scale bars for photogrammetry: user guideculturalheritageimaging.org/what_we_offer/gear/scale... ·...

ScaleBarsforPhotogrammetry:UserGuideJuly2019:DocumentVersion3.0

Toenablethemeasurementofanobjectorasitein3D,youplacescalebarson,around,ornexttotheimagingsubject.Whenyouprocessyourseriesofimagestocreatea3Dmodel,theprocessingsoftware—CHIusesAgisoftMetashapePro—findstheexactcenterofthetargetsonthescalebarsinthesetofphotos.Youthenentertheprecalibrateddistancebetweenthetargetsonthescalebartosetthescaleoftheproject.Oncethescaleisset,youcangetahighlyaccuratemeasurementofanyfeatureoftheimagingsubject.

Tableof Contents AbouttheScaleBars...........................................................................................................................................2TargetTypes...........................................................................................................................................................................2ScaleBarsSpecifications....................................................................................................................................................3

HowtoUsetheScaleBars.................................................................................................................................4PlacingtheScaleBars.........................................................................................................................................................4HowManyScaleBarstoUseinYourProject............................................................................................................6

ProcessingDatawithScaleBars.....................................................................................................................7SettheScaleBarAccuracy................................................................................................................................................8DetectingMarkers................................................................................................................................................................8ExaminingandEditingMarkers..................................................................................................................................11CreatingScaleBarObjects.............................................................................................................................................13

ReferenceInformation....................................................................................................................................16PhotogrammetryResources.........................................................................................................................................16HowScaleBarsAreCalibrated....................................................................................................................................16ThermalExpansionoftheScaleBars........................................................................................................................17Acknowledgments.............................................................................................................................................................18CulturalHeritageImagingLimitedWarranty.......................................................................................................18

ScaleBarsforPhotogrammetryUserGuide,v3©CulturalHeritageImaging2015-2019-AllRightsReserved 2

About the Scale Bars YoursetofprofessionallycalibratedscalebarsfromCulturalHeritageImagingisuniquelysuitedtotoday'spracticeofclose-rangephotogrammetry.

The10scalebarsinthesetareflat,lightweight,linearbars.Thesearecalibratedscalebarswithprintedtargetsseparatedbyaknowndistance.Therearetwoeachof5sizes,with6usingcodedand4usingnon-codedtargets.

Thescalebarsare:

• DesignedbyseniorphotogrammetryexpertsattheUSBureauofLandManagementforhighlyaccuratemeasurementandscientificimaginginphotogrammetryprojects.

• IndividuallycalibratedbyCHIstafftoaprecisionof1/10mmorbetteraccuracy.

• Printedon4mmALUPANEL®,atough,durablecompositematerialconsistingof2aluminumsheetsbondedoneithersidetoasolidpolyethylenecore.

• Notsubjecttodelaminationorwarping,optimizedforlong-termoutdooruse,water-andenvironment-resistant.

• Madewithagrayscalebackgroundwithhigh-frequencydetailstoenablethegenerationofameasurable3Dmodeloftheentirescalebar.

Werecommendthatyouprotectthescalebarsbystoringthemintheheavyplasticbagstheycomein.Ifneeded,cleanthemwithadampcloth.

Target Types

Therearetwobasictypesofscalebartarget,codedandnon-coded.

• Codedscalebarstargets

Threesizesofthescalebarshave12-bitcodedtargets.Thecodedtargetsareautomaticallydetectablebyseveralphotogrammetrysoftwarepackages,includingAgisoftMetashapePro,andautomaticallynamed(representedbynumbers).Thenumbersofthecodedtargetsaredifferentonallthebarsinaset,andCHIoffers5differentsets,eachwithuniquecodedtargets.Thisisusefulifmorethanonesetofscalebarsisneededinaproject.Thecodedtargetdesignensuresthattheprocessing


softwarecanrecognizeeachtargetasuniqueandnotconfusethemwithothertargetswithinthesame3Dproject.

