Guidelines for Calibrated Scale Bar Placement and Processing AuthoredbyCulturalHeritageImaging

Tosupporthighlyaccuratemeasurementof3Ddataofanobjectorasite,itisessentialforpractitionersofphotogrammetrytolearnthecorrectplacementanduseofcalibratedscalebars.

Asyoupreparetoimageasubject,youstartbyplacingthescalebarson,around,ornexttotheobjectorsite.Whenyouhavetakenaseriesofoverlappingimagesthatincludethescalebars,youarereadytoprocesstheimagesettocreatea3Dmodel.AgisoftPhotoScanProfindstheexactcenterofthetargetsonthescalebarsinyourphotos,enablinghighlyaccuratemeasurementofanyfeatureoftheimagingsubject.

Scale Bars

NOTE:Thisdocumentprovidessometipsthatworkwithanytarget-basedcalibratedscalebars.ThesetofscalebarsshownhereisavailablefromCulturalHeritageImaging(CHI).Thisuniquescalebardesignwasdevelopedandfield-testedbyaseniorteamofphotogrammetryexpertsfromtheUSBureauofLandManagement.ThesebarsarecalibratedbyCHIstaffto1/10mmaccuracyorbetter,withtheactualcalibratedmeasurementbetweeneachtargetwrittenonthescalebar.Ifyouareinterestedinpurchasingthisset,youcanorderthemfromtheCHIwebsite.

Guidelines for Calibrated Scale Bar Placement and Processing, Version 2.0 © Cultural Heritage Imaging 2015, All Rights Reserved 2

How to Place Scale Bars Placethescalebarsinasceneoraroundanobjectofinterestsothetargetscanbeclearlyseeninmultipleoverlappingimages.Youshouldconsiderthescalebarstobepartofthe“areaofinterest”foryourphotogrammetryproject,andmakesurethatyoursetofoverlappingimagesextendsbeyondtheplacementofthescalebars.

Thescalebarshavetargetsthatareusedasmarkerswhenthebarsareplacedinasceneoraroundasubject.Whenyoursetofoverlappingphotosincludesthebars,thesemarkersserveasreferencepointsforscaledefinition,andalsohelptheprocessingtoolalignimagesaccurately.

Thescalebarsdonotneedtoappearineveryphoto.Theyshouldbewellcoveredbyoverlappingphotoswithgoodgeometrysothattheircalibratedvaluescanbeusedtoprovideaccuratescaleforthewholephotogrammetryproject.

ImportantNote:Donotmovethescalebarsduringphotogrammetricimagecaptureofyourproject.Ifascalebarisaccidentallybumpedormoved,removeitfromthescene.Donotattempttoplaceitbackinthescene,asitwillnotbeinexactlythesameplace.Scalebarsmustbeinaconstantlocationrelativetoyourprojectinordertobeusedaccurately.

Examples of Placement

Thefollowingfiguresshowsomewaysofplacingscalebars.

Figure 1: On the left, three scale bars in place around a smaller subject. On the right, two scale bars placed on the outside of a large painting. Two additional scale bars (not shown) are placed on the far side of the painting.

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Figure 2: Scale bars in place around a small kiosk-style building. Some are placed on the ground around the subject, and some are on the subject.

Whenusingscalebarsplacedontheground,outsidetheprimarysubjectarea,youmusttakeadditionaloverlappingphotosthatincludethescalebarsinordertousethemforaccuratemeasurementinyourproject.

Inlargerprojects,placethescalebarsasfarawayfromeachotheraspossiblesothatyoucanvalidatethescaleacrosstheentireproject.

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Figure 3: Four scale bars in place near an area of rock art and inscriptions to be imaged. These scale bars are placed near, but not covering, the inscriptions that appear above them on the rock.

How Many Scale Bars to Use in Your Project

Whilephotogrammetrysoftwarecanaddscaletoyourprojectwithasinglescalebar,itisadvisabletoplaceaminimumofthreescalebarsinyourproject.Werecommendfour.Twobarsprovideabasiccheck,andthreeormoreaddconfidenceandstatisticalreassurance.Usingfourbarsisideal,especiallyforlargerprojectsandoutdoorenvironments,becauseitisalltooeasyforascalebartobemovedbythewind,kicked,orotherwisedisplacedduringcapture.Ifthishappens,theprojectwillstillhavetheintegrityofthreevalidscalebars.

