数字地形模型 (4).pdf

1 Digital Terrain Modelling Chapter 3 Techniques for Acquisition of DTM Source Data Contents 3 1 Data sources for digital terrain modelling 3 2 Photogrammetry 3 3 Radargrammetry and SAR Interferometry 3 4 Airborne laser scanning Lidar 3 5 Cartographic digitisation 3 6 GPS for direct data acquisition 3 7 The comparison between DTM data from different sources 3 1 Data sources for digital terrain modelling Data sources means the materials from which data can be acquired for terrain modelling and by DTM source data means data acquired from data sources of digital terrain modelling Different techniques for different sources Field surveying by using total station theodolite and GPS global positioning systems for direct measurement from terrain surfaces Cartographic digitization by using existing topographic maps and digitisers Photogrammetry by using stereo pairs of aerial or space images and photogrammetric instruments Laser scanning actively providing its own illumination in the form of lasers SAR radargrammetry interferometryand radarclinometry actively providing its own illumination in the form of microwaves 2 From terrain surface The continents occupy 29 2 of the earth s surface Relief varies from place to place and is covered by natural and cultural features apart from water Different measurement techniques may be used because some techniques may be less suitable for some areas From aerial and space images Aerial images are the most effective way to produce and update topographic maps the most valuable data source for large scale production of high quality DTM taken by metric cameras mounted on aerial planes Aerial photographs can be classified into different types based on different criteria Based on colour colour true or false and monochromatic photographs Based on the attitude of photography vertical i e main optical axis vertical tilted 3 and oblique 3 photographs Commonly used aerial photographs are tilted photograph Based on angular field of view normal wide angle and super wide angle photography From aerial and space images Types of aerial photographs based on angular field of view 3 H f Aerial photo negative Aerial photo positive Perspective centre lens Main optical axis Aerial camera and aerial photography a An aerial Camera b Geometry of aerial photography the scale of the aerial photograph Form in analogue form recorded on films and in digital form scanned CCD charge coupled device camera Acquisition photogrammetry airborne scanners scanned space images radar From existing topographic maps Every country has topographic maps and these may be used as another main data source for digital terrain modelling These form a rich source of data for digital terrain modelling But for some developing countries the data sources maybe poor A topographic map From existing topographic maps The largest scale of topographic maps which cover the whole country with contour lines is usually referred to as the basic map scale It indicates the best quality of DTM that can be obtained from existing contour maps Some basic map scales China UK USA 1 50 000 1 10 000 1 24 000 topographic maps the metric quality contour map density of contour lines and the accuracy of the contour lines themselves 4 Topographic maps at different scales Map scales and commonly used contour intervals Map scales and commonly used contour accuracy In general it is expected that the height accuracy of any point interpolated from contour lines will be about to 1 2 to 1 3 of the contour interval CI 3 2 Photogrammetry The word photogrammetry comes from Greek words photos meaning light gramma meaning that which is drawn or written and metron meaning to measure It originally signified measuring graphically by means of light Whitmore and Thompson 1966 Photogrammetry and Remote Sensing is the art science and technologyofobtaining reliable information from noncontact imaging and other sensor systems about the Earth and its environment and other physical objects and processes through recording measuring analyzing and representation http www isprs org isprs html The development of Photogrammetry Photogrammetryhas undergone four stages of development each of these cycles are approximately 50 years long Plane table photogrammetry 1850 1900 Analog photogrammetry1900 1960 Analytical photogrammetry1960 present Digital Photogrammetry1990 present In 1849 A Laussedat an officer in the Engineering Corps of the French Army is regarded by many as the father of photogrammetry 5 a Optical plotter b Optical mechanical plotter c Analytical plotter d Digital photogrammetric workstation The characteristics of the four stages of photogrammetry Basic principles The fundamental principle of photogrammetry is to make use of a pair of stereo images or simply stereo pair to reconstruct the original shape of 3D objects Lateral overlap forward overlap A pair of aerial photographs with 60 overlap