• Non-codedtargetscomeintwostyles:

• Non-codedcirclesareonthelongmeterbars.Theseareusefuliftheimagingresolutionofthecodedtargetsonthescalebarisinsufficientforautomateddetection(MetashapeProrequiresaminimumof10pixelsfromthecentertotheedgeofthetargettodetectacodedtarget).Thiscanoccurwhenyouusethescalebarswithlarge-scalesubjectslikesitesandlandscapes,wherethecameramaybedistantfromthesubject.Bysettingthetargetdetectionmodeto“Circlenon-coded”,thesoftwarecanlocatethetwolargeblackcirclesonthemeterscalebar.Thecirculardesignwithawhitedotinthecenterenablesthesoftwaretolocatethecenterofthetargetautomatically.

• Non-coded,cross-typetargets(alternatingquadrantsofblackandwhitewithinacircularboundary)areonthesmallscalebars(18cmand5cm).Thecrossdesignallowssomesoftwarepackages(includingAgisoftMetashapePro)todetectthemautomaticallyastargets,placingamarkeratthecenterofthecirclewheretheblackandwhitesectionsintersect.

Scale Bars Specifications

ThescalebarsarecalibratedbyCHIstafftoaprecisionof1/10mmorbetter,withtheactualcalibratedmeasurementbetweeneachtargetwrittenonthescalebar.

Meter Scale Bars (2)

Include3codedand2non-codedcirculartargets.Centersofeachtargetareapproximately25cmapart,withatotaldistancefromtargetsoneachendof1meter.Thebar’soveralldimensionsare108cmlongx6cmwide.

Half-meter Scale Bars (2)

Include3codedtargets.Centersofeachtargetareapproximately25cmapart,withatotaldistancefromtargetsateachendof1/2meter(or50cm).Thebar’soveralldimensionsare60cmlongx6cmwide.


Quarter-meter Scale Bars (2)

Include2codedtargets.Centersofeachtargetareapproximately25cmapart.Thebar’soveralldimensionsare35cmlongx6cmwide.

18cm Scale Bars (2)

Include2non-coded,circularcross-typetargets.Centersofeachtargetareapproximately18cmapart.Thebar’soveralldimensionsare23cmlongx6cmwide.

5cm Scale Bars (2)

Include2non-codedcircularcross-typetargets.Centersofeachtargetareapproximately5cmapart.Thebar’soveralldimensionsare10cmlongx6cmwide.

How to Use the Scale Bars Tosupporthighlyaccuratemeasurementof3Ddataofanobjectorasite,itisessentialforphotogrammetrypractitionerstolearnthecorrectplacementanduseofcalibratedscalebars.

Placing the Scale Bars

Asyoupreparetoimageasubject,youstartbyplacingthescalebarson,around,ornexttothesubjectorsite.Youshouldconsiderthescalebarstobepartofthe“areaofinterest”foryourphotogrammetryproject,andmakesurethatyouroverlappingimagesetextendsbeyondthepositionofthescalebars.

Placethebarssuchthatthetargetscanbeclearlyseeninmultipleoverlappingimages.Whenoverlappingphotosaretakenofthesceneorobject,thetargetsonthebarsbecome


markers.Themarkersserveasreferencepointsforscaledefinition,andalsohelptheprocessingtooltoaligntheimagesaccurately.Thescalebarsdonotneedtoappearineveryphoto.Theyshouldbewellcoveredbyoverlappingphotoswithgoodgeometrysotheircalibratedvaluescanbeusedtoprovideaccuratescaleforthewholephotogrammetryproject.

ImportantNote:Donotmovethescalebarsduringphotogrammetricimagecaptureofyourproject.Ifascalebarisaccidentallybumpedormoved,removeitfromthescene.Donotattempttoplaceitbackinthescene,asitwillnotbeinexactlythesameplace.Tobeusedaccurately,thescalebarsmustbeinaconstantlocationrelativetoyoursubject.

Thefollowingfiguresshowsomewaysofplacingscalebars.

Figure 1: On the left, four coded marker scale bars in place around a smaller subject. On the right, four non-coded cross-style scale bars are placed around the subject. In the Metashape Pro software, there is a limit of a 30-pixel radius (60-pixel diameter) to detect the coded markers. Placing the non-coded cross style markers eliminates this problem when using high-resolution cameras and smaller subjects.