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Processing Data with Scale Bars TheprocessingworkflowinAgisoftPhotoScanProVersion1.2includesthesebasicsteps:

• Setthescalebaraccuracy

• Detectthetargetsonthescalebarsintheimagesettogeneratemarkersineachphotothatincludesscalebars.

• Createscalebarsobjectsbetweenspecificmarkers.

• Addthecalibratedscalebardistancedataforthescalebarobjects.

Eachofthesestepsisdescribedfurtherbelow.MoreinformationonplacingmarkersandcreatingscalebarscanbefoundintheAgisoftPhotoScanProUser’sGuide,Chapter4,ReferencingandMeasurement.

Set the Scale Bar Accuracy

Beginningwithversion1.2ofPhotoScanPro,itispossibletosetthescalebaraccuracy.TheScalebarsfromCulturalHeritageImagingarecalibratedto1/10mmaccuracyorbetter.YoushouldsettheScaleBarAccuracyto0.0001ifyouareusingthesescalebars.Otherwise,setthisvaluetowhateveraccuracyiscorrectforthescalebarsyouareusing.(thedefaultvalueis0.001)

Detecting Markers

Afteraligningphotos,andoptimizingyourproject,usetheDetectMarkersdialogtodetectthetargetsonthescalebars,andusethemtoautomaticallygeneratemarkers.MakesuretheReferencePaneisvisibleandchooseTools>Markers>DetectMarkers.

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Figure 4: Detect markers for scale bars.

TIP:Wesuggestthatyoustartwithatoleranceof20,andmakeithigherifnotalltargetsaredetected.

Thisgraphicalsoshowsthedifferentkindsofmarkersthatmayappearonscalebars.

ThescalebarsfromCHIincludeallthreekindsofmarkertargetssupportedbyPhotoScan,asshowninthefigure.Markertargetscanbecircularcoded,circularnon-codedornon-codedcross.

Anadvantageofcodedtargetsisthattheyarealwaysnamedconsistently.Codedtargetsarenamed“targetxx,”wherexxisthenumberofthecodedtarget,printedonthescalebar.Non-codedtargetsarenamedsequentiallyintheorderthesoftwareidentifiesthem,andlabeled“pointxx,”butthenumberingisnotreproducibleindifferentprojects.

TIP:Ifyouareusingbothcircularcodedandcircularnon-codedtargets,youshoulddetectthecodedmarkers(thecircular12-bitmarkerinthisexample)beforeyoudetectanynon-codedcircularmarkers.Thisisbecausethecenterofthecircularcodedmarkerisacircle,anditwillbedetectedasanon-codedcircleifthereisnotalreadyamarkerpresentfromdetectingthecodedmarkers.Thenwhencodedmarkersaredetected,youmayhaveduplicationofmarkers.

Thecentersofcodedandnon-codedcirculartargetsareplacedwithhighprecisionbecausethecenterofthecirclecanbedetectedalgorithmically.

Thecentersofnon-codedcross-typetargetswithcalibrateddistances,aswellasthecornersofsmallscalesmadeupofblackandwhitesquaresareplacedwithhighprecisionbecausethecenterpointofthecrosscanbedetectedalgorithmicallybythesoftware.

Althoughyouwillrarelyusethesescalemarkersforsettingscaleinyourproject,youshouldretainanydetectedscalemarkers.Theymarkadditionalknownpointsthatcanaidthesoftwareinthephotoalignmentandcameracalibration.

Toseethemarkersthataregeneratedineachimage,clicktheEditMarkersiconinthetoptoolbar(theblueflag).Eachmarkerislabeledasatargetorpoint,withanindexnumber.

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Figure 5: Detected coded and non-coded cross targets. Corners in the centimeter scale are also detected.

Formoreinformationonmarkers,seetheAgisoftPhotoScanProUser’sGuide,Chapter4,ReferencingandMeasurement.

