Photogrammetry and Real-Time GraphicsEngineering ApplicationsHoward TurnerPurdue University, West Lafayette, IN 47907
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ABSTRACT: Real-time three-dimensional graphics interfaces are examined for mapping engineering application.s. A system developed and implemented in the Photogrammetric Analysis Laboratory at Purdue UniversIty IS descrIbed. Examples of engineering applications are given and illustrated, and the problem of data integration from different datasources is addressed in one example.
INTRODUCTION
REAL-TIME THREE-DIMENSIONAL GRAPHICS INTERFACES havea high potential in the fields of photogrammetry, digital
mapping, and spatial information systems, and they are particularly useful in mapping for engineering applications. However, commercial photogrammetric instrument manufacturersand research institutions have been slow to exploit this potential. Many on-line digital mapping systems have been described(Hoehle and Jakob, 1981; Erez and Dorrer, 1984; Marckwardtand Murai, 1984), but the profession has been slow to acceptreal-time interfaces which fully utilize feedback concepts inphotogrammetry, digital mapping, and geographical information systems. Photogrammetry has been used off-line for manyyears in engineering applications (e.g., Veress and Sun, 1978;Faig, 1972). More recently, on-line and real-time measuring systems have been used (Powell, 1984; Armenakis, 1984; Armenakis and Faig, 1987; El-Hakim, 1986; Haggen and Leikas, 1987).Real-time graphical interfaces can only be fully exploited in conjunction with analytical plotters or all-digital systems. This typeof interface can be of value in two subject areas. First, in thegraphical display of data collected on an analytical plotter, andsecond, in the superimposition of graphical data back into thephotogrammetric model.
Chen (1986) has described interactive graphics interfaces developed to operate with a Wild BC2 and a Zeiss PLANICOMPanalytical plotters. Real-time graphical superimposition has beendescribed by Uffenkamp (1986) and Reinhardt (1986) for theZeiss PLANICOMP analytical plotter. Real-time stereo graphical superimposition has been discussed by Beerenwinkel et al.(1986) and by Schneeberger and Burgermeister (1987) for theWild SYSTEM 9 analytical stereoplotter. Greve and Molander(1987) have described the APPS-IV system with stereo graphicalsuperimposition.
The aim of this paper is to examine the principles involvedin the use of real-time three-dimensional graphics interfaces indata collection and quality control in photogrammetry, digitalmapping, and spatial information systems. Emphasis will beplaced on three-dimensional interfaces, while graphical superimposition, which is a two-dimensional interface will not bediscussed. This paper will examine, discuss, and illustrate someresults, obtained for engineering purposes, using a three-dimensional graphics interface implemented in the Photogrammetric Analysis Laboratory at Purdue University.
ON-LINE AND REAL-TIME SYSTEMS
There is a fundamental difference between on-line and realtime systems, and the difference between the two is frequentlymisunderstood. Systems can be on-line and real-time at thesame time, but not every on-line system operates in real-time.
One essential difference between on-line and real-time sys-
PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING,Vol. 54, No.9, September 1988, pp. 1313-1317.
terns is one of response time to a query. El-Hakim (1986) hasdefined a real-time system in photogrammetry as "a systemwithout interruptions, or appreciable time lags, between ac-quiring the image and the final product.". .
Another important difference between real-time and on-lmesystems is in the feedback domain .. Fe.edback IS a n~cessary
component of a real-time system whIle It IS not e.ssentIal to anon-line system. Erez and Dorrer (1984) have recogruzed the valueof feedback in real-time systems by stating, "Data have to beverified during the process of digitizing, ~herever possible. A'fully interactive system' allows the correc~l?n ?f err?r~ m realtime. A non-interactive system makes venficatIon difficult andinconvenient." A non-interactive system may be representativeof on-line or off-line processing.
A REAL-TIME PHOTOGRAMMETRIC MAPPING SYSTEM
The photogrammetrist working in a pro?uction environmentmay be faced now or in the near future With the purc~ase andimplementation of graphical interfaces. One of the pnme .concerns is system configuration. Another concern IS the mamtenance of maximum quality control in a production environment.
