DR. T. J. BLACHUT

National Research Council of CanadaOttawa, Ontario, Canada

The Stereo-OrthophotoTechnique in Cadastraland General Mapping*

The orthophoto and its stereomate offer a means forperforming cadastral and general integrated large-scalemapping from aerial photography using simplified techniquesand equipment.

T HE STEREO ORTHOPHOTO TECHNIQUE is anovel concept in the field of mapping,

conceived in view of the many new and press­ing needs in conventional cartography andnumerous earth sciences. As the authorpointed out in his basic publication on thesubjectl, it is important that at least somephotogrammetric techniques provide simpleanswers to numerous measuring and map­ping operations. In recent years, earth scien­tists, natural scientists, geographers, en­vironmentalists, and many others have re­quired a great amount of mapping informa­tion of specific interest to them. Plotting isbeing done from aerial photographs, but theusers concerned do not like to be troubledwith the complexity and cost of the classicalphotogrammetric processes. Complex photo­grammetric plotters, relative and absoluteorientation of stereopairs. control points, etc.,are beyond the interest of the geologist, for­ester, or geographer, who is more concernedwith the specific information contained -in aer­ial photographs than with the rigorous,geometric location of selected points.Simplicity and efficiency of plotting are ofparticular interest and value in this type ofwork.

Obviously, simplicity and efficiency arethe most desirable features in any kind ofoperation, and this is precisely the reasonwhy the orthophoto technique found such abroad acceptance in the conventional map­ping field. The differentially rectifiedphotographs have the accuracy of the most

* Presented at the Pan American Conference onThe Role of Integrated Land Surveys in the De­velopment of Countries, Bogota, February 9-13,1976.

precise maps, but their production is muchmore rapid because it can be automatized toa high degree, in the same process also pro­viding precise contour lines. Furthermore,the orthophoto technique has another impor­tant characteristic: a rectified picture con­tains a wealth of information not present inany map. Much of this information is directlydiscernible in the picture; consequently, theamount of symbolization which must beadded in order to convert an orthophoto intoan orthophoto map is reduced and this, inturn, speeds up the orthophoto mappingprocess and makes it less expensive.

The presence of complete information inorthophotos is of great importance to users in

. and outside of the traditional cartographicfield. Both groups, traditional cartographersand mappers and the very large group ofearth and natural scientists, have an obviousinterest in the orthophoto product. However,in order to fully understand the approachthat should be taken to exploit the or­thophoto technique properly as a generaltool in mapping and land inventory, somerestrictive features inherent to single or­thophotos must be mentioned.

The first concerns their limited inter­pretability. With the exception of very fewand rather general applications, singlephotographic (orthophoto) pictures are notsufficient in themselves to reliably identifyall details, frequently minute at the scale ofthe photographs. This is why, in more exact­ing photo interpretation work, stereo­pictures are always used. This is also an im­portant part of the overall economy of aplanned mapping operation: the improvedreadability of stereo-pictures permits the useof smaller scales, one of the basic considera­tions in any photogrammetric mapping or

1511PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING,

Vol. 42, No. 12, December 1976, pp. 1511-1519.

1512 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

photointerpretation work. Only when usingstereo-viewing is a more reliable and com­plete extraction of information possible, inan economically advantageous and techni­cally correct mode of operation.

There is, however, another equally restric­tive shortcoming: single orthophotos notonly do not convey the general concept ofthe vertical configuration of the terrain, butalso do not permit vertical measurements ofthe terrain and its man-made or natural de­tails. This is indeed a very severe limitationof the conventional, single orthophotos,which rules them out as a self-containedmapping or photo interpretation technique.

The stereo-orthophoto technique over­comes these limitations without affecting theessential simplicity of the orthophoto ap­proach. Through the introduction of artificialhorizontal parallaxes which are proportionalto height differences, the single orthophotosare replaced by stereo-orthophoto pairs inwhich one picture is a normal orthophotoand the second (stereo-mate) is an or­thophoto modified by artificially introducedhorizontal parallaxes. Geometrically, thisprocess has a simple explanation. Whereasan orthophoto is the result of an orthogonalprojection of the terrain model on the hori­zontal reference plane, the stereomate isformed through an oblique projection on thesame reference plane, as shown in Figure 1.In the actual production, the oblique projec­tion can be obtained through an appropriatehorizontal shift of the photographic filmplaced on the reference plane. The Spanishreader can find more details on the stereoor­thophoto technique in Amaro2 and Routin3 •

From the point of view of discussion, it isimportant to understand the following basicoperational differences between the stereo­orthophoto approach and the conventionalstereo-photogrammetric methods.

