ASPRS Guidelines for the In Situ Geometric Calibration of the Aerial Camera System DRAFT

8/12/2019 ASPRS Guidelines for the In Situ Geometric Calibration of the Aerial Camera System DRAFT

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GUIDELINES FOR THEIN SITUGEOMETRIC CALIBRATION OF THE

AERIAL CAMERA SYSTEM

[Draft, October 15, !1"

1. Purpose

This document is intended to serve as guidance for the processes involved in the airborne in situ

geometric calibration of vertically-oriented digital and film-based frame type cameras. It is

prepared to assist those who are concerned with airborne applications of both digital and film-

based cameras for surveying and mapping purposes. As a system approach to calibration,

results are intended for use in the geospatial applications where high accuracies are required. It

is understood that the guidelines provided here represent only one bundle-block adjustment

approach to system calibration. In future revisions, these guidelines may cover a broader range

of camera types and include such support devices as the I!.

These guidelines are concerned with the camera airborne system consisting of the camera,

mount, "#$, aircraft, and their spatial relationships. %alibration results include both interior and

e&terior orientation parameter values along with estimates of their accuracies after a bundle-block

adjustment. These guidelines also describe pre-flight, flight and post-flight processes and a

possible calibration range design. 'efinements in the guidelines may evolve with future

developments.

The guidelines described here are based on a benchmark document by (isenhart )*+, then at

the !$ /ureau of $tandards, which provides the concept of measurement 0system1 calibration.

The preparation of guidelines described here was guided by (isenhart2s concept of measurement

0system1 calibration. 3hen comparisons are made to laboratory methods of calibration )i.e.,

camera only, significant improvements in geospatial accuracies are evident )erchant et al,

4556.

The concept of in situsystem calibration for aerial optical cameras has been clearly demonstrated

over the last several decades. Today, most final system calibrations provided by aerial camera

manufacturers, particularly for digital cameras, are produced by aerial, 0 in situapproaches.(&amples of geospatial accuracies provided by in situcalibrations may be found in /rown

)*++, erchant et al )4556 and erchant )45*4. %amera calibration reports provided by

digital aerial camera manufacturers also provide a rich source of high quality results. 7or a

benchmark document developing the concepts and theory of analytical photogrammetry see

/rown )*++.

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5. Terms and Definitions

AC% F aircraft

A$= F above sea level

A"= F above ground level

sCn - serial number

"$9 F distance measured at the nadir point subtended by one image pi&el

"8$$ F "lobal 8avigation $atellite $ystem

I! F Inertial easurement !nit

=* and =4 frequencies - (ach "#$ satellite transmits data on two frequencies, =*

)*@:@.64 hE and =4 )*44:.5 E

$A% F $imultaneous ulti-%amera Analytical %alibration

7or additional terms, definitions, commonly used symbology, abbreviated terms and notation see

the A$#'$ anual of #hotogrammetry )c"lone, 4556.

7. Specific Reuirements

Airborne cameras considered by this guideline are limited to the rectangular frame type that

provides either digital or film-based images. The digital camera arrays may be treated either as

individual arrays or as some form of merged arrays into a common frame. This includes the

merging of multi-frames into a common frame )e.g., GCI 9% or icrosoft !ltra%am which is

then presented to the user. The linear array )i.e., push- broom configuration is not considered by

this guideline at this time.

>bservations of aircraft position relative to the calibration field should be made by a "8$$

receiver collecting at least =* and =4 frequencies in a manner useful for post processing.

(&posure event markers should be provided by the camera and recorded in "8$$ time within an

accuracy of 5.555* seconds depending on the geospatial accuracy requirements.

The aircraft is modified to provide a camera port, either open or with a suitable window.

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The "8$$ antenna should be mounted in the vicinity of the vertical projection of the camera2s

optical a&is, limited by the structural requirements of the aircraft. It is preferred, but not required,

to use a twin-engine aircraft to avoid possible deformation of the air within the camera2s field of

view due to engine cooling air stream and e&haust gases.

7.1 Pre!Arri"a#Hbefore the aircraft and camera system and crew arrive at the airport in the vicinityof the calibration range

The data provider is e&pected to equip the aircraft with camera, "8$$, and support equipment.In addition to an approved camera system installation, the provider should determine, with the aidof the camera manufacturer, the location of the entrance nodal point with reference to sometangible point on the camera. 7inally, the provider should install a simple e&terior-temperaturemeasurement device with probe located in the pro&imity of the entrance node of the lens.

To insure that the camera system to be calibrated can be replicated for subsequent application ofcalibration results, a detailed system specification is to be prepared. This specification shouldinclude every element of the measurement system in accordance with (isenhart2s concept of

calibration of measurement systems )(isenhart, *+. 9uring subsequent operations of thiscamera system, the provider should assure that the system specifications are applied if thecamera calibration results are to be relied on.

