DEM of the year 1996 served as reference DEM for the evaluation of the laser and radar DEM. It is based on nine “Trigonometric Points” (TP), system PrLA, of the official network of the LVA-NRW. These adjusted points owe an inner accuracy of up to ±28 cm within the GK system. The transformation points were remeasured by GPS in 2002 and all used ground control points (GCP) were transformed into the system DSK-Ruhr-23 (see figure 2). 2.2
Laser DEM
In spring 1998 a laser DEM for area of 100 km² was recorded and processed using the TopoSys Laser Scanner system. Most parts of the area are covered with forest. In 1999 this DEM was examined and compared to the photogrammetric DEM [KOCH 1999]. As a result of the analysis a shift in height of about ±30 cm was detected. Nearly the same amount was detected by comparing the photogrammetric DEM with the airborne radar DEM (see chapter 2.3).
The large area comparison of the photogrammetric and the laser DEM showed differences in position of about ±2 m to ±3 m that occurred as differences in height what can be seen for embankments and slopes (see figure 3, left).
influence of poor pixel data. The whole-area comparison of the photogrammetric and the radar DEM showed a good conformity in position but an offset of about ±35 cm in height for areas that were not influenced by mining subsidence. The P-band data is mostly influenced by soil moisture where no backscatters occur. Compared to the photogrammetric DEM dense forest is 6 m to 10 m higher, clearances or less dense forest lies approximately 2 m to 5 dm deeper, in maximum 2 m (see figure 3, right). The advantage of airborne radar-DEM for the monitoring of mining induced surface movements lies in the all-weather capability. For a use in monitoring tasks still passive systems like aerial photography or digital camera systems have the advantage of the high resolution image data. 2.4
Figure 3: Height differences between photogrammetric DEM 1996 and laser DEM 1998 (left); photogrammetric DEM 1996 and combined X-P-band radar DEM 2001 (right).
The protocols showed that the georeferencing of the laser data was done using two GPS receivers as temporal reference points – but the points were not signalized or located on already known GCP. The GPS coordinates were transformed to the GK system via DREF transformation parameters. Furthermore the protocol shows the loss of the GPS L2 phase for parts of the flight campaign. Later the whole processed DEM was georeferenced to the “Deutsche Grundkarte 1:5000” (DGK5) with offsets of -2,1 m in X-, -1,3 m in Y-direction and -1,35 m in height regarding the measurements on local levelling points. So this DEM is influenced by several factors during the registration and post-processing. 2.3
Comparison of photogrammetric and laser DEM
In December 2002 the nine TP for the transformation of the photogrammetric GCP for the flight campaign 1996 from GPS system to GK were re-measured by GPS in the system ETRS89 with geocentric (X,Y,Z) coordinates in the SAPOS system. These points were transformed to the system DSK-Ruhr-23 using the program TRABBI-3D of the LVA-NRW. With the program TRAN3D of the BLUH software the GCP were transformed from the TP-System to DSK-Ruhr-23 (see figure 2) and in the next step the whole photogrammetric DEM 1996 was transformed. Then the laser DEM 1998 was transformed to the system DSKRuhr-23. First the offsets in position and height were revoked and 64 representative points were taken to transform the DEM from GK to ETRS89 with the DREF parameters, and from ETRS89 back to the system DSK-Ruhr-23. Both DEM should now be located in one common comparable coordinate system.
Airborne Radar DEM
In April 2001 two DEM for an area of nearly 100 km² were produced with the airborne radar system AeS-1 with X- and Pband registration. The X-band, with a bandwidth of some cm, leads to a DEM of the real surface, corresponding to the “firstpulse” of laser systems. The P-band data, with a bandwidth of some dm, should lead to a nearly ground surface DEM. It was expected to detect “no-data” areas for wet soil or open water courses in forested areas, what occurs for parts of the landscape where the groundwater level reaches the earth surface due to mining induced subsidence. The accuracy in height was estimated to ±20 cm for the X-band DEM and to ±1 m for the P-band DEM. The weather conditions during the flight campaign were extremely bad with stormy winds and rapidly following rain fronts. The georeferencing was performed by using 10 corner reflectors and a GPS transmitter located on a GCP of the aerial flight campaign of 1996 and one GPS receiver located at the airport. And finally the GPS measurements were processed by using three SAPOS stations of the LVA-NRW. The radar data for the X- and P-band was delivered in nonprocessed raw-format. The post-processing was done by using small filter algorithms like median, size 3x3, only to delete the
Figure 4: Height differences between photgrammetric DEM 1996 and laser DEM 1998 (left); shift-corrected laser DEM 1998 (middle) and laser DEM 1998 transformed to the system DSK-Ruhr-23 (right). Grid distance: 500 meter.
The comparison of the DEM (see figure 4) presents the height differences between DEM 1996 and DEM 1998. The left part shows the height differences of the first DEM, the mid part presents the differences between photogrammetric DEM and the offset-corrected laser DEM and the right part the differences between both DEM located in the common coordinate system DSK-Ruhr-23.