Whenplacingscalebarsontheground,outsidetheprimarysubjectarea,youmusttakeadditionaloverlappingphotosthatincludethescalebarsinordertousethemforaccuratemeasurementinyourproject.Theseadditionalimagesshouldincludephotosthataredirectlyoverthescalebars.Ifyouonlytakephotosofaverticalsubjectwhenthescalebarsarehorizontalontheground,thenthescalebartargetswillbeobliqueinyourimages,andtheymaynotbedetected,ortheymaybedetectedwithhigherrors.Makesuretotakeoverlapping,“bridging”photos,tiltingyourcameranomorethan15degreesatatimesothatallthephotoscanaligntogether.

Inlargerprojects,placethescalebarsasfarawayfromeachotheraspossiblesothatyoucancontrolthescaleacrosstheentireproject.Forsomesubjects,itmaybeconvenienttoplacethescalebarsoverthesubject,butyoumaynotwanttohavescalebarsinyourfinalmodel.Inthiscaseyoucantakeimageswiththescalebarsoverthesubjectalongwiththesurroundingarea.Then,removethescalebarsandtakephotosofthatareaagain.Alignallphotostogether.Whenprocessingyourproject,youcanincludethescalebarimagesandthen“disable”(inMetashapesoftware)thescalebarimagesbeforebuildingyourdensecloud,mesh,orotherproducts.Thescale


barswillhavedonetheirjobofscalingthesubjectorscenebutwillnotappearinyourfinalmodel.

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. 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.

How Many Scale Bars to Use in Your Project

Whilephotogrammetrysoftwarecanaddscaletoyourprojectwithasinglescalebar,itisadvisabletoplaceaminimumofthreescalebarsinyourproject.Werecommendfour.Twobarsprovideabasiccheck,andthreeormoreaddconfidenceandstatisticalreassurance.

Usingfourbarsisideal,especiallyforlargerprojectsandoutdoorenvironments,becauseitisalltooeasyforascalebartobemovedbythewind,kicked,orotherwisedisplacedduringcapture.Ifthishappens,theprojectwillstillhavethestatisticalintegrityprovidedbythreescalebars.


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.

Processing Data with Scale Bars Thereareanumberofsoftwarepackagesavailableforprocessingyour3Dproject.Thescalebarsinyoursetcanbeusedwithanyofseveralsoftwarepackages,thoughnotallpackageswillautomaticallydetectallofthetargetsinthesescalebars.CulturalHeritageImaging(CHI)usesAgisoftMetashapePro,andtheseinstructionsarebasedonthe1.5versionofthatsoftware.ThescalebarprocessingworkflowinAgisoftMetashapeProVersion1.5includesthesebasicsteps:

• Setthescalebaraccuracy.

• Detectthetargetsonthescalebarsintheimagesettogeneratemarkersineachphotothatincludesscalebars.

• Createscalebarobjectsbetweentherelatedmarkers.

• Addthecalibratedscalebardistancedatabetweentherelatedmarkerstothescalebarobjects.


Eachofthesestepsisdescribedfurtherbelow.MoreinformationonplacingmarkersandcreatingscalebarscanbefoundintheAgisoftMetashapeUserManual,ProfessionalEdition,Version1.5,Chapter4,“Referencing.”

Set the Scale Bar Accuracy

ThescalebarsfromCulturalHeritageImagingarecalibratedto1/10mmaccuracyorbetter.Thescalebarsincludescribeddatato1/100mm.Theextrainformationistoeliminateroundingerrorswhenusinglongerscalebars,suchasthehalf-meterbars.YoushouldsettheScalebaraccuracy(m)to0.0001(thedefaultvalueis0.001).

Figure 4: Set scale bar accuracy.

Detecting Markers

Afteraligningphotosandoptimizingyourproject,usetheDetectMarkersdialogtodetectthetargetsonthescalebars,andusethemtoautomaticallygeneratemarkers.MakesuretheReferenceSettingspaneisvisibleandchooseTools>Markers>DetectMarkers(Figure5).


Figure 5: Detect markers for scale bars.

TIP:Wesuggestthatyoustartwithatoleranceof50,andmakeithigherifnotalltargetsaredetected.

Figure5alsoshowsthedifferentkindsofmarkersonthescalebars.

