CREATING AND EVALUATING HIGH RESOLUTION DEM S FOR AN URBAN ENVIRONMENT FROM DIGITAL CARTOGRAPHIC PRODUCTS

CREATING AND EVALUATING HIGH RESOLUTION DEM S FOR AN URBAN ENVIRONMENT FROM DIGITAL CARTOGRAPHIC PRODUCTS Carter, J.R. 1 and Tripathy, D. 2 1 Geography-Geology Department, Illinois State University, Normal, Illinois, 61790-4400, USA. E-mail: jrcarter@ilstu.edu 2 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, USA. E-mail: dj_tripathy@yahoo.com ABSTRACT The local metropolitan government contracted for a digital GIS database in the mid-1990 s. There were many layers in that database but no DEM layer. A study was undertaken to determine if the cartographic databases could be used to create high resolution DEMs for the as-built, urbanized areas that could then be used to model hydrologic flows. The study area is one of low relief with an urbanized population of 110,000 persons. ESRI ArcInfo software was used to process the data and create the DEMs. The first step was to evaluate how well the contour data captured the details on the urban, as-built environment. In general, it was found adequate to the task. Both gridded DEMs and TINs were constructed for test areas. Because breakline and mass point data were not uniformly available no further work was done with TINs. Gridded models were generated at 1.5m, 3m, 5m, 7m, 10m and 15m resolutions. These models were examined for their ability to portray significant characteristics in the urbanized environment. DEMs of 3m, 5m, and 10m resolutions were used to delineate watersheds in the urbanized area. In general, the process gave satisfactory results, but along some of the interfluves there were differences in boundaries depending on the resolution of the DEM. In places, man-made barriers and underground drainage resulted in small, unconnected watersheds being defined independent of the larger watersheds of which they were a part. This study showed that large-scale DEMs can be derived from the cartographic layers. However, such models have significant limitations for hydrological modeling because important man-made structures are not part of the data and no provision exists in these models for subsurface drains. This study provides but one piece of many that are needed to understand and model the behavior of water in the as-built environment of an urbanized area. 1. INTRODUCTION The local metropolitan government has a good GIS but there is no DEM layer in that system. A study was undertaken to determine if the cartographic databases could be used to create high resolution DEMs for the as-built, urbanized areas that could then be used to model hydrologic flows. The study area is located in central Illinois and therefore it was covered by both Illinois and Wisconsin continental glacial advances. Glacial till, recessional moraines and loess characterize the surface of the area. The City of Bloomington and Town of Normal abut each other and have a combined population of 110,000 persons spread over an area of some 96 square kilometers. The surface topography is gently rolling with elevations ranging between 225-255 meters above sea level. The cities are located in the headwaters of three watersheds and thus much of the area consists of interfluves between the many streams. The primary goal of the study [1] was to gain a better understanding of the surface hydrology of this urban, as-built environment using data from the local GIS database. The computing environment for data processing and analysis was at Illinois State University using ESRI ArcInfo software. This work was done in cooperation with an engineer with the City of Bloomington who provided access to the digital datasets, which were organized into Sections. The Section is a basic unit of land measure in the Township/Range Public Land Survey System in this part of the United States. A Section is a square area 1 mile on a side and thus 1 square mile in area (2.59 sq km)). The first Section examined covered the area in and around the campus of Illinois State University. Proceedings of the 21 st International Cartographic Conference (ICC) Durban, South Africa, 10 16 August 2003 Cartographic Renaissance Hosted by The International Cartographic Association (ICA) ISBN: 0-958-46093-0 Produced by: Document Transformation Technologies

