Aerial photogrammetric mapping is a well-established industry. Its methods have been evolving progressively but not explosively over many decades. Entrenched, reliable procedures have been developed to overcome pitfalls along the processing chain, allowing the delivery of high quality mapping products

Skaloud, J.

Infoscience - École polytechnique fédérale de Lausanne

P H O T O G R A M M E T R I C  E N G I N E E R I N G  &  R E M O T E  S E N S I N G M a r c h  2 0 0 2 207Direct GeoreferencingDirect Georeferencing in Aerial Photogrammetric MappingBy Dr. Jan Skaloud, SwitzerlandPreambleAerial photogrammetric mapping is awell-established industry. Its methodshave been evolving progressively but notexplosively over many decades. En-trenched, reliable procedures have beendeveloped to overcome pitfalls along theprocessing chain, allowing the deliveryof high quality mapping products. Un-derstandably this has resulted in the en-trenchment of most people thinking asthey design, capture and adjust blocks ofphotographs. With Integrated GPS/INStechnology, however, one can step out-side of this “blocked-block” philosophysince the parameters of a camera’s Exte-rior Orientation (EO) are computed di-rectly. In many cases this means the pho-togrammetric procedures that were oncefollowed verbatim can now be by-passedcompletely. The application of naviga-tion technology to directly georeferenceimagery data is booming and the indus-try tends to furnish systems that requireminimal input from the user. As these“measuring boxes” turn into black boxesand the length of the processing chainextends, the users may quickly find them-selves lost in the complexity when some-thing does not stand up to their expecta-tions. Hence, understanding the technol-ogy fundamentals as well as its limits maysave time and effort and may avoid sur-prises. The goal of this column is to facili-tate the transition process when startingto apply direct georeferencing by GPS/INS to aerial film-based imagery.Simple ConceptRecall the algebraic relation that trans-forms the image observed co-ordinates to the mapping co-ordinates  (as pre-sented in the first-column in the Octoberissue of PE&RS, pp1105-11):where  comes from image measure-ments,  is solved for the stereoscopicprocessing,  and  are derived fromthe system calibration and the translation vector and the rotation matrix  aredelivered by the GPS/INS on-board mea-surements. What more does the user needto know?In theory nothing, but when it comesdown to the error propagation and thusthe overall performance of the system,users should definitely educate them-selves about the type of instrument used,how the inertial system is aligned andhow its performance is coupled with op-erational maneuvers and the gravity field.Users also should be aware of the influ-ence of the sensor placement on the ac-curacy of EO parameters. It also may beuseful to realize that the GPS/INS outputmay not be directly linked to the desiredmapping frame as suggested by the aboveequation. Finally, it is important to un-derstand that in many cases the overalllimiting factor on system accuracy is notthe GPS/INS but rather the calibration ac-curacy of ,  and the parameters ofcamera interior orientation. Before look-ing into some of these points in moredetail let me say a few words about theGPS/INS technology itself.Enabling TechnologyGPS found its way into airborne mappingright from its deployment. It is taking alittle bit longer for the inertial technologyto do so, given that it has already beenevolving for over 40 years (a brief historyof which was presented in an earlier col-umn). The most promising technologiesenabling direct measurement of cameraorientation came with the concepts ofring laser gyros (RLG) and fiber optic gy-ros (FOG), as well as the later evolution ofa strapdown dry tuned gyros (DTG). Theorientation accuracy potential of thesetechnologies is summarized in Table 1.Most of the numbers indicated in the tablehave been confirmed experimentally dur-ing numerous tests conducted in the lastthree to five years.Although the RLG is a conceptuallyolder technology, its accuracy remainsunbeatable thanks to its inherently andextremely good bias and scale factor sta-bility. At very low rotation rates little com-plication arises that needs to be circum-vented by other means. The gyro is usu-ally sensitive to vibrations and thereforethe instrument cluster needs to bemounted in a mechanical frame designedto mitigate them. Depending on theadopted mount for the camera, this typeof vibration damping may affect the in-stantaneous attitude observation with re-spect to the imaging device. However,its low 0.002 to 0.01 deg/hour drift ratesare advantageous and the