Computer-Assisted Geologic PhotogrammetryK. s. Dueholm' and C. L. Pillmoreu.s. Geological Survey, Denver Federal Center, MS 913, Denver, CO 80225

ABSTRACT: Methods have been developed that aid geologic interpretation, structural analysis, and geologic mappingon a computer-assisted photogrammetric plotting instrument. Software, called the GEOPROGRAM, is used to digitizegeologic data from the stereoscopic model and to make various direct geologic measurements. The GEOPROGRAM alsoallows the integration of mathematical structural models generated in the computer with the visual stereoscopic imageof the terrain so that surfaces can be calculated, projected, and used for interpretation. The intersection of a computer­calculated plane that represents a geologic surface and the perceived stereoscopic image of the terrain can be tracedwith a computer-guided floating mark and plotted. Used with the basic elements of interpretation - tone, texture,shape, and topographic expression - these projected surfaces provide a powerful capability for geologic interpretationof aerial photographs. Some aspects of the method treated herein are strata and fault-throw measurement, fault inter­pretation, profiling, gridding and structure contouring, and isopach mapping.

INTRODUCTION

G EOLOGIC MAPPING can be characterized as the art of un­derstanding, modeling, and portraying the three-dimen­

sional (3-D) relationship between rock units at and beneath thesurface of the Earth. During fieldwork, the geologist graduallydevelops a "conceptual geologic model" that he/she intellec­tually compares and adjusts to continuing field observations.The geologist's ground-level perspective necessitates consider­able imagination and talent to envision and model the 3-D re­lationships of the rocks. Observations and measurements arecommonly difficult to correlate from this limited surface viewand lack of overview.

The synoptic view of the "stereoscopic model" of overlappingaerial photographs provides supplemental information valuableto the understanding of the 3-D geologic relations. By use ofphotogrammetry (pillmore, 1989), very precise measurementscan be performed in the stereoscopic model. To the degree thatgeologic structures can be interpreted from the photographs,photogrammetric point measurements further aid the geologistin developing a conceptual model and ultimately in controllingthe final map product.

The interfacing of a computer to a photogrammetric instru­ment allows the 3-D point measurements to be recorded digitallyand, by means of computer programs, to be transformed intogeological parameters such as strike and dip of bedding. Basedon the measured points and the calculated parameters, math­ematical models of the geological setting are developed.

Three models are available to aid the geologist in understand­ing and illustrating the geology: the hypothetical model in themind of the geologist, the stereoscopic model perceived byviewing photographs under a stereoscope or in a photogram­metric instrument, and the mathematical computer model. Thesemodels are equally important to the mapping process but alsohave completely different functionality: (1) The hypotheticalmodel is flexible, but without consistent scale or quantitativeaspects; (2) the stereoscopic model created by photogrammetricinstrumentation is a precise, scaled model of the terrain; three­dimensional point measurements and projection of traced linesto map manuscripts are possible but limited to what can be seenand interpreted by the geologic operator; and (3) the mathe­matical model interpolates and extrapolates the measurements;through mathematical formulas the model objectively conforms·0 the underlying observations. Today, mathematical models

'From September 1987 to August 1989 on leave from the Institute ofSurveying and Photogrammetry, Technical University of Denmark, DK2800 Lyngby, Denmark under contract with the U.S. Geological Survey.

PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING,Vol. 55, No.8, August 1989, pp. 1191-1196.

are confined to relatively simple structural surfaces as comparedto the commonly complex, real-world geology; therefore, themodels have to be compared to nature and adjusted, preferablyin an interactive process.

The computer-assisted photogrammetric mapping system(CPMS) - described by Dueholm (1979) and Pillmore et al. (1981),and further discussed in this paper- provides an interactivetool for the geologist to build mathematical structural modelsbased on measurements from the field, the photogrammetricplotting instrument, and a topographic base map registered ona digitizing tablet. A unique feature of the CPMS is the abilityto project the mathematical model, which is based on calculatedplanes and surface grids, back into the stereoscopic photo­graphic model. The geologist can then visually measure andcompare geologic features with the stereoscopic image of theterrain, and interpolate or extrapolate visible outcrops accordingto the projected mathematical model. This has proved to be avery powerful tool for forming and testing hypothetical modelsand for subsequent mapping of geologic horizons.