overlappingfiducial mark 6 To measure the 3 D coordinates of the objects on the stereo model 3D model Two overlapping photographs stereocomparator S1 S2 a A Z X Y a 333 222 333 111 SASASA SASASA SASASA SASASA ZZcYYbXXa ZZcYYbXXa fy ZZcYYbXXa ZZcYYbXXa fx A stereo model is formed by projecting images points from a stereo pair The mathematical express Collinear equation In the mathematical expression XYZ is a geodesic coordinate system and i 1 2 3 are the functions of the three angular orientation elements i e as follows coscos cossincossinsin sinsincoscossin sin coscos sincos cossin cossinsinsincos sinsinsincoscos 3 3 3 2 2 2 1 1 1 c b a c b a c b a About the mathematical expression Basic orientations Interior orientation using fiducial marks image coordinates to unify the image coordinate system Relative orientation to restore the stereo model by removing the Y parallax using at least 6 points observations Absolute orientation to scale and orient the stereo model based on the GCP 7 Epipolar geometry Each pair of bundle rays must be coplanar with the base DPW Digital Photogrammetric Workstation Automatic image processing matching VirtuoZoVirtuoZo GPS INS for georeferncing On the fly processing Real time photogrammetry Towards 3 3 Radargrammetry and SAR Interferometry Radargrammetryacquires DTM data through the measurement of parallax InSAR acquires DTM data through the determination of phase shifts between two echoes Radarclinometry acquires DTM data through shape from shading Space Shuttle Endeavour 8 Synthetic aperture imaging radar SAR is a microwave imaging radar developed in the 1960 s to improve the resolution of traditional real aperture radar It receives and records echos reflected by the target and then maps the intensity of the echo into a grey scale to form an image It is able to take clear pictures day and night under all weather conditions Synthetic Aperture Radar SAR Radar imaging geometry The angular fields in the flying direction and the cross track direction are related to the width and the length L of the radar antenna Synthetic Aperture Radar SAR Flying track Orbit Antenna Cross track Nadir Footprint Mid slant range Rm Projected Orbit X Y Flying Height L w Swath Nadir ef Image Plane Y Antenna Slant rangeR Cross Track Far slant range EF H Near slant range Projection of radar image The resolution of the radar image The minimum distance between two distinguishable objects the most important measure of radar image quality It is defined by the azimuth resolution in the flying direction x and by the slant range resolution in the slant rage direction R or the ground range resolution in the cross track direction y y decreases near to the nadir it is the reason why SAR is always side looking Resolution of radar images The azimuth resolution x is dominantly determined by the position and size of the antenna If a C band microwave 5 66cm real aperture radar onboard the satellite is employed to take images with an azimuth resolution of 10m from 785km away the required length of its aperture is longer than 3km Imaging geometry of SAR The principle of SAR imaging 9 The azimuth resolution x of the synthetic aperture radar SAR is much improved based on the principle of the Doppler frequency shift caused by the relative movement between the antenna and the target Indeed it means that the azimuth resolution x of a SAR is only determined by the length of the real aperture of an antenna independent of the slant range R and the wavelength As a result it is possible to acquire images with 5m azimuth resolution by an SAR with a 10m real aperture length onboard ERS 1 2 The principle of SAR imaging Azimuth Direction of slant range Pixel i ebaiba 22 An example of the SAR image the plane coordinate system of the SAR image and the complex number expression of the pixel It is the use of phase information that makes InSAR technology special Radarclinometry Shade shadows and occluded areas are familiar phenomena that can help judge size and shape of objects by providing an impression of convexity and concavity Shadowing method providing localized cues along special contours is principally used to derive relative elevation of a specific target A pair of SAR images of the same area taken at slightly different positions can be used to form an interferogram and the phase differences recorded in the interferogram can be used to derive topographic map of the earth s surface This technology is called Interferometric SAR InSAR or SAR interferometry Interferometric SAR InSAR 10 Principles of InSAR InSAR is a signal processing technique rather than an instrument at the present time It derives heightinformation by using the interferogram Principles of InSAR The process of DTM data acquisition by InSAR An example of InSAR interferogram Longitude Latitude 11 Contour diagram of DTM of the same area produced from DTM generated by InSAR Coast line Taiwan Straits