user shouldexpect better overall performance withcontinued on page 209Table 1: Inertial attitude determination performance with GPS aidingP H O T O G R A M M E T R I C  E N G I N E E R I N G  &  R E M O T E  S E N S I N G M a r c h  2 0 0 2 209Direct Georeferencingcontinued from page 207respect to other strapdown technologiesin the airborne environment especiallywhen long flight lines are flown.The FOG and DTG seem to be themost popular technologies currently usedin airborne mapping thanks to their bet-ter cost-performance ratio. The IMU’s builtaround these gyros usually belong to tac-tical-grade class, although at least onenavigation-grade FOG product is nowavailable on the market. Although lessstable than RLG, their strengths for directgeoreferencing come in low sensitivenessto vibrations (FOG), better instantaneouspointing accuracy (DTG), long life spanand small size and weight. The dimen-sional aspects allow the IMU to bemounted directly on most existing aerialcameras, which helps in preserving theneeded assumption of a rigidly mountedsensor block. Small IMU size also clearsthe way to new specialized applicationslike the hand-held, anywhere pointingsystems operated from the helicopter.What is (not) Solved by theIntegration?GPS and inertial systems are ideal syner-gistic partners, as their error dynamicsare different and uncorrelated. The fol-lowing are the main and well-known ad-vantages:l The integration with GPS solves theproblem of “calibrating” the instru-ment errors (i.e. residual gyro andaccelerometer biases, scale factorsetc.) in a strapdown system.l Similarly, the GPS provides a meansof “in-flight alignment,” removingthe need for the aircraft to be heldstationary due to “north-seeking”process prior to flight.l The inertial system provides a meansof smoothing the noisy velocity out-puts from the GPS, and a continuoushigh-bandwidth measurement of po-sition and velocity.There is no such thing as a perfectinstrument and as strong as it is, the inte-gration cannot completely eliminate allof the errors. In other words, the data in-tegration handled by a Kalman filter/smoother cancels only the non-overlap-ping part of the sensor’s error budget.Thus the ‘width’ of the error cancellationmay overlap only partially with the mo-tion of interest as a function of instru-ment type and precision and the dynamicof an aircraft. For that reason, de-noisinginertial data prior to mechanization hasproven in some cases to be indispens-able for attitude determination and ef-fective procedures have been developedfor that purpose. Another significant por-tion of the residual orientation errors ismost likely to be affected by the qualityof the in-flight alignment. Usually, thedata integrating filter/smoother keeps onrefining the inertial platform all along theflight. The strength of this process is in itsability to decorrelate the misalignmenterrors from other error sources when suf-ficient dynamic is encountered. Its weak-ness lays in the susceptibility to be influ-enced by the changes of the accelerom-eter errors and the anomalous gravityfield. Both act as a wrongly sensed accel-eration that gets “eliminated” by read-justing the previously aligned platform.Dropping the coupling with the acceler-ometers is possible once the platform isaligned and high accuracy gyros areadopted (i.e. 0.002-0.01 deg/h). As thehigh frequency part of the anomalousgravity field is likely to remainunmodeled, this concept may be appeal-ing for certain types of applications whenoperating over a “rough, unknown” grav-ity field or when flying long survey linesof constant velocity.The positioning performance is ob-viously mainly governed by the accuracyof differential GPS and the ability to re-solve correctly the ambiguities. Hence,the absolute positioning accuracy is in arange of 0.05-0.5m, depending on thebaseline-length and differential atmo-spheric modeling although the relativeshort-term accuracy is much better.Validity of the ConceptAs new as it is, direct georeferencing inaerial photogrammetry already has itsown history. Great ideas usually simmersimultaneously in different places andtherefore it is hard to single out one indi-vidual who initiated the concept. Never-theless, it can be safely claimed that theconcept was studied for a long time bythe academic community before the firstcommercial product arrived on the mar-ket. Therefore it may come as a surpriseto somebody that a group at the Univer-sity of Calgary successfully applied thisconcept almost ten years ago. It may beworth mentioning these already histori-cal results, especially in light of the manytests devoted to the proof of the conceptever since.At that time, an RLG-based IMU andgeodetic type GPS