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Subscribe whatwhenhow In Depth Tutorials and Information Examining GPS Data (GPS and GIS) Part 4 An Aerial Photo That Can Be Used as a Map:The Digital Orthophoto A digital orthophoto image is an aerial photograph, in digital form, with a very useful special property: It has been calibrated so that it may be used as a map; the coordinates of the map are precisely specified. As you may know, most aerial photographs, even when taken with the camera pointed vertically downward, have considerable distortion, due to several factors such as uneven terrain and lens distortion. Thus, if you measure a length on a conventional photograph that corresponds to a distance on the ground, an identical length measured elsewhere on the photograph usually will not correspond to the same distance on the ground. Said concisely, a standard aerial photograph does not have a consistent scale. In a digital orthophoto image, however, objects shown on the photograph at ground level are related by a points on objects above ground level (for example, on the roofs of buildconsistent scale, even if the terrain is dramatically uneven. (The locations of ings) are not, however, shown in their correct locations.) Digital orthophotos may come in many forms. The one we look at here is called a "tiff" (which stands for "tagged image file format." Pathfinder Office has the capability of displaying such files as background files. The extension of tiff names is usually "tif." If a tiff file is keyed to a geographical location it may be called a "geotiff"; geotiffs contain the parameters to anchor the image to the proper points on the earth’s surface. If a tiff image is keyed to a geographic location but contains no geographic parameters then it must be accompanied by a world file; world files usually end in the extension "tfw." Pathfinder office requires that a tiff be a nongeotiff and that it be accompanied by a world file.18 In the directory ByeByeSA are the following files: No_sa_ncr_doq.tif—a tagged image file—the special kind of digitized aerial photograph that can be used as a map, showing the highway the automobile was traveling at the time of transition. The term DOQ in the name stands for digital ortho quadrangle; quadrangle is just another term for an "almost rectangular" map.19 No_sa_ncr_doq.tfw—the "world file" for No_sa_ncr_doq.tif containing numeric parameters that tell Pathfinder Office where to place the image No_sa_ncr_doq.tif. NoSA_Moving.SSF—a subset of a file taken in a moving automobile covering the minutes of transition when SA was discontinued. {__ } Open __:\GPS2GIS\ByeByeSA\NoSA_Moving.SSF. Represent the file with a medium yellow line. Set the Time Zone to Eastern Daylight. Bring up the time line and note the time span of the file. If you look at Position Properties you will note that a number of fixes have been deleted. The idea was to capture just the time surrounding the transition. {_} Prepare the map of the GPS track so that the digital orthophotowill be properly positioned: Before the aerial image can be displayed the geographic parameters of the map must be properly set. The DOQQQQ20 (stands for digital ortho quarter quarter quarter quadrangle—since it is one sixtyfourth of a standard USGS 7.5 minute quadrangle) is in the U.S. State Plane 1983 coordinate system, Kentucky North Zone (1601). The datum is NAD 1983 (Conus). The coordinate units, altitude units, and distance units are Survey Feet. Using the Units and Coordinate System choices under the Options menu, make sure these parameters are set up correctly. {__ } Bring up a digital orthophoto image as a background to thefiles you have selected: From the File menu select Background. In the Load Background Files window remove any files that show up and then click Add. The window that appears (Add Background Files) might or might not represent the folder __ : \GPS2GIS\ByeByeSA. (If it doesn’t, navigate to that folder.) Select the file no_sa_ncr_doq.tif and click on Open. Read and okay a reminder if it shows up. Back in the Load Background Files window review the System, Zone, and Datum specifications. It probably appears that all is well, but it may not be. Click on Change to examine and correct parameters so they match the ones given in the previous step. Be particularly careful regarding Coordinate Units. In Explore New Zealand Temukan Keindahan Alam Kami dan Nikmati Liburan Impian Anda di Sini newzealand.com/NewZealand Facebook® Account Sign Up The World's #1 Online Community. Join for Free & Enjoy the Benefits! facebook.com

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Examining GPS Data Part 4

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  • 6/29/2015 ExaminingGPSData(GPSandGIS)Part4

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    InDepthTutorialsandInformation