ThescalebarsfromCHIincludethreekindsofmarkertargetssupportedbyMetashape,asshowninFigure5.CHIusesmarkertargetsthatarecircular12-bitcoded,circlenon-codedornon-codedcross.

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

TIP:Ifyouareusingbothcircularcodedandcircularnon-codedtargets,youshoulddetectthecodedmarkers(thecircular12-bitmarkerinthisexample)beforeyoudetectanynon-codedcircularmarkers.Thisisbecausethecenterofthe12-bitcodedmarkerisacircle,anditwillbedetectedasanon-codedcircleifthereisnotalreadyamarkerpresentfromdetectingthecodedmarkers.Thenwhencodedmarkersaredetected,youmayhaveduplicationofmarkersinthecentercircleofthecodedmarkers.


Figure 6: Detected coded and non-coded cross targets. Corners in the centimeter scale are also detected.

TIP:Ifyouarephotographingclosetoasmallersubjectwithahigh-resolutioncamera,andthesoftwaredoesnotdetectthecodedmarkers,itmaybethattherearetoomanypixelsacrossthemarkerinyourimages.ThelimitinMetashapeProv.1.5isa30-pixelradiusfromthecenterofthetargettotheedgeofthetarget(60-pixeldiameter).Inthiscaseyoucandetectnon-codedcircles,andchecktheboxfor“Inverted”,todetecttheinnercircleofthecodedmarkerasanon-codedcircle.

Themarkersoncodedandnon-codedcirculartargetsareplacedwithhighprecisionbecausethecenterofthecirclecanbedetectedalgorithmically.

Thecentersofnon-coded,cross-typetargetswithcalibrateddistances,aswellasthecornersofthesmallscalesmadeupofblackandwhitesquares,areplacedwithhighprecisionbecausethecenterpointofthecrosscanbedetectedalgorithmicallybythesoftware.Althoughyouwillrarelyusethescalesmadeupofblackandwhitesquaresforsettingscaleinyourproject,youmayretainanydetectedscalemarkersiftheyhavelowerror.Theymarkadditionalknownpointsthatcanaidthesoftwareinthephotoalignmentandcameracalibration.

Toseethemarkersthataregeneratedineachimage,clicktheEditMarkersiconinthetoptoolbar(theblueflag).Eachmarkerislabeledasatargetorpoint,withanindexnumber.

Formoreinformationonmarkers,seetheAgisoftMetashapeProuser’smanual,Chapter4,“Referencing.”


Examining and Editing Markers

Afteryoudetectthemarkers,youcanseeinformationabouteachmarkerintheMarkersareaoftheReferencepane,asshowninFigure7.

Figure 7: The Markers area of the Reference pane after markers have been detected.

• TheError(pix)valueistheestimatederrorinunitsofpixelsforthatmarker’splacement.Youshouldcheckthesevaluestoevaluatethequalityoftheautomaticmarkerdetection.

• TheProjectionsvalueisthenumberofphotosinwhichthemarkerwasdetected.Werecommendaminimumofthreeprojectionsforanymarkerthatisusedtosetscalefortheproject.

Check the Error for Each Marker

Youcansortbyerrortofindthemostextrememarkererrorsinyourproject.Youcanreviewthemarkersonaphoto,removethem,orchangetheirpositions.

Ifyouintendtouseamarkertosetscale,werecommendthatyouremoveprojectionsonphotoswiththemosterror,orremoveprojectionsuntilthemarkererrorisbelow0.4pixels.(See“CreatingScaleBarObjects”belowforinstructions)

InMetashapePro,itispossibletoseeinformationaboutalltheimagesthathaveaprojectionforanygivenmarker,andtoseetheerrorinpixelsforeachprojectiononeachphoto.Toviewthisinformation,right-clickonamarkerintheworkspaceorintheReferencepane,andchooseShowInfo…fromthemenu.


Figure 8: “Show Info…” box for a marker.

Youcansortthe“Value”columnforpixelerrorsintheShowInfo…dialogbox.Youcanremovethemarkerfromphotosthathavehighpixelerror.Youcandouble-clickonanimagenameintheShowInfo…dialogboxandthatimagewillappearinthemainwindow.Werecommendthatyouhaveaminimumof3projectionsforanymarkeryouplantousetocreatescale.(Seebelowonhowtoaddandremovemarkers.)