2. EVALUATING THE GIS LAYERS There are a number of layers in the GIS database, but no DEM layer. The contour layer was designed for display at a 0.5m contour interval. No metadata are available to gain more insight into the quality of the data. There are streams and lakes layers and in some Sections there are layers with breaklines and mass points. Given this data we set out to determine if large-scale DEMs could be created that might be useful in studying the hydrologic characteristics of this urban environment. One goal of the study was to test whether a TIN model or gridded DEM model would give the better representation of the surface. TIN models can be created if there is good contour, breakline and mass point data and normally the TIN model will give a better representation of areas in and around stream courses. Indeed, we did find that the TIN models were very good, but then we found that breakline and mass point data were not available for all Sections, so we abandoned the work with TINs. While the TINs can create a more precise definition of stream courses and their channels, TINs cannot be used for defining watersheds and drainage networks and accumulating streamflow in the ESRI software suite [2]. Gridded DEMs are necessary for those functions so all subsequent work was done with gridded DEMs. The foundation for the creation of the gridded DEMs was the contour database. So, a primary task was to assess if this database was of sufficient quality to permit the creation of appropriate large-scale DEMs. Contour maps were made of portions of the Section in the area of the University campus. Consideration was given to deriving a quantitative measure of how well the global contour surface fit reality, but an RMSE based on a scatter of elevations would not tell us how well the contour patterns replicated the real surface. Instead, a visual analysis was conducted. With large-scale data covering an area we could frequent everyday, we began to focus on small details of real surfaces and the digital representations of those surfaces. During heavy rainfall events we visited specific sites to watch water flow over the surface. We set out to determine if there is a minimum size of feature that can be neglected when considering the surface flow of water in an urban area. The closer we looked, we found ever smaller details that affect the flow of water. Of particular note was an ephemeral streamlet that passes within a meter of a drain but does not flow into the drain (Fig. 1). It would take a very precise model to capture and represent this situation. We concluded that there will always be many smaller features of hydrologic importance but in reality one cannot capture all of those details. Thus, depending on the purpose of the study some features must be neglected although they might be significant. Intermittent stream Drain Figure 1. Intermittent streamlet that passes within a meter of a drain but does not flow into the drain. When the contours were compiled they were carried through buildings in a best-guess approximation of the surface were the structure not there. The foot prints of the buildings do not appear in the digital contour layer, but in many cases the presence of large buildings is reflected in the shape of the contours. We overlaid the orthophoto base to get the footprints of the structures but the actual shape of the land next to buildings is not well represented in the contour layer. In the open areas we found one place where two contours seemed to be too close together indicating a steeper gradient than we thought we saw. The actual gradient was measured by constructing a transect across the contours. Indeed, we

were correct, but that glitch was very localized and similar problems were not evident. Other small variations were detected, but in total we concluded the quality of the contour data was adequate to permit us to generate DEMs to model watersheds and drainage networks. 3. CREATING AND EVALUATING DEMS Starting with the digital data, it is possible to make DEMs at any level of resolution. Given our computing resources and working with an entire Section of data, the finest resolution of DEM that could be generated was of 1.5 meters. To evaluate various combinations of resolution and file size DEMs were also constructed at 3m, 5m, 7m, 10m and 15m resolution. We then examined how well the DEMs represented the shape of the original land surface and the patterns of surface drainage. A number of cross sections along streams and roadways were examined to see how well the DEMs at different levels of resolution captured these areas. Sampling theory tells us that more coarse models will be able to capture fewer of the details of the surface. We established cross section transects by surveying using a hand level and stadia rod. The results were as expected. The profiles based on DEMs of 1.5m resolution have far more detail than those based on more coarse resolutions. In the process of creating gridded DEMs from contours, it is common to have sinks in the surfaces. Sinks are low points surrounded by higher elevations on all sides. While these sinks may exist in reality, most are considered to be artifacts of the gridding process and therefore provision exists in the software to remove the sinks. This is an iterative process that gives a surface that generally meets our expectations. But, in the process some generalization will be introduced. The next test of the quality of the DEMs was based on the quality of a network of drainage lines generated from the models. First flow direction grids were generated, linking each DEM cell to its steepest downslope neighbor. Then flow accumulation grids were calculated to show the routes surface water would follow as it moves downstream. Next, a network of drainage lines was delineated by setting a threshold value such that a digital stream will come into existence only after the threshold number of cells feed into that cell. Figure 2 shows the drainage lines that emerged using a threshold of 100 cells for two different levels of DEMs. Note that these drainage lines may not represent existing streams but rather where water might concentrate and flow across the surface. N a) 3 meter cells b) 7 meter cells Figure 2. Stream courses in the square mile Section covering the Illinois State University Campus. Both models are based on a threshold of 100 cells draining into a point before a stream is shown. These two maps of potential drainage lines proved to be good representations of the way water flows. Observing actual surface flow during heavy rains showed that the patterns of flow are well represented by these models. Many of the drainage lines run north--south and east--west. These lines follow the streets where water is constrained by curbs and gutters. In some cases the streets are shown by double-width lines, which are quite bold on the maps. With large-scale DEMs both sides of a single street can be represented by separate grid cells. Observe that there is much finer detail on the map created from the 3m DEMs, because there are so many more cells in the entire Section at that level of resolution. Based on the DEMs of 7m resolution, the pattern of drainage lines does not

