The use of the Official Digital Terrain Model of the Republic of Croatia in Projects for Water Drainage System Construction Karlo Šimek 1, Damir Medak 2, Ivan Medved 3 1 Šimek Ltd., Rizzijeva 103, Pula, Croatia, karlo@geodezija-istra.com 2 Faculty of Geodesy, University of Zagreb, Kačićeva 26, Zagreb, Croatia, dmedak@geof.hr 3 Croatian Forest Research Institute, Cvjetno naselje 41, Jastrebarsko, Croatia, ivanm@sumins.hr Abstract. Due to the unequal development of cities and drainage systems, noncompliance of the natural drainage basins and the creation of artificial basins, there is a growing demand for the rapid geodetic maps, that can be used for the preparation of project documentation of the water drainage system construction. The reason for the inability of creating geodetic maps only for a specific critical location is that the reconstruction of a single weak link violates the conditions of the infrastructure in the surrounding area. The study and the project documentation cannot be resolved within the limits of the intervention without previously established conditions of the surrounding areas. The goal of this paper is to present the results of the use of the official digital terrain model of the Republic of Croatia combined with cadastral polygon points in the preparation of project documentation for water drainage system construction. The research and the analysis will be carried out for the area of the City of Pula. Since the development in vertical sense is higher in the Centre of the city compared to the parts of the city with lower development intensity, the accuracy of the model within the centre will be presented. It will also be shown, whether the model accuracy depends on the higher coverage of vegetation compared to the places with low vegetation and construction. The paper will present whether the DTM accuracy meets the requirements in areas with a small slope, where the focus is in the preparation of project documentation for water drainage system construction. Keywords: basin, designing, development, drainage, DTM, slope, vegetation. 1. Introduction When decreasing representation of natural environment and increasing the amount of constructed area in the parts of cities where the steep slope leads to a depressed point, and simultaneously disregarding the increasing number of drain channels and diameter sewers, critical areas in water drainage system occur. 313
SIG 2016 International Symposium on Engineering Geodesy, 20 22 May 2016, Varaždin, Croatia In the preparation of project documentation for large areas, the input data are supposed to be collected as cheap and fast as possible. The example of that is the preparation of project documentation for water drainage system construction both in urban and rural areas. This is why more and more digital terrain models (DTM), developed for large areas, are used in this type of projects. This research will show how processing and analysis of digital terrain models, derived from different sources, coincide with real situation on the field and whether they manage to satisfy the accuracy of use in the preparation of such projects. When DEM is used in terrain analysis, like in automatic drainage basin delineation, errors of the model are collected in the analysis results. Investigation of this phenomenon is known as error propagation analysis, which has a direct influence on the decision-making process based on interpretations and applications of terrain analysis [Oksanen 2006]. As the most accurate data we used the official data of the State Geodetic Administration, vector DTM, obtained by photogrammetric restitution. We also used the official cadastral polygon points as the base for height interpolation. On critical areas of the City of Pula, so called hot spots, vector DTM will be analysed using terrestrial survey measurements, which can be considered flawless for the purpose of this analysis. The analysis will examine the dependence of the accuracy of digital elevation models, considering the slopes, vegetation and buildings on some critical points in the City of Pula. 2. Digital terrain model There are many assumptions made in the basic drainage model, yet they have proven to be of considerable value. These assumptions include: (i) uniform precipitation; (ii) flows take place entirely across surfaces that they do not alter, and are unaffected by absorption (notably different soil or rock types) or groundwater; (iii) flows grow as a linear function with distance and are not altered by the slope values, just by the direction; (iv) there are no barriers to flow; and (v) the study region is complete and meaningful in the context of the analysis [de Smith et al. 2015]. Digital terrain model is a statistical representation of the continuous surface of the ground by a large number of selected points with known X, Y, Z coordinates in an arbitrary coordinate field [Miller & Laflamme 1958]. A digital terrain model presents the terrain or surface topography of the Earth. The identification between digital terrain models and a digital elevation model can be found in many literature sources. However, a digital terrain model (DTM) differs from a digital elevation model (DEM). DTM is a representation of a continuous surface as if all objects are removed, and DEM represents a surface where objects are included in the surface measurement. A DEM can be represented by a raster or a vector data. These days, there are many methods for obtaining elevation data to create DEM. In this research we used three methods: Shuttle Radar Topography Mission, digital terrain model obtained by photogrammetric restitution, and terrestrial survey method using GPS and total station. 314