Longitude Latitude Radargrammetry Similar to photogrammetry radargrammetry is to form a stereo model for 3D measurement In radargrammetry two SAR images collected with the same side or opposite side geometry are used to form the stereo model DEM based on RADARSAT stereopair Principles of Radargrammetry 3D reconstruction relies on determining the sensor object stereo model searching for corresponding pixels from two overlapping SAR images using imaging matching techniques and determination of 3D coordinates by solving the intersection problem 12 Stereo configuration of radargrammetry 1S2S 1S 2S 1 V 1 R 2 R P P X Y Z O 2 V terrain features such as topographic slopes geographical conditions and geometric distortions in relation to radar look angles and intersection angles opposite side stereo configuration is superior to the same side stereo Factors affect accuracy of DTM by radargrammetry Hong Kong ERS 1 SAR images 113 8113 9114 0114 1 22 322 422 522 6 0 100 200 300 400 500 600 700 800 a on 2 Mar 1996 b on 18 Mar 1996 DTM generated by a and b 3 4 Airborne laser scanning Lidar ALS system airborne LIDAR LIght Detection And Ranging An example of 3D city model acquired by Lidar 13 Airborne laser scanning system a laser range finder LRF a computer system to control the on line data acquisition a storage medium a scanner and a GPS INS system for determining the position and orientation of the system Airborne laser scanning system It is the type of materials hit by the pulses The wavelength of the laser lies in or just above the visual range of the electromagnetic spectrum i e in the range of 1040 1060 nm Range and range resolution Pulse laser R is the range distance between sensor and object in meters m R is the range resolution in centimetres cm t is the time interval between sending and receiving a pulse or echo in ns c is the speed of light 300 000 km s t is the resolution of time measurement in ns Range and range resolution CW laser Continuous Wave f Hz is the frequency in Hz is the phase for CW lasers in rad is the phase resolution for CW lasers in rad 14 Maximum unambiguous range For pulse lasers they depends on two major factors the maximum range number of bits of the time interval counter and the pulse rate CW laser Where refers to the long wavelength corresponding to the low frequency of a CW laser Accuracy of laser ranging For pulse laser For CW laser is the pulse laser ranging precision in m is the rise from 10 to 90 of its maximum value time of the pulse in ns is the noise input bandwidth pulse lasers in Hz is the peak power applies only to pulse lasers in Watt is the CW laser ranging precision in m is the noise input bandwidth CW lasers in Hz and is the average power applies to pulse and CW lasers in Watt is the short wavelength corresponding to the high frequency of a CW laser From laser point cloud to DTM The process of acquiring ALS data filtering noise outliers or gross errors classification buildings or vegetation and modelling Accuracy height accuracy is in the range of 10 60 cm planimetric accuracy is 0 1 3m Obtained DTM from DSM using filtering Remove the vegetation buildings 15 3 5 Cartographic digitisation Cartographic digitization methods Automated Manual Line following Scanning Manual line following Automated line following Manual scanning Automated scanning Line following digitalization a mechanically based digitisation system a solid state digitising tablet An example of tablet digitizer manual line following digitisation operator doesn t need to do the line following data redundancy and the fidelity of the results to the original line semi automated line following devices An operator is still required to supervise the system execute various operations such as the initial positioning of the device on contours guide the device through areas of closely packed contours and cliffs insert contour elevation values etc the system is very expensive 16 Raster scanning make fully automated digitisation possible Each line scan is divided into resolution units 0 if nothing is present 1 if there is a line Vectorisation follows can be manual on screen digitization or automated drum scanners flat bed right scanners Map Y X Scan Head Map X Y Scan Head Examples Original raster map Vector contour lines 3 6 GPS for direct data acquisition The GPS satellite constellation About GPS Global Positioning System Full name NAVSTAR GPS NAVigation Satellite Timing And Ranging Global Positioning System Developed and owned by the US Department of Defense DoD Provides 24 hour world wide positioning capability 17 GPS Segments Space segment GPS satellites orbiting around the earth and sending GPS signals to the earth Control segment Stations on the earth that monitor and control the satellites User segment Any body who has the devices GPS receivers to receive GPS signals The principles of GPS measurement Principle range intersection the positions of