include as many streets but does show lines in areas where no streams exist. Throughout the range of scales, the direction of the flow lines seems to be correct. 4. DELINEATING WATERSHEDS Based on these tests of the fidelity of the contoured surfaces and on the ability of the DEMs to capture the form of the surfaces, it was concluded that the contour database was of sufficient quality to be used for modeling the surface hydrology in this urban environment. The next step was to see how the DEMs could represent a watershed. Based on the practicality of working with the data it was decided to make DEMs at 3m, 5m and 10m resolution. The High School Branch of Sugar Creek watershed selected for study has many commercial structures, parking lots, residential areas, a golf course and a large school complex. This area extends over six Sections so the data had to be assembled into a single model. To delineate a specific watershed, a pour point or a low point along a stream course is identified and the software then finds the perimeter of the area that drains into the point, thus demarcating the interfluve of the watershed. It proved to be difficult to find a pour point near the mouth of High School Branch that would define the same watershed at the three levels of resolution. We resolved the problem by extending the beginning point upstream a few meters. A visit to the mouth of the stream revealed that the area was so engineered with streets and drains that there is no obvious interfluve separating the different watersheds. With the large model composed of the combination of the six Sections, the High School Branch watershed was delineated at the three levels of DEM resolution 3m, 5m and 10m. Figure 3. The High School Branch watershed as delineated by using 3m DEMs. North is at the top of this annotated orthophoto image. The mouth of the stream is at the northwest corner of the watershed. Note that the mouth of the watershed does not extend all of the way to Sugar Creek. At the east side of the watershed the interfluve follows the north-south highway. Landuse in the watershed includes many shopping centers, a golf course, a large complex of school buildings and recreational space and many residential areas. The three versions of the outline of the watershed were similar but not exactly coincident (Fig. 4). Along the east side of this watershed the 3m and 5m resolution models showed that a major highway formed the interfluve while the watershed delineation based on the 10m DEM showed a small area extending east of the highway.

Figure 4. Three variations in the delineation of the High School Branch watershed. North is at the top. Along the east boundary the green triangle picks up a small portion of the watershed that extends east of the highway complex. An even larger area should have been included in the watershed as shown in Figure 5. To the south the area of variation is in a residential area of very little slope. When examining all watersheds delineated in the large model, it was found that there were a number of mall, isolated watersheds east of the highway (Fig. 5). Visiting the area it was found that there are a number of small areas east of the highway that drain into the High School Branch watershed through conduits under highways. The highway complex is so wide that it formed a barrier that was detected by the DEMs as a watershed divide. We were not the first to recognize such problems for Another complication arises where water flow paths intersect with major highway interchanges, and a significant effort is required to define what happens under the highway. [3] Figure 5. The 3 meter DEM delineated four distinct watersheds that should be part of High School Branch watershed but because of highways they were not linked to each other or to the primary watershed. The red outline is a portion of the watershed boundary of High School Branch of Sugar Creek as delineated based on the 3 meter DEM.