(1) In the conventioanl approach, each usermust first go through the whole process ofproperly forming the oriented stereo­models of the terrain based on the originalphotographs. In the stereo-orthophoto ap­proach the users are presented with theoriented stereo-models of the terrain inthe simple form of stereo-orthophotos.Any number of stereo-orthophotos can be

FIG. 1. Geometry underlying the orthophoto andits stereomate.

contact-copied fro'm the original trans­parencies.

(2). The quality of conventional plotting op­erations depends upon the type and qual­ity of the stereoplotters and the knowl­edge and skill of the operators. In thestereo-orthophoto approach, the basicquality is built into the stereo-orthophotosand the subsequent plotting operationshave little effect on the final quality ofthe plots. It follows that the quality ofmapping products obtained through thestereo-orthophoto approach is uniform,and depends chiefly upon the easy-to­control quality of the original stereo­orthophotos.

(3) The conventional methods can supplyonly numerical products, or graphicalproducts in a line-drawn form. In contrast,the stereo-orthophoto approach providesa pictorial presentation, in addition tonumerical and graphical products, includ­ing contour lines as a by-product of thebasic production process. As a matter offact, the stereo-orthophoto technique alsooffers a fourth possibility-the truethree-dimensional map which is an al­together new form ofcartographic presen­tation1

(4) As mentioned at the outset, the stereo­orthophoto approach requires very littlespecialized training by the users. There isalso very little time lost in the preparatoryset-up before the actual plotting can bestarted. In comparison, the conventionalmethods always require the establish­ment of a properly oriented model (rela­tive and absolute orientation) before theplotting can be started, which is time­consuming.

(5) The stereo-orthophoto technique is basedupon transformed original photographs;consequently, the measurements per­formed on orthophotos are always less pre­cise than the numerical values derivedfrom conventional photogrammetric pro­cesses.

There are other significant differences invarious phases of the actual execution ofmapping and photointerpretation projects.Some of them may be better understood andprojected into the general operational struc­ture of the stereo-orthophoto system afterbasic information concerning the instru­ments involved is given.

Generally speaking, any orthophoto pro­jector designed for the production of singleorthophotos can be modified or adapted forthe production of stereomates. This is par­ticularly true of the orthophoto projectors thatwork in an off-line mode. As an example,MATRA in France, with its "third camera"system, has already been advertising, forsome time, the capability of stereo-

THE STEREO-ORTHOPHOTO TECHNIQUE 1513

orthophoto production. There is, however,an important economic advantage in produc­ing stereo-orthophoto pairs in a single­production step. In certain solutions thereare also valid technical considerations forproduction of stereo-orthophoto pairs fromoriented stereograms and not from consecu­tive single photographs. This approach hasbeen taken by Gestalt International Limited,which has equipped its highly automatizedGestalt Photo Mapper system with the capa­bility to produce stereomates simultane­ously. Since the Gestalt Photo Mapper is al­ready used in large mapping centers in sev­eral countries, the addition of the stereo­orthophoto capability will no doubt speedup the introduction of the stereo-orthophotoconcept into national mapping programs.Users of this powerful system can produceone orthophoto (or, perhaps, stereo­orthophoto) every 60 to 75 minutes or pro­cess 11 to 14 stereo-models in a doubleshift/day, not exceeding 14 hours in totalworking time. Assuming that, in developingcountries, 1:30,000 scale photographs aresatisfactory for the final 1: 10,000 scalestereo-orthophotos, one can producestereo-orthophotos covering a 160 km2 to 200k 2 territory each day. Obviously, the numberof shifts or working hours per day is quite anarbitrary matter, but one can imagine that itcould be extended up to 16 hours a day inthree shifts without imposing undue hard­ship on the working personnel.

These production figures are given only toprovide some general ideas on the matter. Asmentioned, other instrument systems alsocould be considered. Moreover, one can ex­pect further spectacular advancements in theorthophoto technique.