THE COMPUTER-ASSISTED PHOTOGRAMMETRICMAPPING SYSTEM

The basic component of the CPMS is a Kern PG-2t analogphotogrammetric plotting instrument with digital output. Thefloating mark in the stereoscopic model is guided in all threecoordinates by means of a tracing assembly operated by thegeologist. The current coordinates of the floating mark are dig­itized incrementally and transmitted to the host computer. Fromthe computer the coordinates transformed to a map projectionmay be plotted on an attached pen plotter. A stepping motoris mounted on the Z-wheel of the tracing assembly, which al­lows the computer to control and position the floating mark onprecalculated elevations. A digitizing tablet is used to integratecoordinate data digitized from a map. A technician orients thestereoscopic model in the photogrammetric instrument, regis­ters a base map on the digitizing table, and prepares a manu­script sheet on the pen plotter. Typically, ground control forthe orientation of the stereoscopic model is digitized from themap with sufficient accuracy for geologic mapping.

The CPMS is designed so that a geologist can operate the sys­tem. By means of the GEOPROGRAM, geologists can digitize geo­logic data from the stereoscopic model, the base map, or fromfield sheets as desired. Elevations interpreted from well logsand from observations noted at field stations marked on pho-

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and Remote Sensing

1192 PHOTOGRAMMETRIC E GINEERING & REMOTE 5E 51 G, 1989

tographs or topographic maps are entered into the system bymeans of the keyboard. The digitized data can be used in thecalculation of mathematical structural models, plotted on thepen plotter, or stored in data files, simultaneously or individ­ually. The data can be gridded and contoured to show the shapeof geologic surfaces (structure contour maps) and used to com­pute attitudes (dips and strikes) of geologic surfaces, bounda­ries, or strata.

The projection of mathematical structural models into the ste­reoscopic model is achieved by the so-called Z-drive functionof the GEOPROGRAM, which automatically drives the Z motionof the tracing assembly on mathematical planes and surfacesthat represent geologic contacts.

REVIEW OF THE Z-DRIVE FACILITY

In the conventional topographic use of photogrammetricplotting instruments, the movements of the floating mark maybe fixed at selected elevations to trace contours along horizontalplanes. Therefore, photogrammetric plotting instruments aredesigned to facilitate the guiding of the floating mark in such amanner. Although some geologic boundaries are planar surfaces,they are rarely horizontal; they dip gently or steeply, or areoverturned completely. Thus, tracing the intersection ofhorizontal planes with the terrain is of limited value to geologists;instead, geologists need to be able to trace or follow inclinedplanes or irregular surfaces that represent contacts or geologichorizons. Following such contacts or horizons with the floatingmark presents no problem in areas where exposed rocks arevisible in the stereoscopic model, but photogeologic mappingis of limited value where rocks are obscured by soil andvegetation. The Z-drive facility guides the floating mark onprecalculated mathematical surfaces through these obscuredareas.

In order to guide the floating mark on inclined planes orirregular surfaces, the GEOPROGRAM continually compares thecurrent elevation measured in the stereoscopic model with theelevation calculated from the mathematical surface. The steppingmotor mounted on the Z-wheel continually adjusts the floatingmark to the elevation of the surface as the tracing assembly ismoved around the stereoscopic model by the geologist.

Using the Z-drive, isolated exposures may be compared withthe precalculated structural projection of a bed, thus permittingrecognition and correlation of unidentified or misidentifiedoutcrops. Geologic boundaries can be projected across valleysor from one side of a ridge to another and mapped where theycannot be traced readily through covered areas by visualinspection alone. The pen plotter tracks the movement of thetracing assembly at any scale while the Z-drive is operating, sothat the apparent intersection of the projected mathematicalsurface with the terrain observed in the stereoscopic model canbe mapped. In this manner, contacts obscured by talus, soil, orvegetation can be mapped in the stereomodel nearly as routinelyas contours are mapped in conventional topographic mapping.

Subsurface elevation data on particular geologic horizons canbe entered from the digitizer to compute mathematical surfacesand to control the Z-drive. Thus, buried horizons can be projectedfrom the subsurface and their intersection with the ground surfacecan be mapped. Also, the elevation of the horizons of interestcan be displayed at any selected position in the model; thisfeature allows the geologist to predict the position or depth ofa particular horizon in adjacent proposed drill holes for thepurpose of correlating rock units or lithologic beds.

