GIS for Coastal Zone Management - Chapter 3

CHAPTER THREE A Comparative Study of Shoreline Mapping Techniques Ron Li, Kaichang Di, and Ruijin Ma 3.1 INTRODUCTION The shoreline is a unique and complex feature and has been recognized as such by the International Geographic Data Committee (IGDC), which designates it as one of 27 key distinct features on the earth’s surface. A shoreline is defined as the line of contact between land and a body of water. Although it is easy to define, the shoreline is difficult to capture because of the natural variability of water levels. An accepted method of capturing this line of interface is to determine the tide-coordinated shoreline, which is the shoreline extracted from a specific tide water level. NOAA uses Mean Lower Low Water (MLLW) and Mean High Water (MHW) in this way to map shorelines that can be georeferenced. Both the MLLW and MHW are calculated from averages over a period of 18.6 lunar years. Like the shoreline, shoreline mapping techniques are changing and being improved upon. At present, photogrammetric techniques are employed to map the tide-coordinated shoreline from aerial photographs that are taken when the water level reaches the desired level. Aerial photographs taken at these water levels are more expensive to obtain than remote sensing (RS) imagery. With the development of remote sensing technology, satellites can capture high-resolution imagery with the capability of producing stereo imagery: one such source is the IKONOS satellite images. The question is: is it possible to use RS imagery to map the shoreline so that we can reduce the costs and improve mapping efficiency and accuracy? During our study we extracted shorelines from aerial photos, simulated and actual IKONOS imagery, and the intersection between a Coastal Terrain Model (CTM) and the water level. We estimated the accuracies of these shorelines and analyzed the potential of these techniques for practical shoreline mapping by comparing the extracted dataset with the shorelines from the United States Geological Survey (USGS) topological maps and National Oceanic and Atmospheric Administration (NOAA) nautical charts. © 2005 by CRC Press LLC 3.2 SHORELINE EXTRACTION To evaluate the new shoreline mapping techniques it is necessary to compare the results with a current shoreline product. In our study, the shoreline extracted from the tide-coordinated aerial photos was used as the reference shoreline and the other shorelines were compared with it to assess their relative accuracies. Shorelines from USGS Digital Line Graphs (DLG) and a NOAA Nautical Chart were digitized to facilitate the evaluation of the new results. The extraction methods used are discussed in the following sections. 3.2.1 Shoreline from aerial photos A shoreline was digitized from tide-coordinated aerial orthophotos. This product was chosen as the baseline in our analysis due to its high accuracy. The aerial photos were commissioned by NOAA in 1997 when the water surface reached the specified level (MLLW). The photo scale is about 1:20,000 and the pixel size is about 0.6 meters on the ground. This shoreline is tide-coordinated to the mean lower low water level. A Global Positioning System (GPS) survey was carried out in 2000 to obtain control points for the bundle adjustment. Eight control points were used in the adjustment. The Root Mean Square (RMS) numbers indicate a good precision in this adjustment: 0.106, 0.107 and 0.073 meters in the X, Y and Z directions, respectively. Based on the bundle adjustment results, Digital Terrain Models (DTMs) and orthophotos were generated in sequence. Then, the orthophoto mosaic was created with a resolution of 1 meter. The shoreline was digitized manually from this orthophoto mosaic. The task was accomplished using ERDAS software. 3.2.2 Shorelines from simulated and actual IKONOS images IKONOS imagery has two resolutions, 1-meter panchromatic and 4-meter multispectral. We first obtained 4-meter multispectral geo-referenced imagery for our project. The stereo 1-meter images were still pending when we produced the shorelines in this paper. To study the potential of 1-meter stereo imagery in shoreline mapping, we simulated the imaging procedure and obtained the simulated IKONOS 1-meter stereo imagery using rational functions (RFs). The shoreline extracted from aerial orthophotos was used for back-projecting to the simulated IKONOS raw backward- and forward-looking images. The resulting shorelines on IKONOS raw images show that RFs work very well. This indicates that we can use RFs to extract high accuracy 3-D shorelines from real IKONOS 1-meter imagery provided that we can find well-matched conjugate points on raw IKONOS images along the shorelines. To improve the accuracy of 4-meter IKONOS real data, 8 control points were used to perform planar polynomial geo-correction. The RMS in the X and Y directions are 1.048 meters and 1.1028 meters respectively. A shoreline was extracted from the refined IKONOS image. © 2005 by CRC Press LLC 3.2.3 Shoreline from the intersection between a CTM and the water level This idea comes from the definition of a shoreline as the contact or intersection line between the water level and the land. We tried to obtain the land model and water surface model (WSM) and then compute the intersection line of these two models. This method is intuitive to people’s perception of shoreline (See the profile in Figure 3.1 and the colour insert following page 164). The CTM depicts the elevation and bathymetry in the coastal area. The water surface model is obtained from the Great Lakes Forecasting System at The Ohio State University (OSU). To extract the