    ExaminingGPSData(GPSandGIS)Part4

    AnAerialPhotoThatCanBeUsedasaMap:TheDigitalOrthophoto

    Adigitalorthophotoimageisanaerialphotograph,indigitalform,withaveryusefulspecialproperty:Ithasbeencalibratedsothatitmaybeusedasamapthecoordinatesofthemaparepreciselyspecified.Asyoumayknow,mostaerialphotographs,evenwhentakenwiththecamerapointedverticallydownward,haveconsiderabledistortion,duetoseveralfactorssuchasuneventerrainandlensdistortion.Thus,ifyoumeasurealengthonaconventionalphotographthatcorrespondstoadistanceontheground,anidenticallengthmeasuredelsewhereonthephotographusuallywillnotcorrespondtothesamedistanceontheground.Saidconcisely,astandardaerialphotographdoesnothaveaconsistentscale.Inadigitalorthophotoimage,however,objectsshownonthephotographatgroundlevelarerelatedbyapointsonobjectsabovegroundlevel(forexample,ontheroofsofbuildconsistentscale,eveniftheterrainisdramaticallyuneven.(Thelocationsofings)arenot,however,shownintheircorrectlocations.)

    Digitalorthophotosmaycomeinmanyforms.Theonewelookathereiscalleda"tiff"(whichstandsfor"taggedimagefileformat."PathfinderOfficehasthecapabilityofdisplayingsuchfilesasbackgroundfiles.Theextensionoftiffnamesisusually"tif."Ifatifffileiskeyedtoageographicallocationitmaybecalleda"geotiff"geotiffscontaintheparameterstoanchortheimagetotheproperpointsontheearthssurface.Ifatiffimageiskeyedtoageographiclocationbutcontainsnogeographicparametersthenitmustbeaccompaniedbyaworldfileworldfilesusuallyendintheextension"tfw."Pathfinderofficerequiresthatatiffbeanongeotiffandthatitbeaccompaniedbyaworldfile.18

    InthedirectoryByeByeSAarethefollowingfiles:

    No_sa_ncr_doq.tifataggedimagefilethespecialkindofdigitizedaerialphotographthatcanbeusedasamap,showingthehighwaytheautomobilewastravelingatthetimeoftransition.ThetermDOQinthenamestandsfordigitalorthoquadranglequadrangleisjustanothertermforan"almostrectangular"map.19

    No_sa_ncr_doq.tfwthe"worldfile"forNo_sa_ncr_doq.tifcontainingnumericparametersthattellPathfinderOfficewheretoplacetheimageNo_sa_ncr_doq.tif.

    NoSA_Moving.SSFasubsetofafiletakeninamovingautomobilecoveringtheminutesoftransitionwhenSAwasdiscontinued.

    {__}Open__:\GPS2GIS\ByeByeSA\NoSA_Moving.SSF.Representthefilewithamediumyellowline.SettheTimeZonetoEasternDaylight.Bringupthetimelineandnotethetimespanofthefile.IfyoulookatPositionPropertiesyouwillnotethatanumberoffixeshavebeendeleted.Theideawastocapturejustthetimesurroundingthetransition.

    {_}PreparethemapoftheGPStracksothatthedigitalorthophotowillbeproperlypositioned:Beforetheaerialimagecanbedisplayedthegeographicparametersofthemapmustbeproperlyset.TheDOQQQQ20(standsfordigitalorthoquarterquarterquarterquadranglesinceitisonesixtyfourthofastandardUSGS7.5minutequadrangle)isintheU.S.StatePlane1983coordinatesystem,KentuckyNorthZone(1601).ThedatumisNAD1983(Conus).Thecoordinateunits,altitudeunits,anddistanceunitsareSurveyFeet.UsingtheUnitsandCoordinateSystemchoicesundertheOptionsmenu,makesuretheseparametersaresetupcorrectly.