PROJECTION OF GEOPLANES

The Z-drive was first developed to aid in geologic mappingof sub-horizontal sedimentary formations in North Greenland

(Jepsen and Dueholm, 1978). Therefore, elaborate planemanipulations are included in the CPMS system. The derivationof this capability is described in detail by Dueholm (1981), soonly an overview of the geologic capabilities is presented here.

By means of a least-squares adjustment, planes called"geoplanes" are fitted to coordinate data measured point bypoint or incrementally along a visible contact in the stereoscopicmodel, elevations from drill holes, or attitude data from fieldobservations (Figure I). Strike and dip and standard deviationsare computed and displayed or printed. The geoplanes are storedin the system and may be later combined in different ways toaid geologists in the measurement of structural parameters. Theperpendicular distance between geoplanes representing separategeologic horizons or from the floating mark to any geoplanecan be calculated and displayed. Such measurements representstratigraphic thickness (Figure 1). Two geoplanes derived fromthe same geologic horizon on opposite sides of a fault can beused to determine the amount of fault displacement. Theorientation and plunge of fold axes also can be calculated bycombining two or more geoplanes. The parallel shift functionraises or lowers the geoplane to the position of the floatingmark. This function enables a geoplane calculated from readingstaken on a visible dipping bed or contact in the stereomodel to

FIG. 1. Photograph showing a geologic surface (contact) represented bya shaded plane (geoplane). Geoplanes are calculated from points mea­sured on geologic horizons (beds) in the stereomodel of a photogram­metric plotter. The Z-guiding mode of the GEOPROGRAM maintains thefloating mark of the plotter on that calculated surface throughout the model,enabling the geologist to correlate outcrops within the model and to mapthe trace of the geoplane across covered areas.The reconstructed profile beneath the photograph shows the geoplane.The vertical distance (v) from the floating mark (F) to the geoplane andthe stratigraphic thickness (s) can be automatically calculated by the sys­tem.

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FIG. 2. Distances from geoplanes projected from geomorphic surfaces tomodern stream levels can be accurately measured and surfaces corre­lated by projecting the geoplane.

COMPUTER-ASSISTED GEOLOGIC PHOTOGRAMMETRY 1193

FIG. 3. (a) A structure contour map computed by the Surface DisplayLibrary of Dynamic Graphics, Inc. from drill-hole locations and outcrops.(b) Terrain profile showing the trace of the gridded surface drawn byGEOPROGRAM along line A-A'. (D is drill hole.) (c) The same terrain profileas in (b), showing the trace of a surface identical in form to the griddedsurface of (b) but raised a vertical distance e (i.e., a stratigraphic heighte).

surface (depth), which can be used to evaluate overburden.Numbers may be added to or subtracted from the grid toeffectively raise or lower the gridded surface so that geologic"parallel" surfaces above or below the gridded datum can beinvestigated and mapped (Figure 4). In effect, contacts can bemapped throughout the stereomodel or the outcrop area byraising the gridded surface to the altitude of a known point ona contact in the model. Integration of the computer model withthe visual model is accomplished by observing ledges, cliffs,breaks in slope, and other topographic or geologic features inthe visual model and using them as guides to mapping. Knownchanges in thickness of a stratigraphic unit within the stereomodelmay be accounted for by adding or subtracting wedges fromgrids. Finally, at any specified point on the gridded surface, afunction of the GEOPROGRAM permits plotting of the strike anddip of the surface.

The implementation of gridded surfaces is very useful in gentlyundulating, non-faulted to moderately faulted terrain, especiallywhere subsurface information is available. Use of the Z-grid

be shifted to another contact or to a field station and projectedat that level parallel to the original geoplane. An example ofuse of geoplanes for evaluation of geomorphic surfaces such asalluvial terraces is shown in Figure 2.

The plane-projection function integrates statistical error­surfaces on each side of the geoplane, which allows the geologistto evaluate new outcrops recognized during Z-driving that donot fall exactly at the level of the calculated plane.

In sub-horizontal planar bedding, the geoplane functions allowmarker horizons to be measured with accuracies around 0.1degree on dip, and projected throughout the mapping area asa valuable aid in the structural interpretation. Fluctuations instratigraphic thickness can be monitored over large distances,and even small and varying displacements along minor faultscan be readily computed. In intensely folded, thrusted, andoverturned strata, the geoplane functions can aid the computationof local attitudes, which later may be combined to describe themajor axes involved.

IRREGULAR SURFAGES

Functions for gridding irregular surfaces from random datapoints and projection of curved surfaces using a Z-grid drivewere added to the USGS system in response to requirements forcoal-bed mapping in continental sedimentary deposits.