Examining and Editing Markers

Afteryoudetectthemarkers,youcanseeinformationabouteachmarkerintheMarkersareaoftheReferencePane,asshowninFigure6.

Figure 6: The Markers area of the Reference Pane after markers have been detected.

• TheError(pix)valueistheestimatederrorinunitsofpixelsforthatmarker’splacement.Youshouldcheckthesevaluestoevaluatethequalityoftheautomaticmarkerdetection.

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• TheProjectionsvalueisthenumberofphotosinwhichthemarkerwasdetected.Werecommendaminimumofthreeprojectionsforanymarkerthatisusedtosetscalefortheproject.

Check the Error for Each Marker

Youcansortbyerrortofindthemostextremeonesinyourproject.Youcanreviewthemarkersonaphoto,removethemorchangetheirposition.

Ifyouintendtouseamarkertosetscale,werecommendthatyouremoveprojectionsonphotoswiththemosterror,ormoveprojectionsuntilthemarkererrorisbelow0.4pixels.(See“CreatingScaleBarObjects”belowforinstructionsoncreatingthescalebarsandsettingtheirdistancevalues)BeginningwithPhotoScanversion1.2,itispossibletoseeinformationaboutalltheimagesthathaveaprojectionforanygivenmarker,andtoseetheerrorinpixelsforeachprojectiononeachphoto.Toviewthisinformation,rightclickonamarkerintheworkspaceorinthereferencepaneandchoose“ShowInfo…”fromthemenu.

Figure 7: “Show Info…” box for a marker.

Youcansortthe“Values”forpixelerrorsintheinfodialogbox.Youcanremovethemarkerfromphotosthathavehighpixelerror.StartinginPhotoScanversion1.2.5,youcandouble

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clickonanimagenameintheinfodialogboxandthatimagewillappearinthemainwindow.Werecommendthatyouhaveaminimumof3projectionsforanymarkeryouplantousetocreatescale.(Seebelowonhowtoaddandremovemarkers)

TIP:Aftersortingthepixelerrorsforeachphotowithaprojectionforamarker,youcanremovethemarkerfromanyphotothathasahighpixelerror.Removingthemarkeraffectsthenumberofprojectionsforthatmarker,sobesuretocheckthatyouhaveenough.

Youcanalsofindalltheimagesthathaveprojectionsforaspecificmarkerbyright-clickingonthatmarkerandchoosingFilterPhotosbyMarkers.Thencheckthemarkerplacementfortheimagesthathaveagreenorblueflag(indicatingthemarkerisbeingused).Todothis,double-clickontheimageinthePhotosPane,andthenzoominonthetargetinthemainwindowarea.

Whenyouhavemadeanychangetoamarkerinaphoto,clickUpdateintheReferencePanetoolbartoupdatethepixelerrorestimate.

Check the number of projections for each marker

Markersshouldappearinthreeormoreimagesifyouhaveagoodimagesetwithproperoverlappingimages.Ifyouseefewerthanthreeprojectionsforamarker,youcanplacethemarkerbyhandonadditionalphotos.Alternatively,youcanchoosenottousethatmarkerforcreatingscalebars.Asdescribedabove,therearetwowaystofindthephotosthathaveprojectionsforagivenmarker.YoucanuseFilterPhotosbyMarkersorthe“ShowInfo”dialogbox.Youcanthenremovemarkersthathavehigherror,orrepositionthem.Ifyoudonothaveenoughmarkers,youcanplacethembyhand.

PhotoScansometimesdetectsadditionalphotosthathaveamarkerandshowsthemwithagrayflag,becausethereisnotenoughcertaintyabouttheposition.Thesepotentialmarkersarenotusedintheprojectunlessyouplacethemarkerexplicitlyintheadditionalphotoorphotos,asshowninFigure8.Ifyoudon'thaveenoughprojections,onewaytoaddmoreisbyexplicitlyplacinganypotentialmarkersinaphoto.

Thegreenflagbyphoto13inFigure8indicatesthatthemarkerfortarget72isbeingusedinthisphoto.Thegrayflagindicatesthatapotentialmarkerwasdetected,butitisnotbeingused.