Figure 1 shows conceptually the config~ration of a. syste~.The analytical plotter is shown in the configuratIo~WIth a dI~
tributed computer system. The burden of the analytical plotter sreal-time loop is, therefore, taken off the host processor. Ideally,the host processor should run under a multi-tasking .operatingsystem to address the analytical plotter an? the graphIcs systemwithin a short time frame. In the mappmg process, a threedimensional database is stored in the host computer and trans-ferred to the graphics system in real-time. .
In each model, the assumption is made that a real-time photogrammetric model exists on the analy~ical plotter, an~ thatthe photogrammetric model is fully onented. Three-dlme~
sional data are digitized on the analytical plotter and stored ma three-dimensional data base. It is also assumed that the graphics system operates in the same coordinate system tha.t is usedin the analytical plotter and that the photogrammetnc modelcoordinate system of the analytical plotter prOVIdes the reference system for the whole network. Furthermo~e, the real-timeinterface system to the analytical plotter should Insure adequateresponse time, less than a few seconds, for qualIty assurancemodeling.
GRAPHICS SYSTEMS
Graphics systems can be classified as two-dimensional or .threedimensional by the way they display info~matio.n on a displayscreen. Display screen surfaces are two-dImensIOnal. The realworld which is mapped in surveying is three-dimensional, ~utthe map produced of the real world is a two-dimen~ional entitywith the third dimension (2) represented as an attnbute on themap.
FIG. 3. Additional views used in three-dimensional graphics.
coordinates. Clipping ensures that nothing is displayed outsideof the viewport.
One of the benefits of computer graphics is the ability to drawpictures of three-dimensional objects. The basic problem inmaking a picture of a three-dimensional object is representingthe third dimension, depth, on a two-dimensional screen. Thecommon approach is to make a perspective or isometric drawing of the object. In a three-dimensional system, the user canusually understand the object in the perspective view, but it isdifficult to obtain precise distance and depth relationships froma single view. In engineering, it is common practice to generatethree additional views-a top view, a side view, and a frontview - which are shown in Figure 3.
There is a tendency in production environments to use online mapping with the digital map displayed as a two-dimensional entity on a two-dimensional display screen. While thistype of system has been common in the past with analog plotting instruments, there is an increasing tendency for productionenvironments to use analytical plotters. Systems which haveworked well in the past with analog instruments and will stillwork with analytical plotters may require some redesign foroptimum production.
The introduction of the analytical plotter also brings the concept of feedback into the mapping domain. Maximum production from an analytical plotter can only be achieved by fullyutilizing feedback. The ability to use feedback depends on theconfiguration of the graphics system.
Feedback enters the system because it is desirable to correctany plotting error seen on the graphics screen in situ as closeto the error source as possible. The error is corrected by pointingto the mistake with the graphics pointer. With a three-dimensional graphics file, the stage plates of the analytical plotter aredriven to the point where the mistake was made. No parallaxappears in the real-time photogrammetric model and the modelremains intact and complete. The mistake can, therefore, beimmediately corrected. On the other hand, a two-dimensionalgraphics file does not have the same feedback capabilities. Whenthe stage plates are driven to the point of error, parallax appearsin the photogrammetric model because the Z value cannot berecovered from a two-dimensional file where it has been converted into an attribute.
The interface between the analytical plotter and the graphicssystem attempts to solve five of the problems in digital mapping. First, data are collected in their true geometric form andused in a three-dimensional database; second, the collection ofraw data can proceed at a rate much faster than traditionalanalog mapping; third, some blunders can be detected in situ
FOUR PARAHETEA
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LEFT
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FIG. 2. Transformations from global coordinate system to display coordinate system.
A two-dimensional graphics display is, therefore, a replication of the analog mapping process which has been practicedfor many years. In a two-dimensional graphics system, the endproduct, a digital map, is displayed in two-dimensions on atwo-dimensional display screen. An orthogonal transformationis used to reduce the three-dimensional real world to a planeprojection with elevation data shown as a labeled attribute. Afour-parameter transformation is then used to convert the planeprojection mapping into the display system. Figure 2 shows adiagram of the operations involved. The mapping is describedby a window in ground coordinates and a viewport in display
PHOTOGRAMMETRY AND REAL-TIME GRAPHICS 1315
ISOMETRIC
FRONT
TOP
SIDE
n • s
FIG. 4. Contour map of Gais area of Switzerland displayed in a three-dimensional graphics file.
at the' time they are created; fourth, all data collected are referenced to a geodetic framework; and fifth, edge matching between map sheets is performed at the time of data collection.