2.1. Vector DTM obtained by photogrammetric restitution As our most accurate data for a large area of interest, we used the official digital terrain model of the Republic of Croatia that we obtained from State Geodetic Administration. It is a vector DTM, obtained by photogrammetric restitution [URL 1]. This data is formed by 3D polylines, 3D lines and 3D points. DTM turned out to be very interesting for our research, because one of the layers of the 3D polylines representing the pipelines, is created over the city streets, as the highest percentage of line pipes, not only for water drainage system, are located directly under the streets [Figure 2.1]. Figure 2.1 Vector DTM overlaid on DOF5/2011 Consequently, using the right interpolation model that uses these 3D polylines in process, will leave their height unchanged. 2.2. SRTM data The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a global scale. The Shuttle Radar Topography Mission is a global project spearheaded by the U.S. National Geospatial-Intelligence Agency and the U.S. National Aeronautics and Space Administration. Previously, SRTM data for regions outside the United States were sampled for public release at 3 arc-seconds, which is 1/1200th of a degree of latitude and longitude, or about 90 meters. The new data are being released with a 1 arcsecond, or about 30 meters, sampling that reveals the full resolution of the original measurements [URL 2]. DEM provided from SRTM data is often used in hydrological studies and for delineation of drainage network. So far, it has not provided satisfying accuracy 315
SIG 2016 International Symposium on Engineering Geodesy, 20 22 May 2016, Varaždin, Croatia for such studies, but it is included in processing and analysis in order to compare results with other input data. 3. Height deviation between vector DTM and cadastral survey points The accuracy of the points available in the height models are only one aspect of the quality of the DEMs; along with the point spacing being important, they both lead to the morphologic quality, visible at contour lines [Sefercik et al. 2007]. One of the input data was cadastral survey network points. All the 2D points and all the points with the height that would have given wrong information in the process of interpolation (churches), had been removed from the list of points. All the points that were left after the process were converted to raster. Vector DTM was converted to raster DEM by firstly creating triangulated irregular network (TIN). The first accuracy indicator was the height deviation between raster DEM, created from vector data obtained from State Geodetic Administration, and raster created from cadastral survey network points. 316 Figure 3.1 Height deviation between vector DTM and cadastral survey points The analysis has shown that over 5100 cadastral polygon points were in the range of ±1 m in height deviation. Also, height deviation indicated that almost 40% of points were in the range of ±0.5 m. It can be noticed that the mean value
of all height deviations is about 0.15 m [Figure 3.1]. This is the first indicator that this kind of DEM can be used in the preparation of the project documentation for the water drainage system construction or at least for the studies that precede it. To eliminate even more errors, weighted interpolation of the vector DTM with cadastral survey points was used. 3.1. Height accuracy of vector DTM compared with SRTM data DEM provided from SRTM data is one of the most frequently used datasets in hydrological studies, as well as for delineation of drainage network. Usually, the results of studies and analysis were not accurate enough. We compared SRTM digital elevation model (30 m sampling) with the vector DTM and cadastral survey points. The first analysis has shown that, even though mean height deviation was also just under 0.15 m between vector DTM and DEM from SRTM data, most values were inside range of ±3 m [Figure 3.2]. To further determine that DEM derived from SRTM data does not meet the requirements of accuracy, we analysed height deviations between the DEM derived from SRTM data and the raster derived from cadastral survey points. The results have shown that height deviation is versatile and that they do not meet the requirements of accuracy. Figure 3.2 Height deviation between vector DTM and DEM-SRTM 317