STRUCTURE-CONTOUR STUDIES

The configuration of geologic horizons can be shown by meansof structure contours - lines that join points of equal elevationon any geologic surface, such as a formation boundary. TheCPMS can be used to produce structure-contour maps from bothsubsurface data and outcrop data. Point elevations are enteredmanually through the keyboard, or automatically from thephotogrammetric instrument. A GEOPROGRAM function forstoring drill-hole information allows the operator to enter thecollar elevation, followed by the depths of the rock units orbeds identified in the drill records. Depth data are convertedautomatically to elevations of rock units. The elevations of pointsmeasured along outcrops, as observed in the stereoscopic model,also can be entered and coded for later retrieval.

Once the coded rock data are entered into the computer,separate files are made for each rock or bed code, so that allthe elevation data, both outcrop and subsurface, for each geologichorizon can be handled as a single data set. Points can be deletedor inserted, and the XYZ data of each point can be edited. Afterediting, the data are transferred to the Surface Display Library(SOL) of Dynamic Graphics, Inc., where they are interpolated,gridded, and contoured by a main-frame computer holding theSOL procedures and transmitted back to the CPMS's onboardmicrocomputer. (The communication procedures were writtenby David Lake, a contact software specialisL)

After gridding and contouring, the grids and contours areretrieved and stored in the CPMS. The structure contour mapsand data points can be plotted by the pen plotter in correctposition on the base map (Figure 3a). If the shape of the gridded,contoured surface is not realistic and does not fit the geologist'shypothetical model, points can be checked and edited oradditional points can be added to the file on an interactive basis.The interpolated grids can then be retrieved separately for usein the Z-grid drive function as described below.

Z-GRID DRIVE

In the Z-grid function, the computer maintains the Z-motionof the tracing table on the gridded surface by bilinearinterpolation. The function allows continual readout of elevationsfrom the gridded surface; it also can display the distance fromthe floating mark placed on the ground surface to the gridded

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1194 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSI G,1989

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FIG. 4. Computation of a curved gridded surface (A) from drill-hole ele­vations (e) and points measured on outcrops (0). The sketch illustrateshow the surface can be raised to describe another level and how it tracesoutcrops on the terrain. The Z-grid function keeps the floating mark of thephotogrammetric instrument on the surface while the operator traces theintersection with the terrain.

drive can indicate areas of stratigraphic thinning, channeling,and possible faulting.

ISOPACH MAPS AND OVERBURDEN MAPS

Elevations on tops of geologic units such as formations, coalbeds, and coal interbeds may be entered from drill-hole data.Grids that represent the top and bottom of a bed are calculatedfrom these data. The grids are then subtracted and the differencecontoured, thus making an isopach map, which shows changesin thickness of the geologic bed or unit. Interburden mapsshowing thickness of strata between coal beds can be computedin a similar manner.

Overburden maps can be constructed by subtracting thegridded geological horizon from a digital elevation model (OEM)of the surface. The OEM may be calculated from points digitizedin an arbitrary pattern from the stereoscopic model, or it couldbe one generated by the USGS National Mapping Division.

An overburden map to be used as a guide to surface miningcan be easily prepared using the 2-grid drive function of theGEOPROGRAM. Open-pit mines are usually planned on the basisof ratios between thickness of the bed to be mined (a coal bed,for instance) and the rocks and soil above it (overburden) thatcan be economically removed. By gridding the surface of thecoal bed to be mined from drill data, required overburden-limitmaps can readily be constructed by adding designatedoverburden increments to the coal-bed grid and tracing thefloating mark along the terrain surface as the Z-grid drive functionmaintains the proper vertical increment.

PROFILES AND CROSS SECTIONS

Terrain profiles are used to construct geologic cross sectionsthat show the relationship of rock units and geologic structuresintersected along the line of profile. Cross sections representthe geologist's concept of the geometry beneath the surface ofthe profile. The system allows profiles to be drawn in any di­rection and displayed at any scale within the limits of the plot­ter. They can be plotted as true length profiles along any curvedpath of data gathering (e.g., stream profiles) or projected to aprofile line defined by the starting and stopping points of data.The vertical (2) element of the profile can be exaggerated asdesired to enhance topographic relief that might aid in geologicin terpretation.