    {__}Bringupadigitalorthophotoimageasabackgroundtothefilesyouhaveselected:FromtheFilemenuselectBackground.IntheLoadBackgroundFileswindowremoveanyfilesthatshowupandthenclickAdd.Thewindowthatappears(AddBackgroundFiles)mightormightnotrepresentthefolder__:\GPS2GIS\ByeByeSA.(Ifitdoesnt,navigatetothatfolder.)Selectthefileno_sa_ncr_doq.tifandclickonOpen.Readandokayareminderifitshowsup.BackintheLoadBackgroundFileswindowreviewtheSystem,Zone,andDatumspecifications.Itprobablyappearsthatalliswell,butitmaynotbe.ClickonChangetoexamineandcorrectparameterssotheymatchtheonesgiveninthepreviousstep.BeparticularlycarefulregardingCoordinateUnits.In

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    KentuckytheyareinSurveyFeet!OkaytheCoordinateSystemwindow.OkaytheLoadBackgroundFileswindow.AnimageshouldappearintheMapwindow.

    {__}MakethemainPathfinderOfficewindowoccupythefullscreen.MaketheMapwindowwideasinFigure38,soyoucanhavePositionPropertiesandTimeLineupalso.ZoomuponthepartoftheyellowtrackthatcrossestheDOQ.LookattheGPStracksthatrepresentthepartofthetriponthedividedhighwaythattookplaceduringthetransitionfrom"SAon"to"SAoff."Firstthetrack,comingfromthewest,iswayofftheroad,thenyouseesomejaggedspikesthatoccurredprobablyduetothetransition,andfinallythetraceisatleastintherightofwayofthehighway.WithPositionpropertiesyoucanascertainthatthetransitionwascompletedatabout13secondsafter12:01AMon2May2000.

    PROJECT3EYourData

    NowthatyouhaveagoodideaofhowtousePathfinderOfficetodisplayandanalyzedatatakenwithaGPSreceiver.

    {__}Beforebeginningthecomputerwork:

    Unlessyourinstructortellsyouotherwise,youmustusethemachineontowhichyouloadedyourdata

    Figure38.Watchselectiveavailabilitygoaway.

    RecallthenameoftheProjecttowhichyoutransferredyourdata.ItisprobablyDATA_yis(where"yis"indicatesyourinitials).Thefolderinwhichyouwillworkis__:\GPS2GIS\DATA_yis.

    Bringwithyouthefullname(s)ofthefile(s)youcollected.AlsohavewithyouthevaluesyoucalculatedbyhandinProject1C.

    Bringthedetailedmap(s)ofthearea(s)whereyoutookthedatasoyoucancomparewhatappearsonthescreenwiththefeaturesshownonthemap.

    {__}Ifnecessary,startthePathfinderOfficesoftware.SelectProjectsintheFilemenu.MakesurethattheProjectnameis"DATA_yis."ClickontheOpenFoldericonontheProjecttoolbar.Clickthe"Details"buttontoseethedatesonthefilesandfolders.Reviewthefilesthataretheretobesurethatyouareintherightfolder.YoushouldseeaBackup,Base,andExportfolderaswellasyourfiles.Select,withasingleclick,thefirstofthefilesyoutook.Lookattheinformationaboutthatfileatthebottomofthewindow.Closethewindowwithoutopeningthefile.

    {__}ExaminethemapyouusedinProject2Atodeterminethecorrectunits,coordinatesystem,anddatumtouse.Setupthesoftwarewiththeseparameters.

    {_}Therepresentationnowonthescreenshouldbeofthepoints(fixes)youcollected,connectedinthetimeorderinwhichtheywereloggedbythereceiver.

    Thefollowingstepsassumethatyoutookpointdata.

    {__}YousettheGeoExplorerroveroptionsbeforethatsessionaccordingto:

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    Coordinates~Deg&Minutes(anddecimalfractionalpartsthereof)

    Datum:(settoyourmap)

    Units~Custom~Distance:(settoyourmap)

    Units~Custom~AltitudeUnits:(settoyourmap)

    Units~Custom~AltitudeReference:Geoid(MSL)

    Date&Time~SetLocalTime:(adjusttolocaltime)

    SetthePathfinderOfficeoptionstotheequivalents.

    {__}SelectPositionProperties.Clickonafixthatistowardthemiddleoftheclusterofpoints.Readitslocationatthebottomofthescreenandwriteitdownhere.