TIP:Aftersortingthepixelerrorsforeachphotowithaprojectionforamarker,youcanremovethemarkerfromanyphotothathasahighpixelerror.Removingthemarkeraffectsthenumberofprojectionsforthatmarker,sobesuretocheckthatyouhaveenough.

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

Whenyouhavemadeanychangetoamarkerinaphoto,clicktheUpdateiconintheReferencepanetoolbartoupdatethepixelerrorestimate.

Check the number of projections for each marker

Markersshouldappearinthreeormoreimagesifyouhaveagoodimagesetwithproperoverlappingimages.Ifyouseefewerthanthreeprojectionsforamarker,youcanplacethe


markerbyhandonadditionalphotos.Alternatively,youcanchoosenottousethatmarkerforcreatingscalebarobjects.Asdescribedabove,therearetwowaystofindthephotosthathaveprojectionsforagivenmarker.YoucanuseFilterPhotosbyMarkersortheShowInfo…dialogbox.Youcanthenremovemarkersthathavehigherror,orrepositionthem.Ifyoudonothaveenoughmarkers,youcanplacethembyhand.

Metashapesometimesdetectsadditionalphotosthathaveamarkerandshowsthemwithagrayflag,becausethereisnotenoughcertaintyabouttheposition.Thesepotentialmarkersarenotusedintheprojectunlessyouplacethemarkerexplicitlyintheadditionalphotoorphotos,asshowninFigure9.Ifyoudon'thaveenoughprojections,onewaytoaddmoreisbyexplicitlyplacinganypotentialmarkersinaphoto.

Thegreenflagbyphoto13inFigure9indicatesthatthemarkerfortarget72isbeingusedinthisphoto.Agrayflagindicatesthatapotentialmarkerwasdetected,butitisnotbeingused.

Toconvertapotentialmarkertoausedmarker,double-clicktoselectthephotowiththegrayflag.Inthemainwindowarea,hoveroverthebaseoftheflagforthesuggestedmarkeruntilitturnsred.Youcanthenright-clicktobringupamenu(notethatthemenuiscontext-sensitiveandchangesdependingonwhetheryouarehoveringoveramarkerornot).Toplacethemarker,choosePlaceMarkerfromtheright-clickmenu.

Figure 9: Placing markers by hand.


Creating Scale Bar Objects

Theprocessingsoftwareusesascalebarobjecttorepresentthephysicalscalebarthatwasusedtocreatethemarkers.Youmustcreatethesescalebarobjectsfrommarkerpairs,afterallmarkershavebeencreated.

Tocreatescalebarobjects,selecttwomarkersintheReferencepane;thenright-clickandchooseCreateScaleBar.

Eachscalebarobjectyoucreateshouldcorrespondtoaphysicalcalibratedscalebarthatyouusedwhencapturingyourimageset.

Figure 10: Creating a scale bar from target 71 to target 72.

Adding Scale Bar Distances

ClicktheViewSourceiconintheReferencepanetoolbartoviewthescalebarobjectsyouhavecreated.EachscalebarobjectisassociatedwithaDistancevaluethatyoucansettotheexactdistanceinmetersbetweenthetwomarkersonyourphysicalscalebar.Thisistheinformationthatprovidesthescaleforthewhole3Dproject.

TheDistancevalueforascalebarobjectisinitiallyblank.Inordertosetthescalefortheproject,youmustenteradistancevalueforoneormorescalebarobjects.Thedistancebetweenmarkersinthescalebarobjectshouldmatchtheknowndistancebetweentargetsonthephysicalscalebar.Toenteraknowndistancevalue:

• Selectascalebarobject.

• Double-clickintheDistance(m)column.

• Entertheknowndistancebetweentargetsforthecorrespondingcalibratedscalebar.

TIP:Whileaprojectcanbescaledusingtheknowndistancefromasinglescalebar,morescalebarsarestronglyrecommended.Twobarsprovideabasiccheck,andthreeormoreaddconfidenceandstatisticalreassurance.