SIG 2016 International Symposium on Engineering Geodesy, 20 22 May 2016, Varaždin, Croatia 4. Weighted interpolation of the vector DTM with cadastral survey points Interpolation is a procedure used to predict values of cells at the locations that lack sampled points [URL 3]. There are many interpolation methods and all of them depend on the correlation between the near and distant objects. Correlation is defined by the level of interdependence between the objects, the strength of the interdependence and similarity of the objects within the area. The direction in which water will flow from a cell in consideration of its immediate surrounding cells yields the Flow Direction Grid. This is a grid of cells indicating the steepest direction from a cell to the surrounding cells [Akajiaku & Ngozi 2015]. Four groups of data were gathered for creating the triangulated irregular network: 3D polylines that were created over the streets during the photogrammetric restitution, all other 3D polylines and lines from vector DTM, 3D points from 3D vector DTM and cadastral survey points. Due to the nature of this project, 'weighted interpolation method' was used for creating the TIN. Knowing that the highest percentage of pipelines are located directly under the streets, 3D polylines that were created over the streets were set as hard lines, so their height does not change and edges represent distinct breaks in the slope. All other 3D polylines and lines were set as soft lines because we wanted gradual change as the triangulated surface gets converted to the raster. All points were imported in process as nodes. Figure 4.1 TIN of critical area in City of Pula for water drainage system The TIN of one of the critical areas in the City of Pula is shown in [Figure 4.1] above. It is evident that the steep slopes are leading to a depressed area. 318
5. Deviation of the raster DEM compared to data obtained with terrestrial measurements For further analysis, three areas in the City of Pula were measured by terrestrial surveying methods. Two areas represent critical points with their topographical features. For this research all three measured areas were considered flawless. On all of them height deviations were examined given the slopes, representation of natural environment and the amount of constructed area. Raster DEMs have been created from terrestrial measurements using spherical Kriging interpolation method. This method uses a progressive decrease of spatial autocorrelation to a distance, beyond which autocorrelation equals zero. Regardless to the previous results, SRTM data were included into the research. Height deviations between terrestrial measurements and SRTM data were computed and results were not satisfying. On some places height deviations were over 9 meters, therefore the conclusion is that SRTM data do not meet the requirements of accuracy for the execution of such projects. Removing artefacts from digital surface models is a prerequisite for deriving high quality bare-earth digital elevation models [Gallant et al. 2012]. This is the main reason why vector DTM, obtained by photogrammetric restitution, is used for this project. 5.1. Height deviations on terrain with low slope The first of three locations is specific for its topographical features, steep slopes around it leading to a depressed street with a low longitudinal slope. Along the street there is a continuous longitudinal slope of around 1%. Figure 5.1 DEM from terrestrial measurements and height deviation results against vector DTM Height deviations between raster DEM, created from vector data obtained from State Geodetic Administration, and raster DEM obtained from terrestrial measurements were calculated. Results of the research show that 90% of the 319
SIG 2016 International Symposium on Engineering Geodesy, 20 22 May 2016, Varaždin, Croatia values are in the range of -1.3 meters up to +0.30 meters, with mean value of - 0.56 meters and standard deviation of 0.65 meters [Figure 5.1]. 5.2. Height deviations on terrain with high development density The second area is a narrow street with low natural environment and high development density. The research shows very good results, 85% of the values are in the range of ±0.35 meters, with mean value of -0.01 meters and standard deviation of 0.35 meters. This analysis indicates that high development density does not affect the accuracy of vector DTM, and that this kind of digital elevation model satisfies the accuracy requirements for the execution of water drainage system projects on the terrain with this kind of topographical features. 5.3. Height deviations on terrain with high vegetation coverage Final research was conducted on the area covered with high vegetation (forest). The area is specific for its sudden steep slopes, thus poor analysis outcome was predicted. Results and height deviations demonstrate scattered values. Only 70% of the values are in the range of -1.25 meters up to +0.25 meters, with mean value of -0.71 meters and standard deviation of 0.63 meters [Figure 5.2]. Figure 5.2 Height deviations for the forest area model In table [Table 5.1] research results of all three locations are shown. 320