At the south margin of the High School Branch watershed there is an area where the delineation of the interfluve was not consistent over the three levels of DEMs (Fig. 6). This is an older residential area where streets and curbs show the effects of age. The bases of the individual houses are elevated slightly above the sidewalks and streets. Upon visiting the area where the discrepancies were found, the trend of the slope of the streets was not evident. To detect that direction of slope in some areas visits were made during rain events when water could be observed flowing along curbs towards drains. This is a relatively flat area with detailed variations in topography that make it difficult to determine the direction of slope whether using a digital model or walking the ground. Figure 6. Watershed boundaries at the south end of the High School Branch. This is a residential area of complex topography but with little relief. Beyond these problem areas in the delineation of the watershed boundary, no other concerns were identified in the completed model of the patterns of drainage in the High School Branch watershed. One of the things the Engineering Department in the City of Bloomington was looking for from this study was a test of the ability of the models to delineate watersheds. We demonstrated that indeed these models can be used to define the boundaries of small watersheds. 5. SUMMARY AND CONCLUSIONS This study was undertaken to learn more about the nature of watersheds in an as-built urban environment. The work formed the base of a Masters Thesis in the Hydrogeology Graduate Program at Illinois State University [1]. As such, the study proved to be of great educational benefit to the Graduate Program and the GIS community in the area. It also laid the foundation for subsequent studies. The process of creating large-scale DEMs from the contour data proved satisfactory and we anticipate more work will be done with this data, simply because the data are available. However, for areas where good contour data do not exist, LIDAR data has now become the likely basis for creating such largescale DEMs. We also recognize that in the urban environment, buildings and plazas are integral parts of the surface hydrology. Thus, in such environments we see a place for the use of Digital Surface Models (DSMs) in addition to DEMs [4]. At the outset of the study, a search of the literature turned up little reference to documented studies on this topic. Colleagues working in this general area applauded the effort to take on this topic but could point to no published work on the topic. Subsequently, some literature has begun to emerge. Vieux [5] notes that the use of good elevation data and DEMs has been able to make great improvement in the quality of models used in watershed management. However, the watersheds he models are much larger and the DEMs he has been able to use are those created by the U.S Geological Survey. Maidment [6] has developed a framework model to integrate the many aspects of surface hydrology in a GIS. The work we report on here fits into the model outlined by Maidment, but it is only one piece in a complex model. We would like to better understand the urban watershed. As such, we want to know how water moves over the surface or is retained at the surface. But, in the urban environment, we must account for that water that is diverted into drains and sewers. In most cases that water is dumped back on to the surface at some point downstream. To account for this we need to integrate the surface models with utilities. Maidment [6] recognizes that still a different model is needed to account for the diversion of water into and out of utilities found in the urban environment. He also acknowledges that his model has no explicit data structures for aquifers or other groundwater features While Maidment [6] breaks new

ground with his model, that model is only one piece in the multi-dimensional environment of water at the surface and the near sub-surface. This paper is being presented to a community of cartographers. As such it is appropriate to ask what role cartography has in the study of water in the watershed, whether urban or regional. Our answer is that cartography provides much of the base information underlying the foundation of the studies and models that will look at water in the future. It is important that users understand the foundations on which they build their models. Cartographers should also recognize the challenges inherent in such studies for we need to deal with mapping the surface and sub-surface behavior of water through time. Certainly, cartographers should have something to contribute to the advancement of the science of water in the environment. 6. REFERENCES [1] Tripathy, Dibyajyoti, 2002, Creation and Evaluation of High Resolution Digital Elevation Models (DEMs) for Hydrological Applications in an Urban Environment. Masters Thesis, Hydrogeology Program, Illinois State University, Normal, Illinois, U.S.A. [2] Environmental Systems Research Institute (ESRI), Inc., 2001. ArcDoc Version 8.1-ArcInfo Online Manual. [3] Olivera, Francisco, Jordan Furnans, David Maidment, Dean Djokic and Zichuan Ye, 2002, Drainage Systems, in Maidment, David R. (ed.), Arc Hydro: GIS for Water Resources. Redlands, CA: ESRI Press. [4] Maune, D. F (ed.), 2001a, Digital Elevation Model Technologies and Applications: The DEM Users Manual. Bethesda, Maryland: American Society for Photogrammetry and Remote Sensing. [5] Vieux, B. E., 2001. Distributed Hydrologic Modeling Using GIS, Kluer Academic Publishers, Dordrecht, The Netherlands. [6] Maidment, David R. (ed.), 2002, Arc Hydro: GIS for Water Resources. Redlands, CA: ESRI Press.