TIP:Whenyoumakeanychangestomarkersorscalebarobjects,suchasmanuallyplacingormovingmarkersorenteringscalebardistances,clicktheUpdateicontoseetheeffectofthosechangesontheerrorsandestimateddistances.

Figure 11: Updating information about markers and scale bars.

Check estimated scale bar distance values

Inawell-shotprojectwithcrispphotosandproperimageoverlap,thedifferencebetweentheestimateddistancevalueforeachscalebarobjectandtheknowntargetdistanceinthecorrespondingcalibratedscalebarshouldbelessthantheprecisionofthescalebar.InthecaseofthescalebarsfromCHI,thisis1/10mm(0.0001meter).Eachtimeyouupdatethesourcedata,thetoolrefinesthescaleoftheentiremodel.Itthenusesthecurrentscaleandprojectgeometrytoestimatedistancesforalldefinedscalebarobjects.

Toshowtheestimateddistancevaluesthatarebasedonthecurrentscale,clicktheViewEstimatediconinthetoolbar.


Figure 12: Show estimated distance between markers in scale bar objects.

InFigure12,thedistancevaluesyouenteredarereplacedbytheestimatedvaluesbasedonthecurrentprojectscale.Thetoolestimatesvaluesforallscalebarobjects,basedonthecurrentscaleithasdevelopedforthemodel.

• Thecheckmarksshowthoseobjectsforwhichyouhaveentereddistancedata.Fortheseobjects,theestimateddistancevalueisadjustedtoconformwiththecurrentscaleinthemodel.

• Theuncheckedobjectshaveonlyestimateddata,becausenodatahasbeenexplicitlyenteredforthem.

Youcanevaluatetheaccuracyofthecurrentscalebyseeinghowcloselytheestimateddistancevaluesforscalebarobjectsmatchtheknowndistancesbetweentargetsinthephysicalscalebarsthatyouusedtogeneratetheimageset.

TIP:Enterknownmarkerdistancesforallexceptoneofthescalebarobjectsthatcorrespondtoyourphysicalscalebars.Thisenablesyoutocomparethewhollyestimatedvaluewiththeadjustedvalues.

Reference Information

Photogrammetry Resources

• Photogrammetry Information ThereisaphotogrammetrypageontheCHIwebsitewithlinkstoavarietyofmaterials,includinganinstructionalvideoseriesentitledPractical,ScientificUseofPhotogrammetry.CHIhostsfreeuserforumsforpeopleworkinginthefieldofcomputationalphotography,includingaforumforphotogrammetry.


• Photogrammetry Training and Consulting CulturalHeritageImagingoffersa4-dayclassinphotogrammetry.Thishands-ontrainingsessionisheld2or3timesayearinourstudioinSanFrancisco,orwecanmakearrangementstoconductthetrainingatyoursite.

CulturalHeritageImagingalsooffersconsultinginphotogrammetrytomeetawiderangeofneeds.Wecanhandlescientificimagingforyou,helpyouplanprojects,makeequipmentrecommendations,andmore.

How Scale Bars Are Calibrated

TheCulturalHeritageImaging(CHI)calibrationprocessusesphotogrammetry.Wehave"goldenmaster"scalebarsthatwerecalibratedmultipletimesusingdifferentcamerasandlenses,alongwithothergoldenmasterscalibratedbyseniorphotogrammetristsfromtheUSBureauofLandManagement.Weuseatleast4goldenmasterscalebarsinthephotogrammetryimagesetwhenwecalibratenewscalebars.

Inadditiontohowweimagethescalebars,wealsofollowanerrorreductionworkflowintheMetashapeProprocessingsoftware.Wereduceerrorinthephotogrammetryprojecttoarootmeansquareerrorofbetween0.13to0.15pixels.Weuseauto-detectioninAgisoftMetashapeProforplacingthemarkersonthescalebartargets.TheMetashapeauto-detectionalgorithmplacesthemarkerattheleastmeansquarescentroidofthedetectedtarget.Wemanagetheprocess,and,ifneeded,removeerrorfromthemarkers,asdescribedabove,toachieveanestimatederroronthemarkerplacementbelow0.15pixels.Weaddscalevaluestotheprojectfromourgoldenmasterscalebars.InourcalibrationprojectstheMetashapeestimatederror(foralltheestimatedgoldenmastersdistances)isbetween.000005and.00008ofameter(5to80microns).