Compared to the left image the picture in the middle shows some areas with less and other parts with larger differences in height. The right image shows height differences of a higher amount and thus a larger incoherence than the other images before. It is not known if the transformation of the laser DEM 1998 from GPS system to GK was performed using a method with a best fitting ellipsoid like Bursa-Wolf or Molodensky-Badekas. These methods use an ellipsoid with a fundamental point that is not identical with the geocentric origin of the ETRS89, used in the TRABBI software. The needed information for a final exact valuation of the DEM is not available any more. For the determination of systematic effects the DEM 1996 and the DEM 1998 were divided into seven smaller areas. The offsets between both DEM were calculated by a 7-parameter transformation. But the resulting offsets differed that much in amount and direction so that no systematic effect could be detected for the whole overlap area. The conclusion at this point is that data sets processed or derived from different geodetic reference systems can hardly be compared without a sufficient amount of identical 3-D ground control points. A common homogeneous geodetic reference system with standardized transformation parameters is able to serve as the framework for all different sensors and measurement techniques that are to be used for the comparison and evaluation of diverse sensor types.
3. COMPARISON OF SUBSIDENCE MOVEMENTS DERIVED FROM ERS-SATELLITE DINSAR DATA In the last chapter the different reference systems were discussed for DEM derived from aerial flight campaigns. The R&D projects even dealt with ERS satellite data with 10 meter ground resolution. Now it the question raise how the satellite data was affected by these different reference systems. For a city located in the northern Ruhrgebiet dInSAR subsidence movements were derived from ERS-satellite data for 10 data takes from June 1996 until October 1998 (see figure 6) by the Swiss company GAMMA Remote Sensing. The mining activities were monitored by line-levellings from July 1997 until July 2000. For the monitoring tasks of the mine site it was not necessary knowing the absolute position of the levelling points so the points were not exactly determined using a coordinate reference system, but roughly set by hand into a digital map. To receive the accurate DSK-Ruhr-23 coordinates the line-levellings were measured by GPS in the year 2001. The differences between the roughly set and the measured coordinates lay between ±4 m and ±12 m, approximately around ±5 meter (see figure 5). The georeferencing of the ERS dInSAR subsidence models was done using the transformation parameters “Germany National 7PAR” (see table 1). For this reason the point coordinates of the line-levellings had to be transformed to the system “Germany National 7PAR”. A shift in the easting of -0,815 m and in northing of +5,295 m to the system DSK-Ruhr-23 was calculated, according to the half pixel size of the ERS data. The comparison of the levelling points in system DSK-Ruhr-23 and “Germany National 7PAR” showed differences in height of up to 5 mm. The left part of figure 6 shows the dInSAR surface movements.
The area-wide dInSAR surface movements showed offsets up to ±4 cm to the measured subsidence on the levelling lines 1, 2 and 3. With the additional line-levelling information these offsets in height can be corrected for the dInSAR deformation models.
x,y: 5 meter
Figure 5: Coordinate difference between roughly set and measured line levelling points.
For the third monitoring period of 105 days the dInSAR results are not representative because too large subsidence movements occurred within this time span. Again it could be proved that movements of about 10 cm during two ERS data takes are detectable [SPRECKELS 2001, 2002]. For the monitoring tasks of DSK ±5 mm are within the limit for the detection of subsidence movements but under good registration conditions the ERS dInSAR surface movements match to the measured deformation within ±1 cm. So it is even necessary for satellite data – as far as it has to be used for DSK monitoring tasks - to receive the most exact position within the system DSK-Ruhr-23. For future activities this can be realised by using corner reflectors to get the relation between satellite data and the ground reference system.
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Figure 6: Comparison of dInSAR surface movements superposed with active (black) and old mining fields (grey) (left part). Grid distance: 500 meter. Surface movements in the satellite system (magenta coloured lines); in the system DSKRuhr-23 (dark blue lines) and subsidence measured on line levellings (yellow lines).
4. CONCLUSION The R&D projects concerning the detection of mining induced surface movements revealed that the comparison of DEM or deformation models derived from different sensor types only can give conclusions to the inner accuracy when the data is based on a common reference system - or if the data could properly be transformed from one system to the other. If there is not enough available information about the reference system or the data processing, no precise statement can be given for the DEM accuracy. The introduction of standardized transformation parameters from the ETRS89 to the system DSK-Ruhr-23 created a basis for combining different kinds of sensor data. This data may be terrestrial measurements like line-levellings or electro-optical distance measurements, site plans, analytical photogrammetric DEM measurements or DEM derived by other sensor types and even satellite data. These DEM and coordinates can be transformed to or from local surveying networks, more or less local official networks like the cadastral AP-networks or elder network configurations of the Ordnance Survey - if enough identical points or sufficient information are available. It was proved that the transformation of GCP from aerial flight campaigns in the year 2000 (PrLA) and 2003 (DSK-Ruhr-23) could be transformed from the TP system (PrLA) to the system DSK-Ruhr-23 with an accuracy of less than ±5 cm. With this system archived and actual data like point coordinates, DEM and even GIS-data and modern sensor data can be homogeneously collected and conducted based on the new reference frame in the system ETRS89.