Table 5.1 Accuracy indicators on all three locations Mean Locations Results range value Low slope terrain High development density Forrest Standard deviation 90% of the values in range -1.3 m up to -0.56 m 0.65 m +0.3 m 85% of the values in range of ±0.35 m -0.01 m 0.35 m 70% of the values in range -1.25 m up to -0.71 m 0.63 m +0.25 m 6. Conclusion This research demonstrated various results. Firstly, the conclusion is that there is no significant impact of interpolation methods on DEM accuracy. This is because the highest accuracy is needed on streets with low longitudinal slope. However, in this case, it is very important to set weights for input datasets, to eliminate errors in critical areas of research. By comparing height deviations using cadastral survey points with vector DTM, obtained by photogrammetric restitution, results show that over 40% of values are in the range of ±0.5 m, and over 75% are in the range of ±1 m. As expected, SRTM data did not meet the accuracy requirements of this project. DEM crated from vector DTM and cadastral survey points using weighted interpolation method, demonstrated different results on terrain with different topographical features. As expected, lowest accuracy level occurred in the area with high vegetation coverage, the main reason being the lowest density of input data for the certain area. However, this result does not indicate that vector DTM, obtained by photogrammetric restitution should not be used for water drainage system construction. On the terrain with high development density, the research points to very good results with over 85% of the values in the range of ±0.35 m. This is an indicator that high development density and tall buildings do not have influence on the accuracy of vector DTM. However, in critical areas with steady longitudinal slope of around 1%, terrestrial measurements should be provided for projects like this. The research results demonstrated that 90% of the values are in the range of -1.3 meters up to +0.30 meters. Hence, vector DTM should be combined with more accurate input data on the terrain with this kind of topographical features. References Akajiaku, C.C., Ngozi B. (2015). Geographic Information Systems Based Urban Drainage Efficiency Factors, FIG Working Week 2015, Nigeria. 321
SIG 2016 International Symposium on Engineering Geodesy, 20 22 May 2016, Varaždin, Croatia de Smith, M.; Goodchild, M.; Longley, P. (2015). Geospatial Analysis: A Comprehensive Guide to Principles, Techniques and SoftwareTools, 5th Edition, Splint, Leicester, UK. Gallant J.C.; Read A.M.; Dowling T.I. (2012). Removal of Tree Offsets from SRTM and Other Digital Surface Models, International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 XXII ISPRS Congress, Melbourne, Australia. Miller, C. L.; Laflamme, R.A. (1958). The Digital Terrain Model: Theory and Application, Photogrammetric Engineering and Remote Sensing, 24, pp. 433 422. Oksanen, J. (2006). Digital Elevation Model Error in Terrain Analysis, Academic Dissertation, Faculty of Science, University of Helsinki, Publications of the Finnish Geodetic Institute, No. 134. Sefercik, U.; Jacobsen, K.; Oruc, M.; Marangoz, A. (2007). Comparison of SPOT, SRTM and ASTER DEM's, Zonguldak Karaelmas University, Leibniz University of Hannover. URL 1: State Geodetic Administration, Republic of Croatia, http://www.dgu.hr, (21. 2. 2016). URL 2: National Aeronautics and Space Administration, The United States of America, https://www.nasa.gov, (21.02.2016). URL 3: Childs, C. (2004). Interpolating Surfaces in ArcGIS Spatial Analyst, http://webapps.fundp.ac.be/geotp/sig/interpolating.pdf, (23. 2. 2016). 322
Upotreba službenoga digitalnog modela reljefa Republike Hrvatske u izradi projekata za izgradnju sustava odvodnje oborinskih voda Sažetak. Uslijed neusklađenosti razvoja gradova i sustava odvodnje, te nepoštivanja granica prirodnih slivova i stvaranja umjetnih slivova, sve je veća potražnja za brzim geodetskim podlogama koje mogu poslužiti za izradu projektne dokumentacije sustava odvodnje oborinskih voda. Razlog zbog kojeg se na takvim kritičnim lokacijama ne može pristupiti izradom geodetskih podloga samo za određenu lokaciju je taj, da se rekonstrukcijom samo jedne slabe karike narušavaju uvjeti za infrastrukturu u okolici. Projektnu dokumentaciju i studiju nije moguće riješiti unutar granica zahvata bez prethodno uspostavljenih uvjeta okolnog područja. Cilj ovog članka je prezentirati rezultate uporabe službenog digitalnog modela reljefa Republike Hrvatske s bazom točaka geodetske osnove (katastarske poligonske točke) u izradi projekata za izgradnju sustava odvodnje oborinskih voda. Istraživanja i analize provoditi će se za područje Grada Pule. Prezentirat će se analiza točnosti modela unutar centra grada, gdje je izgrađenost u vertikalnom smislu veća u odnosu na manje izgrađene dijelove na periferiji grada, te analiza točnosti digitalnih modela terena na dijelovima grada pokrivenim višom vegetacijom u odnosu na mjesta niske vegetacije i izgrađenosti. Prikazat će se da li DMR zadovoljava točnost na lokacijama manjih nagiba, na koje se stavlja naglasak pri izradi projektne dokumentacije za izgradnju sustava odvodnje oborinskih voda. Ključne riječi: DMR, izgrađenost, nagib, oborine, odvodnja, projektiranje, vegetacija. *scientific paper 323