Once the original stereo-orthophotos areavailable, one can make an arbitrary numberof contact copies and distribute them amongvarious users. Three typical usages can beenvisaged:

(a) the field work(b) in conventional mapping and office

photo-interpretation work(c) in conventional mapping including di­

gitizing of data.Because there is no vertical parallax in

stereo-orthophoto pairs, their use by the re­spective plotting instruments calledStereocompilers are, extremely simple bypresent standards of photogrammetricequipment. Further simplification isachieved by a proper selection ofStereocompilers with features which do notexceed the requirements of the intendeduse. This is practical because even interrupt-

ing and shifting of the plotting job from onetype of Stereocompiler to another does notresult in any significant loss of time or accu­racy. Thus, the work can be distritibed in amanner that permits optimum utilization ofthe equipment. We will mention these in­struments only briefly, referring the readerinterested in more details to other publica­tions 1,4.

(a) Field work. For field work a very sim­ple field table has been developed, equip­ped with a small stereoscope mounted on afriction bracket which assures that thestereoscope remains in the desirable posi­tion. (See Figure 2.) The advantage of usingstereo-orthophotos in field work should beobvious to anyone familiar with the relevantoperations. Not only information marked orplotted in the field is geometrically in a cor­rect form and position-and therefore can betransferred by direct copying or photo­graphic method into the final maps-but theactual interpretation is being done in the op­timum operational mode, by usingstereoviewing. As a result, significant tech­nical and economic advantages are achieved.

(b) Conventional mapping and officephotointerpretation-type work. In this kindof work, graphical plotting and occasionalelevation or height measurements are ofprimary interest. If free-hand plotting is pref­erable and one does not need to plot con­tour lines, a very simple "Minicompiler" canbe used. (See Figure 3.)

For plotting, including the direct scribingof the plotting content, contouring, and forany kind ofphotointerpretation and mappingthat requires a more precise, graphical rendi­tion, a more complex instrument, calledStereocompiler I, should be considered.(See Figure 4.) In essence, this is a completephotogrammetric stereoplotter, designed to

FIG. 2. Field table to be used in conjunction withstereo-orthophotos.

1514 PHOTOGRAMMETRIC ENGINEERING &REMOTE SENSING, 1976

FIG. 3. Minicompiler

be used in conjunction with stereo­orthophotos. However, the Stereocompiler Iis equipped· to measure only thez-coordinates.

(c) Conventional mapping, including di­gitizing of data. The basic instrument,Stereocompiler II, is identical toStereocompiler I. The additional capabilityto measure x- and v-coordinates requires amore precise pantographic arrangement inorder to preserve the parallel position of thecarriage during the plotting operation andthe addition of x- and v-measuring compo­nents. These are technically simple itemsand can be added at any time. Nevertheless,precise measurement of coordinates is rela­tively expensive regardless of whichtechnique is used, particularly if automaticrecording is expected. This additional ex­pense cannot be avoided if the project re­quires numerical data. However, aSterocompiler equipped with a digitizing

FIG. 4. Stereocompiler 1.

capability remains a very simple and effi­cient instrument. Also, contrary to the cur­rent procedures in stereo-photogrammetricwork, when using the stereo-orthophoto ap­proach and Stereocompilers, various phasesof the same plotting job can be assigned todifferent types of instruments, thus assuringthe most efficient and economic use both ofequipment and technical personnel.

There is a great deal of information availa­ble concerning the accuracy provided by thestereo-orthophoto technique. The more pre­cise orthophoto projectors provide aplanimetric accuracy, expressed in x- andv-errors in the plane of original photographs,of the order of 30 J.Lm. This is also theplanimetric accuracy of stereo-orthophotos.The contouring accuracy on a Stereocom­piler can best be illustrated by the result ofone of the test plots in which original photo­graphs in a Wild A7 and the correspondingstereo-orthophotos in a Stereocompiler wereused. (See Figure 5.) The scale of originalphotographs was 1: 10,000 and of the stereo­orthophotos, 1:4,000. The difference in con­tours is indeed insignificant and of the mag­nitude to be expected even if an identicalplotter should be used in two independentcontrouring operations.

This brief information on the instrumentsfor production and processing of stereo­orthophotos and the accuracy achievedshould be helpful in understanding the op­erational structure of this novel system onwhich integrated large-scale mapping, in­cluding cadastre, is based. The unique ver­satility and characteristic of the stereo­orthophoto technique makes it particularlysuitable for larger projects with multipur­pose requirements.