    Latitude_____________

    Longitude____________

    Altitude_

    Isitaboutwhereyouexpectedittobe?CompareitscoordinatevaluesagainsttheaveragedvaluesinPROJECT1Cthatyouwrotedownatthattime.

    WecouldmakeaPoint_genericfile(averageallthefixesintoasinglepoint)aswedidinProject3B,butletslookatanotherwayofgettingtheaverage.ThedatacollectedbytheGPSreceiveranddownloadedintothemachinetaketheformofrecords,whicharenumberedstringsoftext.AgivenGPSfilewillhavequiteafewrecordsatleastoneforeveryfixtaken,andusuallyalotmore.YoucanlookattheserecordswithaTrimbleUtilitycalledtheSSFRecordEditor.WhentheRecordEditorisinvokeditautomaticallyaveragesthecoordinatesofallthefixesandappendsadditionalstatisticalrecordstothefileattheend.InthestepbelowyouwillopenthefileintheRecordEditor,lookbrieflyatafewrecords,andthenreadthecoordinatesofthe"average"point.

    {__}Gointothefilemenuandpick"Close"toclosethecurrentfile.(TheRecordEditorwillnotcalculatestatisticsonafileifthefileisopen.)Youhavealreadysetuptheproperparametersofdatum,coordinatesystem,andunits.IntheUtilitiesmenuselectOther,thenSSFRecordEditor.Theappropriatewindowwillcomeup.UnderFileinthatwindowclickOpen.Youshouldthenbeabletoselectthefileyouhavebeenworkingwithfromthewindowthatshowsup.ClickOpen.MaketheSSFRecordEditorwindowoccupythefullscreen.

    Theresultingwindowisdividedintotwoparts.(Youcanvarythepositionofthedividinglinebetweenthembydraggingitwiththemouse"Split"intheViewmenualsoworks.)Inthetoppartaretherecordsofthefile,beginningwithaheaderrecordwhosethreeletterdesignationisHDR.TheHDRrecordishighlighted.Inthelowerpartofthewindowaredetailsinamorereadableformatrelatingtothathighlightedrecord.Also,somerecords,likeHDR,containinformationnotshownontheupperpartofthewindow.Thisinformationappearsinthebottomportionofthewindow.Makesurethatthebottompartofthewindowislargeenoughtoseeallthetextlines.PleaseseeFigure39.

    MostSSFfileshaveseveraltypesofrecords.Shortlyyouwillseethattowardthemiddleofthefileyouwillfindposition(POS)recordsthattellyouthetimeandpositionofeachseparatefix.Attheendofthefilearestatisticsrecords(STS).

    {__}LookattheinformationintheHDRrecord.Thenusethedownarrowkeyand/orPageDowntolookatrecordsfurtherdowninthefile.Finally,dragtheverticalslidertothebottomsothattheSTSrecordsthatwereaddedtotheendofthefileshowup.Usethe"uparrow"keytoselectthe"LatitudeStat"record.Writethecoordinatesofthatandthefollowingrecordshere.

    MeanLatitude

    MeanLongitude:

    MeanAltitude:21

    {_}FromtheFilemenuexittheSSFRecordEditor.

    {__}Someofthequestionsbelowhaveseveralblanks.AssumingyoutookdataforbothPROJECT1BandPROJECT2Ainthesamelocation,recordthecalculationsyoumadeinPROJECT1C.

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    Figure39.FindingstatisticsfromtheRecordEditor.

    {__}Setupthedatum,coordinatesystem,andunitstocorrespondtothoseofthemapyouusedinPROJECT2B(whereyoumovedtheantennaalongapath).

    {__}BringinthefileofPROJECT2Banddisplaythepathyoutookwhilemovingtheantenna.Nowqueryvariouspointsonthedisplayandcomparetheircoordinateswiththoseonthemap.Doesthedisplaycorrespondtotheactualpathyoutook?

    {__}AddthefilethatcontainsthefixesyoucollectedinPROJECT2A.Arethetwositesinthecorrectrelativeposition?Ifnot,youmaybeusingdifferentprojections,differentdatums,ordifferentcoordinatesystems.

    Nextpost:DifferentialCorrection(GPSandGIS)Part1

    Previouspost:ExaminingGPSData(GPSandGIS)Part3

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