Profiles can be drawn from either the stereoscopic model or

the base map. Profiles are digitally constructed from the modelusing points that are entered incrementally as the tracing as­sembly is moved along the line of section, keeping the floatingmark on the ground surface. The profile points are stored inthe system, and the profile can be drawn on the pen plotter atdesired scales and vertical exaggeration ratios. To draw profilesfrom the contours on the base map using the digitizing table,only the elevations of the start and end points, and the contourinterval of the map need be entered into the system. By pressingkeys on the digitizer cursor that are coded for uphill, level, ordownhill movement, points are digitized where contours crossthe line of profile. Terrain extremes (ridge crests and valleybottoms) may be digitized and elevations entered manually asthey are encountered along the profile.

Once the profile is stored and drawn (using either technique),geoplanes that represent geologic surfaces stored in the systemcan be retrieved and projected into the profile to aid in con­struction of geologic cross sections (Figure 5). The program isdesigned so that geoplanes can also be generated from fieldobservations by entering the strike and dip from the keyboard.These geoplanes can also be plotted on the profile. The trace ofgridded surfaces, generated by the computer from subsurfaceand outcrop information, can be shown on the plotted profile,as well (Figure 3b and 3c); thus, automated construction of geo­logic cross sections is possible.

Stream profiles that accurately portray changes in gradientand knick points along streams can be drawn and plotted astrue length profiles or projected profiles. Stream cross sectionsshow changes in the shape of the channel along the stream thatmay reflect the rock type.

Another function allows the construction of thickness sec­tions. A profile is constructed along a line where a photogram­metrically measured section is desired. After the profile is drawn,either a photogrammetric or a field-measured geoplane is en­tered and projected on the profile (Figure 6). The floating markis then positioned on field stations or model image points alongthe line of section; these stations are then automatically notedand labeled on the line of section at their proper position (Figure6).

COMPUTER-CONTROLLED PHOTOGRAMMETRICSYSTEMS

The CPMS, however versatile, is limited by the analog/me­chanical design of the photogrammetric plotting instrument used(i.e., the Kern PG-2), and many GEOPROGRAM functions arerestricted because of the limited capacity of the computer usedfor the original program development (the HP-9825 desk-topcomputer) and the programming language applied (HPL).

ANALYTICAL PLOTTERS

Analytical plotters reconstruct ground-coordinate positionsmathematically instead of by mechanical analogs. They are notlimited by mechanical linkage, and the photograph stagemovements are computer-controlled. System software maycontrol the movement of the measuring mark in all threecoordinate directions. In the PG-2-based system, only the 2­motion can be driven by the computer, which presents problemswhen the geoplanes used to drive the 2 become too steep. Ingeneral, the PG-2 based system works well if dips are less than60 degrees. The PG-2 can drive the pen plotter, but neither thedigitizer nor the flat-bed plotter can drive the X and Y motionsof the PG-2. This restricts the flexibility in comparing thestereoscopic model with lines drawn on a map, or alreadydigitized data in a file. Such restrictions are not present inanalytical plotters. Some new features of potential importanceto geological mapping are described below.

Points and linework that were measured along an outcrop or

COMPUTER-ASSISTED GEOLOGIC PHOTOGRAMMETRY

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FIG. 6. Cross section showing actual or inferred outcrop points drawn and labeled bythe GEOPROGRAM. Stratigraphic distances to subunit tops (shown after each unit label)are automatically calculated for each point using a projected geoplane as basis (geo­plane 1). If attitudes change along the section, another geoplane may be entered(geoplane 2) and used for stratigraphic distance calculations.

contact in the analytical plotter and stored in the system can bere-occupied automatically and checked. By following contactson a geologic map with the cursor, the floating mark can alsobe driven in X and Y, once the map has been oriented to theaerial photographs. By keeping the dot on the ground as contactsare followed on the map, the operator(s) can visualize what wasmapped originally. This feature could promote data verificationand enable error estimates to be assigned and tracked throughoutthe map compilation process. Likewise, the computer can recorda photogrammetric traverse during mapping and drive thephotograph stages to reverse the traverse or repeat the exactmovements of the cursor, to show how particular features weremapped and indicate any necessary corrections.