5. REFERENCES
ADV (1995): Das European Terrestrial Reference System 1989 (ETRS89). http://www.adv-online.de/produkte/etrs89.htm BLUH (2003): Program System BLUH (BundLe Block Adjustment University of Hannover), http://www.ipi.unihannover.de/html/service/bluh/bluh.htm IPI (2000): INSTITUTE FOR PHOTOGRAMMETRY AND ENGINEERING SURVEYS, UNIVERSITY OF HANNOVER: Final report for the R&DProject: „Nutzung von hochauflösenden Satellitendaten zur großräumigen Überwachung der Umweltauswirkungen bergbaulicher Tätigkeiten im Ruhrgebiet, Arbeitspaket 2: Stereoauswertung“. FuE-Vorhaben 0364000 der Deutschen Steinkohle AG, supported through Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 50 EE 9652. SPRECKELS, V.; JACOBSEN, K., Institute for Photogrammetry and Engineering Surveys (IPI), University of Hannover, 2000, unpublished. IPI (2001): INSTITUTE FOR PHOTOGRAMMETRY AND GEOINFORMATION, UNIVERSITY OF HANNOVER: Status report for the R&D project „Überwachung bergbaubedingter Bodenbewegungen durch hochauflösende Satellitendaten und flugzeuggetragene Scannerdaten“. FuE-Vorhaben 0885000 der Deutschen Steinkohle AG. SPRECKELS, V.; JACOBSEN, K., Institute for Photogrammetry and GeoInformation (IPI), University of Hannover, 2001, unpublished.
IPI (2003): INSTITUTE FOR PHOTOGRAMMETRY AND GEOINFORMATION, UNIVERSITY OF HANNOVER: Final report for the R&D Project „Überwachung bergbaubedingter Bodenbewegungen (MONS)“. FuE-Vorhaben 0995000 der Deutschen Steinkohle AG. SPRECKELS, V.; JACOBSEN, K., Institute for Photogrammetry and GeoInformation (IPI), University of Hannover, 2003, unpublished. JAHN, C.-H. (2001): Ein neues aktives dreidimensionales Bezugssystem für Niedersachsen. In: Wissenschaftliche Arbeiten der Universität Hannover, No. 239, 2001, pp. 63 - 75. KOCH, A. (1999): Analyse und Aufbereitung von LaserScanner-Aufnahmen. Diploma Thesis, Institute for Photogrammetry and Engineering Surveys, University of Hannover, 1999, unpublished. LVA-NRW (2002): TRABBI-3D-Transformationen und Abbildungsübergänge, on CD-ROM. Landesvermessungsamt Nordrhein-Westfalen, 2002. SPRECKELS, V. (1999): Monitoring of Hard Coal Mining Subsidence by Airborne High Resolution Digital Stereo Scanner Data. In: ISPRS Joint Workshop on "Sensors and Mapping from Space", ISPRS WG I/I, I/3, IV/4, Hannover, Germany, 27. – 30. September 1999, on CD-ROM. SPRECKELS, V. (2000): Monitoring of Hard Coal Mining Subsidence by HRSC-A Data“. In: Proceedings of XIXth ISPRS Congress, Amsterdam, The Netherlands, 16. – 23. July 2000, pp 1452 – 1458. SPRECKELS, V.; WEGMÜLLER, U.; STROZZI, T.; MUSIEDLAK, J.; WICHLACZ, H.-C. (2001): Nutzung von InSAR-Daten zur großflächigen Erfassung von topographischen Veränderungen über Abbaubereichen der Deutschen Steinkohle AG (DSK). In: Tagungsband des Deutschen Markscheider Vereins (DMV) 2001, Trier, Germany, 26. - 28. Sept. 2001, pp. 49 - 70. SPRECKELS, V. (2002): Untersuchung operationeller Aufnahmesysteme zur großflächigen Erfassung von Digitalen Geländemodellen und topographischen Veränderungen über Abbaubereichen der Deutschen Steinkohle AG (DSK AG). In: Veröffentlichungen der DGPF-Jahrestagungen, 22. WissenschaftlichTechnische Jahrestagung der DGPF, 24. – 26. Sept. 2002, FH Neubrandenburg, Germany, pp. 67 – 82. SPRECKELS, V. (2003): Einführung fester Transformationsparameter vom ETRS’89 zum Gauß-Krüger-Koordinatensystem über NWREF-Punkte zur einheitlichen Führung der Koordinaten und GIS-Datenbestände der Deutschen Steinkohle AG (DSK AG). In: Veröffentlichungen der DGPF-Jahrestagungen, 23. Wissenschaftlich-Technische Jahrestagung der DGPF, 9. – 11. Sept. 2003, FH Bochum, Germany.