7.2 Post!Arri"a# Hafter aircraft arrives at the range airport

easurements should be made to insure the recorded "8$$ data is accurately related to thecrossroad target system of coordinates. 7or a description of the 0crossroad1 calibration range,see the Appendi&. A bias may e&ist between the provider2s "8$$ measurements and the cross-road range coordinate system )standard methods are available to measure and remove this typeof bias. If a bias e&ists, it will be applied during data processing as a correction to the "8$$-determined geospatial coordinates of each e&posure station.

7inally, a preflight briefing to the crew by someone knowledgeable of the range should be held,informing them of the flight pattern over the range and, to a limited e&tent, the theory of the in situprocedure.

7.3 $#i%ht &ission

Image and data collection over the calibration range should be conducted as closely as possibleto conditions and procedures anticipated during a typical operational mission. It is understoodthat some variations, for instance in altitude and temperature, may be accepted. 9uring thephoto mission, "8$$ observations must be collected at a base station located in close pro&imityto the range center.

The flight pattern for a typical calibration mission will consist of a minimum of si& lines, eachacquiring a single frame over the intersection of roads. The directions of the crossroads shouldintersect at a nominal +5 degrees. 8o orientation with respect to north is required. 7lightdirections would then beD

In the direction of the first roadJ

In the direction 5K off the first roadJ

In the direction 6@K off the first roadJ

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In the direction 5K off the first roadJ

In the direction +5K off the first roadJ

In the direction 44@K off the first road.

These directions are arbitrarily chosen as the minimum number to balance the coverage of targetimages across the image field and to suppress possible remaining systematic errors not

accounted for in the mathematical model. The 6@K and 44@K directions are to insure thema&imum number of target images appear in the frame corners. 7or non-square format cameras,some adjustments to the aEimuths may be necessary to cause target images to appear in thee&treme corners of the image format.

7.4 Post!&ission Hbefore the data provider2s crew leaves the range airport

$patial offsets between the phase center of the antenna and entrance node of the camera shouldbe measured. It is essential that these offsets be measured in a coordinate system parallel to thecamera coordinate system and to within @ millimeter accuracy along each a&is. 7or thesemeasurements, the aircraft must be stabiliEed on the ground, and the camera2s pitch, roll andswing angles measured. ount angles should be set to Eero. These settings facilitate offsetmeasurements along camera a&es. These settings, when compared to the nominal swing, pitch

and roll angles typical of a photo mission for this aircraft, can be used to measure the spatialoffsets between antenna phase center and entrance nodal point of the camera. These commentspertain to both manual and stabiliEed mounts. It is recogniEed that operational departures fromthese values of orientation are a source of system error. To measure these angular values withrespect to the aircraft while operational, additional airborne equipment will be required. 8ote thatthe I!, if present, measures camera orientation with respect to the spatial, not the aircraftsystem of coordinates.

After the flight, it should be verified that the mission has collected adequate and reliable imageryand "8$$ data. These procedures, although time consuming, will assure that data processingcan proceed and no return of the aircraft and crew will be needed.

%omments from the data provider2s crew will be useful in refining all field aspects of thecalibration process.

7.5 Data Processin% and Reportin%

easurement of imagery and processing of the "8$$ data and range coordinates may beconducted by commercially available or other software. $uch software should be capable ofcarrying the parameters of interior and e&terior orientation as parameters with the possibility ofapplication of appropriate weight constraints. (rror estimates after adjustment should beprovided along with the estimated standard error of unit weight.7or the final step, a report of calibration must be produced and distributed for subsequentapplications. The report should include the specifications describing all equipment andprocedures when pertinent )i.e., camera sCn and type, optical filter, film, magaEine sCn,processing, scanning, flight height A"=, A$=, temperature at the lens and AC% cabin, and

antenna to nodal point spatial offsets.

The mathematical model to be used to represent the camera2s interior orientation will depend onthe level of correction that will be applied to the imagery to be provided to the user. In the case ofthe film-based camera, only the scanned image without corrections is normally provided to theuser. In this case, the conventional $A% model or other appropriate model may be evaluatedduring calibration computations resulting in estimates of interior orientation including focal length,principal point coordinates and parameters describing radial and decentering distortions. 7or afull description of the $A% model and theory, see )/rown, *++.

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%ontrol survey should result in a local, three-dimensional )(ast, 8orth, !p local

coordinate system with the same geodetic datum as that to be used to position theaircraft )3"$?6

'elative spatial accuracy along each a&is should be within * cm )5.6 inches in terms of

standard error with respect to the base station)s.

7or flights of higher or lower altitudes, the linear dimensions should be increased or decreased

accordingly. %onsiderations of focal length, pi&el siEe, frame format and camera field angle as

they influence target siEe and distribution should also be made.

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Documents

ASPRS Guidelines for the In Situ Geometric Calibration of the Aerial Camera System DRAFT