1-

THE STEREO-ORTHOPHOTO TECHNIQUE 1515

__ CCIfltour. plott'" on s---.,i".!StMeo-Otthophot.I

- - - e-t_.• plotted on A-7!Originll• ......-,

FIG. 5. Comparison between contours plotted ona Wild A7 and a Stereocompiler. The scale oforig­inal photographs is 1: 10,000 and the scale ofstereo-orthophotos is 1:4000.

In Figure 6, a general scheme of an integ­rated mapping system is presented. Forreasons of brevity and clarity, this schemewill be explained by brief remarks referringto consecutive operational phases.

(1) In addition to the geodetic controlpoints, monumenting and targeting of a lib­eral number of photogrammetric controlpoints should be considered. Thse pointssimplify and improve the photogrammetricoperations and also could be used as refer­ence points in future field surveying work.

(2) The flying height must be selected ac­cording to the intended mapping scale of in-

dividual regions and areas. Obviously, citiesand built-up areas require an accuracy andscale different from remote, uninhabited re­gions.

(3) At present the most efficient andeconomic aerial triangulation can be carriedout on an analytical-type plotter. In additionto the accuracy and speed, all the input datafor the computation is being checked at thetime of actual measurements. Thus, fre­quently encountered blunders are not per­mitted to enter the computation process.

(4) Central production of stereo­orthophotos can be carried out either by agovernmental or a commerical agency. Thisproduction includes:

• original stereo-orthophotos• digital terrain information from which

elevation contours can be computed orotherwise derived

• adequate number of copies (transparen­cies) at required scales

• required number of paper prints.

(5) Cadastral field work is the bottle-neckof the whole operation. Field parties, equip­ped with the field tables for the use ofstereo-orthophotos, must ascertain all the rel­evant cadastral information, as required bythe cadastral register, in addition to carefullymarking the property boundaries on the or­thophotos. Use of stereo-orthophotos in thefield speeds up the work since it helps toidentify the location of boundary points, par­ticularly if they are obscured by vegetationor if the terrain is lacking characteristic fea­tures.

The field party should, at the same time,indicate on the orthophotos all the other de­tails of importance in cadastral and conven­tional mapping (type of houses, footbridges,trails, class of roads, etc.) unless the informa­tion concerning conventional mapping isgathered separately.

Depending upon the terrain, the conditionof the existing cadastral system, and otherfactors, several field parties are necessary tokeep one Stereocompiler fully employed. Itfollows that a large number of field partiesmust be considered in order to keep up withthe production rate of stereo-orthophotos suchas that provided by the Gestalt Photo Map­per. However, there is no other way ofachieving visible progress in establishing afunctional cadastre in American countries.

(6) By using field orthophotos with iden­tified boundary corners and other informa­tion, such as buildings, boundaries betweenrespective soil classes, a manuscript of a"cadastral" overlay is plotted on aStereocompiler II if boundary corners are

1516 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

Q) Block triangulation; on-line Ianalytical trianeulation

~

01 Central production of

~5 tereo-orthophotos

0 0 ! @

General planimetric and I Remaining land inventory

II Cadastral field work I topographical plotting Several MinicompilersSeveral StereocomDilers

Use of stereo-orthophocos •H H H H

0 - - - I - - - - I ~. .~

. . . .H H H H H

~

I"" I ". H I ]i r§" §" §" §" ~

Cadastral mappingI

~ ~I

Srereocompiler II i i I i :~ I

~ J. l I l >: >: >: I >:

_I Photogrammetric cadastral

/o work in buil (-uP areas: on-line analytical or analog

oho top:ramme t rv @

// Editing and Control I0 @ ~ @

IUPdating of cadastral data

IPrinting of large scale maps Land inventory map! I

Cadastral data bank with number of overlays

1@ ® G>I Small scale conventional

I ISmall scale orthophoto maps Land da ta bank I

topographical maps1, 50,000 1,100,0001,50,000 1,100,000

FIG. 6. Operational scheme of an integrated mapping system including cadaster.

recorded numerically. In our opinion, this isworthwhile since coordinates are very con­venient for further manipulation and storageof data. The fact that the coordinates ofboundary points are recorded has no connec­tion with the actual accuracy requirements orwith the legal validity of boundary determi­nation using coordinates. We will return tothis question later on in the paper.

