CH2-1空间信息基础

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1、第二章空间数据结构靖娟利土木工程系主要内容空间信息基础1空间数据的表达2空间数据模型与数据结构3矢量数据模型3.13.2不规则镶嵌数据模型3.3栅格数据模型§2.1空间信息基础空间信息基础地球模型地理参考与坐标系地图投影一、地球模型地球自然表面水准面大地水准面地球椭球体地球形状的概括一、地球模型地球表面水准面大地水准面铅垂线地球椭球体一、地球模型—椭球体SpheroidsandspheresTheshapeandsizeofageographiccoordinatesystem'ssurfaceisdefinedbyasphereorspheroid.Althought

2、heearthisbestrepresentedbyaspheroid,theearthissometimestreatedasaspheretomakemathematicalcalculationseasier.Theassumptionthattheearthisasphereispossibleforsmall-scalemaps(smallerthan1:5,000,000).Atthisscale,thedifferencebetweenasphereandaspheroidisnotdetectableonamap.However,tomaintainac

3、curacyforlarger-scalemaps(scalesof1:1,000,000orlarger),aspheroidisnecessarytorepresenttheshapeoftheearth.Betweenthosescales,choosingtouseasphereorspheroidwilldependonthemap'spurposeandtheaccuracyofthedata.一、地球模型—椭球体SpheroidsandspheresAsphereisbasedonacircle,whileaspheroid(orellipsoid)isb

4、asedonanellipse.Theshapeofanellipseisdefinedbytworadii.Thelongerradiusiscalledthesemimajoraxis,andtheshorterradiusiscalledthesemiminoraxis.Rotatingtheellipsearoundthesemiminoraxiscreatesaspheroid.Aspheroidisalsoknownasanoblateellipsoidofrevolution.Thefollowinggraphicshowsthesemimajorands

5、emiminoraxesofaspheroid.各种地球椭球体模型椭球体名称年代长半轴（米）短半轴（米）扁率白塞尔(Bessel)1841637739763560791：299.15克拉克(Clarke)1880637824963565151：293.5克拉克(Clarke)1866637820663565841：295.0海福特(Hayford)1910637838863569121：297克拉索夫斯基1940637824563568631：298.3I．U．G．G1967637816063567751：298.25埃维尔斯特(Everest)183063772766

6、3560751：300.8一、地球模型—基准面Datumadatumdefinesthepositionofthespheroidrelativetothecenteroftheearth.Adatumprovidesaframeofreferenceformeasuringlocationsonthesurfaceoftheearth.Itdefinestheoriginandorientationoflatitudeandlongitudelines.Geocentricdatumswhichrelatescoordinatestotheearth'scentero

7、fmass.Anearth-centered,orgeocentric,datumusestheearth'scenterofmassastheorigin.ThemostrecentlydevelopedandwidelyuseddatumisWGS1984.Itservesastheframeworkforlocationalmeasurementworldwide.LocaldatumsAlocaldatumalignsitsspheroidtocloselyfittheearth'ssurfaceinaparticulararea