receivers were flownwith an LMK camera in three, about 10kmlong and slightly overlapping lines, at aphoto scale of 1:10,000. The adjustedblock of images served as the compari-son of the GPS/INS attitude performancegiving an RMS agreement in the range of13-35 sec (0.004-0.009 deg). It is inter-esting to note that most results publishedsince then fall more or less into this range,although being carried out over longerperiods, using different equipment etc. Ashort-term accuracy of 3-10 sec (0.0008-0.0025deg) has later been confirmed forthe same type of instrument with specialfiltering procedures in place. It should bementioned, that although no specialmount for the IMU and the camera wasimplemented, other factors such as cali-bration of camera interior orientation andthe procedure of  and  determinationwere carefully looked at. In many cases,these parameters are often estimatedwithout sufficient accuracy, and as a re-sult they hamper the potential of a GPS/INS system for direct georeferencing.The Position and AttitudeTransferIt is true that aerial photogrammetric map-ping cannot be liberated completely fromthe concept of aero-triangulation (AT)when applying GPS/INS. Leaving asidethe reliability issues, the need of precise (boresight) determination calls for atleast a small block adjustment with few butsome ground control points. A practicalexperience showed that a 5x5 regularblock of strong geometry is sufficient forthat purpose. What is sometimes wronglyperceived is the hope that all the calibra-tion parameters are found when their vari-ance decreases under a certain level asindicated in the bundle adjustment. Thereis a strong correlation between these pa-rameters and although operational as-pects (i.e. flying from different directions,etc.) may result in their partialdecorrelation, sorting out as much ascontinued on page 210210 M a r c h  2 0 0 2 P H O T O G R A M M E T R I C  E N G I N E E R I N G  &  R E M O T E  S E N S I N GDirect Georeferencingcontinued from page 209possible by independent means helpsmore. For instance, conducting a littleclose range survey and determining and the IMU-GPS antenna displacementterrestrially will always outperform its in-direct determination within the bundle ad-justment. When ambiguities are fixed andwith  determined the GPS/INS observa-tion of the camera perspective shouldenter the adjustment with heavy weights.With high accuracy photo measurementsand favorable geometry in place thisshould in turn provide sufficiently accu-rate  (i.e. 3-5 sec) even when interiororientation parameters are calibrated atthe same time. Calibration procedures arenow supplied either with certain bundleadjustment software or come as a part ofthe navigation system package. As pow-erful as they can be these products do notspan the whole design/processing chainall the way and therefore cannot guardcompletely against mistreatment or mis-interpretation.The issues concerning stability of themount in the carrier and the effects ofvibrations on the instruments are nottrivial but should worry the developersmore than the user. The same is true forthe correctness of time synchronizationbetween the inertial, GPS and the camerathat is very exigent when dealing withattitude data. Although the technologyshould now be mature enough in this re-spect, a simple check for constant delaysmay be worth implementing, for instanceas a part of the boresight calibration.PerspectivesIn a relatively short time the GPS/INS tech-nology made its way into the spectrumof airborne remote sensing applications.Indispensable in SAR and laser scanningsystems, by design the new digital (lineand frame) cameras have GPS/INS as theprimary orientation system. The technol-ogy acceptance is taking longer in film-based aerial photogrammetry, mainly dueto the progress towards automated AT(AAT). The AAT concept has been, idlingfor a long time as “assisted”AAT and theinclusion of GPS/INS data within a largeAAT adjustment is sometimes perceivedas a possibility to drop the need for op-erator backing. Although such an ap-proach may be great in terms of reliabilityit increases the computational burden anddoes not give the flexibility needed whenworking with individual stereoscopicmodules. On the other hand, as advancedas it is, the GPS/INS data processing stillhas a lot of room for improvements,mainly in the reliability and confidenceissues. Filling these gaps may finally “un-block” the traditional concept of this field.Dr. Jan Skaloud is with The Swiss FederalInstitute of Technology, Lausanne, Swit-zerlandEdited By Dr. Mohamed M. R. Mostafa,Applanix Corporationò

Direct Georeferencing in Aerial Photogrammetric Mapping

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