Analytical plotters may be controlled by modernmicrocomputers (PCs) that have much more capacity than wasavailable in the CPMS computer system, or by multitaskingminicomputers that have virtual memory. Powerful commercialsoftware programs for database manipulations and geographicinformation systems are available for these computers. Theintegration of modern database systems with the CEOPROCRAMfunctions for the storage and retrieval of points, planes, profiles,and drill-hole information will improve the flexibility of the datamanipulations. Geologic structures commonly are morecomplicated than those that can be represented by simple flatplanes and gridded surfaces used for Z-guiding in the CPMS.Software for more complex modeling of geological structures,including mathematically described curved surfaces instead ofthe gridded surfaces and allowing for incorporation of faultsand complicated folds is being researched and developed atmany universities and companies. Such software, in an easy­to-use version, would be an important supplement to the moreprimitive modeling software now used. On the other hand,modeling software for geologic structures would, in fact, benefitfrom the comparison with the stereoscopic model.

A stereo superimposition system is available for some analyticalplotters (Cogan et a!., 1988). By means of this system, the geologistcan observe a 3-D model of digitized lines superimposed on thestereoscopic photographic model. Because any stored lineworkcan be superimposed, complicated mathematical structuralsurfaces can be depicted graphically and projected into the modelas an improvement of the present Z-guiding facility.

GEOGRAPHICAL INFORMATION SYSTEMS

Incorporation of a modern Geographical Information System(GIS) for storing, editing, and drawing of digitized geologic mapdata will enhance mapping capabilities of the systems andimprove the quality of the final map products. Though prototype3·D raster systems exist, commercially available GIS programsare based on two-dimensional topology. Some even considerthe elevation data as an attribute rather than a spatial coordinate.Because the CPMS is a three-dimensional system, such two­dimensional GIS systems would impose limitations that wouldcause problems with many applications of the GEOPROGRAM. Adatabase and GIS system with true three-dimensional topology,where the spatial relationship between points, lines, surfaces,and volumes are solved, would, on the other hand, be theultimate tool when combined with an analytical plotter and thestereoscopic model. Combined with the above describedsuperimposition system, a stereoscopic framework oftopologically arranged data could be compared to the stereoscopicimage of the terrain, thereby allowing a superior check of digitizedand stored data in the GIS system. The ability to access, utilize,and project other data sets resident in the GIS could benefit thephotogrammetric mapping process.

SUMMARY

Though a prototype system, the CPMS has been very usefulin many different geological mapping projects at the GeologicalSurvey of Greenland and the U.s. Geological Survey in Denver,Colorado.

The CPMS is designed so that geologists can operate the sys­tem after a technician has oriented the stereoscopic model tothe base map. The CPMS is interactive and aids the geologist inthe verification of hypothetical structural models by allowingthe formation of three-dimensional mathematical models fittedto measurements, and the projection of these models into thestereoscopic photographic image. Intersection between thesemathematical models and the terrain can be interpreted, veri­fied, traced, and mapped.

The CPMS offers a true three-dimensional mapping capability.Geologic data digitized into a two-dimensional GIS system maysomeday become obsolete because the nature of geology is three

1196 PHOTOGRAMMETRIC E GINEERI G & REMOTE SENSING, 1989

dimensional. Topographic mapping can utilize contours to de­scribe the terrain because all topographic features reside on theground surface - geology extends beneath that surface.

Software for geologic photogrammetry is being developedcontinuously. Analytical plotters greatly expand the horizonsfor development of automated mapping systems for geology,especially when ultimately combined with software to handlea true three-dimensional topological description of the geology.

REFERENCES

Cogan, L., D. Harbour, and C. Peny, 1988. KRISS Kern Raster ImageSuperimposition System, Proceedings of Commission IV, CommissionIV, ISPRS Kyoto.

Dueholm, K. S., 1979. Geologic and Topographic Mapping from Aerialphotographs, Meddelelse Instilullet for Landmaling og Fotogrammelri,Technical University of Denmark, No. 10, 146 p.

--, 1981. Computer-supported geological photo-interpretation,Phologralllmelria, Vol. 36, pp. 159-171.

Jepsen, H. F., and K. S. Dueholm, 1978. Computer supported geolog­ical photointerpretation, Rapporl Cronlands Ceologiske Undersgels, No.90, pp. 146-150.

Pill more, C. L., K. S. Dueholm, H. S. Jepsen, and C. H. Schuch, 1981.Computer-assisted photogrammetric mapping system for geologicstudies-a progress report, PllOlogralllllletria, v. 36, p. 159-171.

Pillmore, C. L.. Geologic photogrammetry in the U.S. Geological Sur­vey, 1989. Phologral11l1letric Engineering and Remote Sensing, Vol. 55,No.8, pp. 1185-1189..

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