Toconvertapotentialmarkertoausedmarker,double-clicktoselectthephotowiththegrayflag.Inthemainwindowarea,hoveroverthebaseoftheflagforthesuggestedmarkeruntilitturnsred.Youcanthenrightclicktobringupthecontextmenu.Toplacethemarker,choosePlacemarkerfromtheright-clickcontextmenu.

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Figure 8: Placing markers by hand.

Creating Scale Bar Objects

Theprocessingsoftwareusesascalebarobjecttorepresentthephysicalscalebarwithtargetsthatwasusedtocreatethemarkers.Youmustcreatethesescalebarobjectsfrommarkerpairs,afterallmarkershavebeengenerated.Tocreatescalebarobjects,selecttwomarkersintheReferencePane,thenright-clickandchooseCreateScaleBar.Eachscalebarobjectthatyoucreateshouldcorrespondtoaphysicalcalibratedscalebarthatyouusedwhencapturingyourimageset.

Figure 9: Creating a scale bar from target 71 to target 72.

Adding Scale Bar Distances

SelectViewSourceintheReferencePanetoolbartoviewthescalebarobjectsyouhavecreated.EachscalebarobjectisassociatedwithaDistancevaluethatyoucansettothe

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exactdistanceinmetersbetweenthetwomarkersonyourphysicalscalebar.Thisistheinformationthatprovidesthescaleforthewhole3Dproject.

Figure 10: Updating information about markers and scale bars.

TheDistancevalueisinitiallyblank.Inordertosetthescaleforthemodel,youmustenteradistancevalueforoneormorescale-barobjects.Thedistancebetweenmarkersintheobjectshouldmatchtheknowndistancebetweentargetsonthephysicalscalebar.

Toenteraknowndistancevalue:

• Selectascale-barobject.

• Double-clickintheDistance(m)column.

• Entertheknowndistancebetweentargetsforthecorrespondingcalibratedscalebar.

TIP:Whileaprojectcanbescaledusingtheknowndistancefromasinglescalebar,morescalebarsarestronglyrecommenced.Twobarsprovideabasiccheck,andthreeormoreaddconfidenceandstatisticalreassurance.

TIP:Whenyoumakeanychangestomarkersorscale-barobjects,suchasmanuallyplacingormovingmarkersorenteringscalebardistances,clickUpdatetoseetheeffectofthosechangesontheerrorandestimateddistances.

Check estimated scale bar distance values

Inawell-shotprojectwithcrispphotosandproperimageoverlap,thedifferencebetweentheestimateddistancevalueforeachscale-barobjectandtheknowntargetdistanceinthecorrespondingcalibratedscalebarshouldbelessthantheprecisionofthescalebar.InthecaseofthescalebarsfromCHIthisis1/10mm(0.0001meter).

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Eachtimeyouupdatethesourcedata,thetoolrefinesthescaleoftheentiremodel.Itthenusesthecurrentscaleandprojectgeometrytoestimatedistancesforalldefinedscale-barobjects.

Toshowtheestimateddistancevaluesthatarebasedonthecurrentscale,clicktheViewEstimatediconinthetoolbar.

Figure 11: Show estimated distance between markers in scale-bar objects.

InFigure11,thedistancevaluesyouenteredarereplacedbytheestimatedvaluesbasedonthecurrentprojectscale.Thetoolestimatesvaluesforallscale-barobjects,basedonthecurrentscaleithasdevelopedforthemodel.

• Thecheckmarksshowthoseobjectsforwhichyouhaveentereddistancedata.Fortheseobjects,theestimateddistancevalueisadjustedtoconformwiththecurrentscaleinthemodel.

• Theuncheckedobjectshaveonlyestimateddata,becausenodatahasbeenexplicitlyenteredforthem.

Youcanevaluatetheaccuracyofthecurrentscalebyseeinghowcloselytheestimateddistancevaluesforscalebarobjectsmatchtheknowndistancesbetweentargetsinthephysicalscalebarsthatyouusedtogeneratetheimageset.

TIP:Enterknownmarkerdistancesforallexceptoneofthescale-barobjectsthatcorrespondtoyourphysicalscalebars.Thisenablesyoutocomparethewhollyestimatedvaluewiththeadjustedvalues.