Tian-Yuan Shih

Lidar-based change detection and change-type determination in urban areas

Change detection of objects, such as buildings, is essential for map updating. Traditionally, detection is usually performed through spectral analysis of multi-temporal images. This article proposes a method that employs multi-temporal interpolated lidar data. The objective of this study is to perform change detection and change-type determination via geometric analysis. A shape difference map is generated between the digital surface models in two different time periods. The areas with small shape differences are treated as non-changed areas and are excluded from the segmentation. The objects properties are then applied to determine the change types. Experimental results demonstrate that the proposed scheme achieves accuracy as high as 80%. Most of the errors from this study occurred in small or vegetation areas.

The Geomorphometry of Rainfall-Induced Landslides in Alishan Area Obtained by Airborne Lidar and Digital Photography

For understanding the distribution of slope angles, OHM and roughness of the rainfall-induced landslides in Alishan Area of Central Taiwan, a survey was carried out with airborne Lidar and aerial digital camera to obtain DEM and DSM of 1 m grid and color orthophotos of 50 cm grid. DEM, DSM and orthophotos are georeferenced and co-registered. The 106 landslide polygons derived from photo-interpretation are used for extracting slopes, OHM and roughness. Results show that the average slope angle of landslides is 41 degrees with a standard deviation of 14 degrees; average OHM is 4.4 m with a standard deviation of 6.3 m; average roughness is 2.05 m with a standard deviation of 2.56 m. It is also observed that scale effects are obvious for roughness but not for slope and OHM when the grid is larger than 40 m, which is the average dimension of landslides. These morphometric parameters can be further applied in the automation of landslide inventory.

Parcel boundary identification with computer-assisted boundary overlay process for Taiwan

This study investigates the design of a process for parcel boundary identification with cadastral map overlay using the principle of least squares. The objective of this research is to provide an objective tool for boundary identification survey. The proposed process includes an adjustment model, a weighting scheme, and other related operations. A numerical example is included.

Lidar DEM for characterizing the volcanic landforms of tatun volcanoes in metropolitan taipei

Tatun volcano group is a cluster of dormant volcanoes surrounding metropolitan Taipei. Rugged terrain, monotonic lithology and dense vegetation covers are adverse factors for mapping the geological structures. In this study, airborne lidar survey was conducted to obtain a bare ground DEM with 2 m grid and with an accuracy of decimeters. Shaded-relief images, pit-patterns and drainage networks are then derived from these DEMs for visual interpretation. 51 volcanoes are thus recognized. Two fissures running through the highest volcano in this area, namely Mount Seven-Stars, are extending 2000 m and 1000 m, respectively. The largest width and depth of the opening of the ruptures is located in the west flank of the volcanic cone. The slope angle of the east-wing of the volcanic cone is 36deg, whereas the angle of the west-wing is only 24.5deg. The opening of the west fissure is larger and its extension is longer than the east one. Thus, the west side can be subjected to an active extensional process of strain. The fissures could be resulted from the ongoing regional extension of the Tatun volcanic area due to the plate subduction and collision of the Eurasian plate and the Philippine sea plate.

Effects of JPEG2000 on the information and geometry content of aerial photo compression

The standardization effort of the next ISO standard for compression of the still image, JPEG2000, has recently reached International Standard (IS) status. This waveletbased standard outperforms the Discrete Cosine Transform (DCT) based JPEG in terms of compression ratio, as well as, quality. In this study, the performance of JPEG2000 is evaluated for aerial image compressions. Different compression ratios are applied to scanned aerial photos at the 1:5 000 scale. Both the image quality measurements and the accuracy of photogrammetric point determination aspects are examined. The evaluation of image quality is based on visual analysis of the objects in the scene and on the computation of numerical indices, including RMSE, entropy, and Peak Signal-to-Noise Ratio (PSNR). The geometric quality of JPEG2000 with different compression ratios is studied for some photogrammetric operations, including interior orientation, relative orientation, absolute orientation, and DSM generation. The objective of this study is to explore the possibility of JPEG2000 for replacing JPEG as a standard in photogrammetric operations.

A QUALITY ASSURANCE APPROACH FOR LAND SUBSIDENCE INTERPOLATION

Abstract Land subsidence in southwest Taiwan is mainly caused by ground-water withdrawal and a monitoring network of benchmarks has been established to observe this subsidence. Using results from differential leveling, the magnitudes of land subsidence from 1992 to 1994 were calculatedfor 50 benchmarks. The calculated maximum magnitude of land subsidence is 15.4 centimeters between these two years. Spatial interpolation is frequently required to provide information for assessing the distribution and extent of subsidence. In this study, geostatistics were applied to describe and characterize the spatial structure of sample data. Thus, an implementation procedure was proposed for the quality assessment of interpolation. The procedure includes geostatistical analysis, outlier detection, and comparison of different estimation techniques. The results show that the approach proposed in this study is effective to ensure the quality of interpolation.

A statistical analysis for characterizing landslide caused by heavy rainfall and severe earthquake

Heavy rainfall and earthquakes are the two major factors inducing landslides in Taiwan. The distribution of area size is the most basic quantitative parameter of landslides. Therefore, the purpose of this study is to characterize the scale and spatial difference of rainfall-induced as compared with those of earthquake-induced landslides. Two representative landslide cases, Toraji typhoon occurred in 2001 and 921 earthquake occurred in 1999, are used to analyze the causes of different kinds of landslide disasters in the paper. The test area for the 921 earthquake case is 3700 km2, the recognized number of landslide in the event is 7279, and total area for the landslides is 14766 Ha. Moreover, the maximum area of one landslide is 532 Ha, the average area for the landslides is 2 Ha, and its corresponding standard deviation is 13 Ha. In the test case of Toraji typhoon, the total study area is 8847 km2, the number of landslides is 10359, and total area for the landslide is 22305 Ha. The maximum area of a landslide is 232 Ha, the average area for the landslides is 2.2 Ha, and its corresponding standard deviation is 6.2 Ha.

Landslide detection by indices of LiDAR point-cloud density

The deliverables of an airborne LiDAR survey usually include all points, ground points, digital surface models (DSM) and digital elevation models (DEM). Indices of point clouds tested in this study include density of all points, density of ground points, density of only returns, and density of multiple returns. Shallow landslides are the most common landslides triggered by torrential rainfalls and explicit fresh scars after rainfall events. Multiple returns in forest area give the possibility of differentiating landslide scars from vegetated lands. Classification results from the indices derived from these four kinds of densities are verified by the result obtained by manual interpretation of the derived nDSM images. The experiment is carried out using the dataset obtained in I-Lan County after Typhoon Kalmaegi on 17 July 2008. The results show that a proper definition of the parameters for the indices is most critical for the detection of shallow landslides.

The performance of GPS Standard Positioning Service without selective availability

Abstract The accuracy of the Standard Positioning Service of GPS has greatly improved after the termination of Selective Availability. Based on GPS observations from permanent GPS-fixed stations, the actual accuracy of GPS without S4 is analyzed. For Station NCTU, the local geodetic coordinates are computed with the CIA code pseudo-ranges, and then transformed into a local grid system with Trans-Mercator projection. From 77984 single epoch observations collected in twenty-nine days, the 95% accuracy is foundto be ∆E: 17.6544m, ∆N: 1l.8236m, ∆H: 31.8018m. For five selected UNAVCO stations, the axes of error ellipsoid are computed.