IMPLEMENTATION OF THE GRAPHICS MODEL
A Kern DSR-1 analytical plotter was interfaced to an Intergraph 751 graphics system. The interface allows data collectedon the analytical plotter to be displayed in real-time on theIntergraph InterMap workstation. The Intergraph design file isconfigured as a three-dimensional file with the top view as thedefault view. The other views can be configured in any order.Two cursors appear on the display screen, one belonging to theanalytical plotter and the other to the workstation. The cursorfrom the analytical plotter is confined to the default top view,while the workstation cursor can operate in any view. Data arestreamed across the interface and recorded in all four viewssimultaneously. Figure 4 shows a contour map of Gais area ofSwitzerland as it appears simultaneously in the top, isometric,side, and front views.
Three-dimensional graphics displays have considerable benefits over two-dimensional displays in the field of digital mapping. Multiple views are a benefit in many engineeringapplications. In some engineering tasks, cross sections are usedfor volume calculations. Cross sections are collected by profiling. In profiling, a two-dimensional graphics display will showa line passing over the windowed area if the cross section isstored in a two-dimensional file. In a three-dimensional file, theside view allows the actual profile to be displayed. This canclearly be seen in Figure 4.
Three-dimensional graphics displays can also be used in coastalengineering models. Figure 5 shows a test plot of a coastal areaof Lake Huron mapped to evaluate the potential of the graphicssystem to provide data for coastal erosion models. In engineering, coastal erosion models have traditionally been tested bycollecting a large number of cross sections in the field.
Data required in the coastal environment are mapped into athree-dimensional graphics file. Photography is flown parallel
to the coast with the maximum amount of land possible on themodel to obtain good restitution in the photogrammetric model.Data are mapped into the graphics system configured as a threedimensional file with the top view as the default view, and theside view, the front view, and the isometric view as additionalviews. In the example shown, the top view represents the traditional two-dimensional map, the isometric view representsthe view that planners and decision-makers prefer, the sideview represents the profile, and the front view represents theview from the lake.
DOCUMENT REVISION AND INTEGRATION
Data used in engineering applications are rarely collected inthe same time frame or with the same spatial precision or onthe same coordinate basis. Integration of different types of datainto a uniform base is a common problem. Frequently, the dataare stored in the analog form of maps, plats, or building plans,and there is no indication as to the quality of the document.The real-time interface between an analytical plotter and agraphics system, described and implemented here, is used totest the validity of existing documents.
Existing maps are digitized into a three-dimensional graphiCSfile. Plats, which rarely show coordinates, are also digitized iffeatures shown on the plat are identifiable in the photogrammetric model. The feature is plotted from the photogrammetricmodel into the three-dimensional graphics file, and coordinatesare obtained from the graphics file using the tentative buttonon the cursor. These coordinates are used as control values todigitize the plat. Three-dimensional digitization can be into anydisplay view but, with a map and a plat, it is more convenientto digitize into the top view. With building plans, where elevation diagrams are given, digitization is easier into the frontand side views.
For a map, the Z value is set for each feature at a differentelevation and digitization is performed in the same manner astwo-dimensional digitization. Once all or part of the map isdigitized into a three-dimensional file, the precision of the doc-
FIG. 5. Three-dimensional graphics display of part of coast of Lake Huron, Michigan.
FRONT
ISOMETRIC TOP
SIDE
FIG. 6. Three-dimensional display of part of Civil Engineering Building, Purdue University.
ument is checked by driving the stage plates of the analyticalplotter, functioning in real-time, to the point of interest. Discrepancies are measured between the digitized existing map orplat and the restituted photogrammetric model. The photogrammetric model can be from the time frame of the documentbeing evaluated or from a different time frame. By analogy,
building elevation plans could be tested by using industrialphotographs of the building restituted in the analytical plotter.