shoreline, these two models were overlaid and the WSM was subtracted from the CTM to get an output layer. Theoretically, the shoreline should be the line with the zero value in the output layer. In practice, the zero pixels do not form a line due to random errors in these two data models introduced from data acquisition and sampling. We chose a small value range for the study and obtained a shoreline strip. We then took one side edge of the strip as the shoreline for our study. We employed this method because the strip is very thin and it is very small compared to the error of the CTM. 3.2.4 Shorelines from other sources Besides the above shorelines, four other shorelines were used for the analysis in our study. Among them, two digital shorelines were provided by the Ohio Department of Natural Resources (ODNR), the third one was extracted from a USGS DLG and the fourth one was digitized from a NOAA Nautical Chart. Figure 3.1 Generation of Digital Shorelines © 2005 by CRC Press LLC 3.3 ESTIMATION OF THE ACCURACY OF THE SHORELINES The accuracy of these shorelines was analyzed in our study by considering the data sources and the extraction methods. More extensive work will be done in the future to further investigate this important issue. From the bundle adjustment and conjugate point matching, the DTM standard deviation derived from the aerial photographs is estimated to be 2.1m in X and Y coordinates. Considering the error introduced by resampling from the aerial photos of a 0.6m resolution, the 1-meter orthophoto has an estimated standard deviation of about 2.1m. The identification error of the shoreline from the orthophoto may introduce 1.5 pixels, that is 1.5m, considering those situations where the water and land separation is very hard to define. Therefore, the shoreline from the aerial orthophoto has an estimated standard deviation of about 2.6m if we apply a simple model of error propagation. The accuracy of the shoreline derived from 1-meter simulated IKONOS imagery should be about 2-4m (Zhou and Li, 2000), considering the fact that the accuracy of 3D ground points reaches 2-3m with GCPs and the accuracy of identifying and locating conjugate shoreline points is about 1.5 pixels (1-2m). The 4-meter IKONOS images have an accuracy of 23.6m (http://www.computamaps.com/ikonos_tech.html). After polynomial georectification using 8 GPS control points, the difference between the GPS and computed coordinates at the control points is about 6m. An optimistic estimation of the shoreline accuracy derived from the 4-meter IKONOS images in this specific case is about 8.5m. The accuracy of the shoreline derived from a CTM and the water level is affected by the accuracy of the CTM and the accuracy of the water level. The CTM comes from the DTM generated in turn from the aerial photos and the bathymetry. The DTM has an accuracy of 2.1m and the bathymetry has an estimated accuracy of 40m. When we merged these two data sets, we select the DTM grid points in the overlapping area. If there is gap between the two data sets, we interpolate the elevations using weights of 2/3 from DTM and 1/3 from bathymetry where the standard deviation of the CTM is about 13.4m. Overall the accuracy of the CTM is from 2.1m to 13.4m for the area where the water surface model intersects with the CTM. The water level data is accurate to several centimeters. Taking 5 degrees to be the coastal slope in worse cases and 5cm the water level error, we can estimate the horizontal accuracy of the intersected shoreline caused by the water level error to be about 0.6m. Therefore, the final accuracy of the digital shoreline is about 13.4m with the CTM contributing the greatest source of error. The NOAA T-sheets have large and medium scales: from 1:5,000 to 1:40,000. Taking 0.5 mm on the map as error source, the accuracy of the digitized shoreline from the T-sheet is about 2.5m to 20m. The shoreline digitized from the USGS DLG (1:24,000) should have an accuracy of within 12m. The ODNR map scale is 1:12,000. Again, using 0.5mm as the shoreline digitizing error, the estimated error of the shoreline is about 6m. © 2005 by CRC Press LLC Table 3.1 Estimated accuracy of the shorelines derived from various sources Shoreline T-sheet USGS DLG ODNR map Orthophoto CTM and water level IKONOS 1-meter simulated image IKONOS 4-meter image Estimated standard deviation 2.5m to 20m depending on scale 12m (1:24,000) 6m (1:12,000) 2.6 meters 2m-13m dep. on CTM quality 2-4 meters 8.5 meters 3.4 DIFFERENCE AND SHORELINE CHANGE ANALYSIS The shorelines were acquired at different times and differences can be seen in the results (Plate 3.1). There are two possible interpretations of the shoreline differences. One is that the shoreline indeed changed in the real world. The other possibility is that the differences were introduced as shoreline mapping errors. From our analysis of the accuracies above, the shoreline from the aerial orthophotos has the highest accuracy, and so we used it as the baseline for the difference analysis. The analysis was performed in raster format because of the efficiency of this method. The first task was to convert the vector shorelines into raster shorelines. The resulting pixel size was 2.5 meters. The procedure is outlined in Figure 3.2. Figure 3.2 Difference Analysis Procedure This analysis involved comparing every point on the test shorelines with the reference shoreline, which was derived from the aerial orthophoto, in a perpendicular direction toward the reference shoreline. In the buffer zone of the reference shoreline we created pixels whose value represented the distance of each © 2005 by CRC Press LLC ... - tailieumienphi.vn