Webelievethatourcalibratedmeasurementsareaccuratetoaprecisionof1/10mmorbetter.Ourtestingshowsrepeatabilitycloserto5hundredthsofamillimeter.Wescribethedataontothescalebarto1/100ofamillimetersoastopreventroundingerrorswhenaddingscalevaluesacrossthelargerscalebarsthathave3or5targets.

Thermal Expansion of the Scale Bars

Thescalebarsareprintedon4mmALUPANEL®.Themanufacturer’spublishedlinearthermalexpansionis2.4mm/mata100°Ctemperaturedifference.

Thistranslatestoalineardimensionalchangeof0.024mm/meterperdegreeCelsius(C)or0.013mm/meterperdegreeFahrenheit(F).Thelongercodedscalebarsarecalibratedevery25cm,sothelinearexpansionforthecalibrateddataforatemperaturechangeof1°C=0.006mmand1°F=0.0033mm. Thecalibratedscalebarsaremarkedwiththetemperatureatwhichtheywerecalibrated(generallybetween62°–68°F,16.7°–20°C).Ifyouwanttocompensateforlinearexpansion(forexample,ifyouareusingthescalebarsoutdoorsinawiderangeoftemperatures),youcanaddorsubtractthefollowingamountsforthetemperaturechangefromthenotedcalibrationtemperature.


Temperature change in Celsius

Add or subtract the following amount for a 25cm calibrated distance

Temperature change in Fahrenheit

Add or subtract the following amount for a 25cm calibrated distance

+/- 5°C 0.00003 +/- 5°F 0.000017

+/- 10°C 0.00006 +/- 10°F 0.000033

+/- 15°C 0.00009 +/- 15°F 0.00005

+/- 20°C 0.00012 +/- 20°F 0.000067

+/- 25°C 0.00015 +/- 25°F 0.000083

+/- 30°F 0.0001

YoucandownloadabrochurefromtheALUPANEL®manufacturerwithadditionaltechnicalinformation:https://www.multipaneluk.co.uk/products/sign-display/alupanel/#panel-overview

Acknowledgments

ManythankstoTommyNobleandNeffraMatthewsoftheUSBureauofLandManagement,NationalOperationsCenterforthedesignanddevelopmentofthescalebars.Theirmanyyearsofphotogrammetryexperienceandseveraliterationsofscalebarshaveresultedinaveryefficientandhighlyaccurateproduct.ThanksalsototechnicalwriterJudyBogartforthedevelopmentandlayoutofthisdocument.

Cultural Heritage Imaging Limited Warranty

CulturalHeritageImagingIncorporated(“CHI”)warrantsthecalibrationdataonourCalibratedScaleBarsProducttobeaccuratewithin1/10ofonemillimeter.Weprovidetheabovewarrantyfor90daysaftertheshipmentoftheproduct.Weprovideaone-timereplacementofanyscalebarthatdoesnothavethislevelofcalibratedaccuracy.TheCustomershouldemailscale-bars@c-h-i.orgwithanyquestionsorconcernsaboutthecalibrationontheirscalebars.CHIwill,atitssolediscretion,determineifthereisacalibrationerror,andwillreplacethescalebarorbarsatCHI’sexpenseifitisdeterminedthereisacalibrationerror.NotethatthescalebarsareprintedonALUPANEL®whichhaslinearthermalexpansionpropertiesdescribedintheScaleBarsforPhotogrammetryUserGuide(theGuide)whichcanaffectthecalibrationdata.HowtocompensateforthermalchangesisalsodescribedintheGuide.

ExclusionsandLimitations

Exceptforthelimitedwarrantysetforthabove,CHIexpresslydisclaimsanyandallotherwarrantiesexpressorimplied,includinganywarrantyofquality,merchantability,orfitnessforaparticularpurpose,andyouspecificallyagreethatCHIshallnotbeliableforanyspecial,incidental,indirect,punitive,orconsequentialdamagesforbreachofanywarrantyofanytypeonanyCHIproduct.

scale bars for photogrammetry: user guideculturalheritageimaging.org/what_we_offer/gear/scale... ·...

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