Figure 6 shows an application of the real-time mapping system discussed in this paper in integrating data. A common requirement is to develop a spatial information system for anindustrial plant or a university. Existing documents, in the form
PHOTOGRAMMETRY AND REAL-TIME GRAPHICS 1317
of building plans, plats, or as built plans frequently exist, butthere is no relationship between these different analog forms.Figure 6 is a three-dimensional plot of part of the Civil Engineering building at Purdue University, which demonstrates thecombination of aerial photographic data and building plan datainto a single file.
Aerial photography was flown of the campus in 1984. Signalized targets were marked on the sidewalks and coordinatedbefore the photography was flown. Aerial photographs wereplaced in the analytical plotter and points on the roof of thebuilding, which could be recognized on the building plans, weredigitized and displayed on the graphics system in a three-dimensional file. The height of the building beneath these controlpoints was determined from the building plans and a verticalline was placed in the graphics file at these points. Controlcoordinates were thus established for points on the top andbottom of the building. The control coordinates were used todigitize the building plans into the three-dimensional file. Digitization can be performed into any view in the graphics system.The building plans were digitized into the top view for plandrawings and into the front or side views for elevation drawings.
CONCLUSIONS
A real-time photogrammetric mapping system developed andimplemented in the Photogrammetric Analysis Laboratory atPurdue University has been described. Several examples of realtime mapping have been given. The potential of such a systemcan be extended to various other applications. The feedbackfrom the graphics can be used to locate and check ground control points in aerial triangulation and industrial photogrammetry. The system can be used in digital terrain modeling. Datapoints can be collected on optimum driving paths and contoursinterpolated as the points are collected. In the future, it is believed that map production environments will change to threedimensional displays of digital data once the advantages of suchsystems have been realized.
ACKNOWLEDGMENTS
The Photogrammetric Analysis Laboratory was partially supported by an equipment grant, number DAA29-85-G-0026, fromthe U. S. Army Research Office.
REFERENCES
Armenakis, c., 1984. Deformation Measurements from Aerial Photographs, International Archives of Photogrammetry and Remote Sensing,Vol. 25, Part A5, Commission 5, pp. 39-48.
Armenakis, c., and W. Faig, 1987. Sequential Photogrammetry for
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Greve, C. W., and C. W. Molander, 1987. The Architecture of the APPSIV Analytical Plotter, Technical Papers of the ASPRS-ACSM AnnualConvention, Vol. 2, Photogrammetry, Baltimore, pp. 208-215.
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Marckwardt, W., and S. Murai, 1984. Computer-Assisted StereoplottingSystem CASP, International Archives of Photogrammetry and RemoteSensing, Vol. 25, Part A5, Commission 2, pp. 361-367.
Powell, G. E., 1984. The Use of Photogrammetry in the Manufacture ofHigh Performance Aircraft, International Archives of Photogrammetryand Remote Sensing, Vol. 25, Part A5, Commission 5, pp. 622-626.
Reinhardt, W., 1986. "Optical Superimposition of Stereo Model andGraphical Information as a Tool for OEM Quality Control, Presented Paper, International Society for Photogrammetry and RemoteSensing, Commission 4, Edinburgh.
Schneeberger, R., and W. Burgermeister, 1987. The New Wild System9 Analytical Plotter Workstation S9-AP, Technical Papers of the ASPRSACSM Annual Convention, Vol. 2, Photogrammetry, Baltimore, pp.180-189.
Uffenkamp, V., 1986. Improvement of Digital Mapping with GraphicsImage Superimposition, Proceedings of the Symposium FROM ANALYTICAL TO DIGITAL, ISPRS, Commission 3, Rovaniemi, Finland,pp. 665-671.
Veress, S. A., and L. L. Sun, 1978. Photogrammetric Monitoring of aGabion Wall, Photogrammetric Engineering and Remote Sensing, Vol.44, No.2, pp. 205-211.
(Received 27 December 1987; accepted 12 February 1988; revised 13 May1988)
Geographic Information Systems Seminar
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