(7) For urban areas we are recommendingfield surveying as the basic approach. How­ever, in view of the fact that American cities(with very few exceptions) do not have anyconsistent surveying system or the existingmaps are not up to date, photogrammetryshould be considered as being able to pro­vide a rapid and economically viable solu­tion. Tall buildings and stringent accuracyspecifications in urban areas create condi­tions with which the conventionalstereophotogrammetric methods can copemore adequately than the stereo-orthophototechnique.

Obviously, stereo-orthophotos at a suita­ble scale also should be produced for urbanareas as an immediate product. They are ex­cellent for all kinds of studies and planning,

and their production is extremely fast. Forthose thousands of cities and towns on theAmerican continents that do not have, orhave only very primitive obsolete maps,stereo-orthophotos could provide a rapid, in­termediate solution until something moreperinanent becomes available.

(8) Cadastre must have an instantaneousupdating feature to be of practical use. Cur­rently in updating, field surveying proce­dures are being used, but the alteration ofproperty boundaries must be immediatelyintroduced into the cadastral overlays andthe respective changes in property rightsand other relevant data must be recorded inthe cadastral register. Establishment of anadequate cadastral register is an essentialpart of any cadastral work and should not re­quire any further explanation here.

Survey documents, maps, evaulation andlegal data form a cadastral data bank. Ad­vanced countries are using computers andmicrofilms for storage and retrieval of relev­ant cadastral information.

(9) General planimetric and topographical(contouring) mapping is being carried out ona Stereocompiler as on any other stereoplot-

THE STEREO-ORTHOPHOTO TECHNIQUE 1517

ter. Therefore, if there should be a require­ment for conventional line-drawn maps, theycan be produced from the stereo­orthophotos, keeping in mind our commentconcerning specific conditions in urbanareas.

However, most of the mapping require­ments of the countries, particularly develop­ing countries or countries with a modestpopulatin density, can be satisfied at pres­ent with orthophoto maps. Since theamount of symbolization required for or­thophoto maps is greatly reduced in com­parison to the conventional maps, an impor­tant gain in time and expense is achieved.

Moreover, one should realize that plotting,particularly of the planimetric contents of or­thophoto maps in the usual stereo-plotters asis frequently done at present, is technicallywrong, inefficient, and expensive. Once theorthophoto map is accepted as a mappingproduct, the pertinent plotting work shouldbe carried out on the orthophotos. Other­wise, unavoidable discrepancies betweenorthophoto image and the plots would occur.This is particularly undesirable in cadastralwork.

General planimetric and topographicalplotting is done in Stereocompiler I, a veryinexpensive instrument. Operation of thisinstrument requires little training and skill.Consequently, one can afford a relativelylarge number of Stereocompilers to turn outorthophoto maps at a rate which wouldmatch the production of stereo-orthophotosin phase 4.

In view of the urgent need for maps inmost ofour countries, a map production rateof several hundred squre kilometers-per-daymust be attempted. This can be achieved rel­atively easily by using the stereo­orthophoto approach. It may not be realistic,however, to expect that cadastral mappingcan proceed with the same speed because ofthe complexity of the field work involved.Therefore a separation of the cadastral com­ponent from the general topographical map­ping ought to he considered so as to maintainthe desirable high production rate of generalmaps. Due to the unique characteristic of thestereo-orthophoto technique, this separationdoes not introduce any additional incon­venience.

(10) The strictly geometric mapinformation-topographic and cadastral­must be complemented by other data such asthat concerning soil, geology, vegetation,use of the land, climate. One should not at­tempt to include too much information ofdoubtful general value.

There is no better approach to this workthan the stereo-orthophoto technique. Theacutal work is done partly in the field andpartly in the office. The equipmentneeded-the field table and theMinicompiler-are extremely simple and ef­ficitmt. The information is mapped withadequate accuracy and in a mode convenientfor further processing find transfer of results.

(11) All the results of the cadastral, topo­graphical, and land inventory mapping mustbe checked and edited for final publicationand storage.

(12) The results are printed as needed inthe form of orthophoto maps, line-drawnmaps, overlays, or any combination of them.In each country or region, experience willtell how many copies of which products arerequired.

(13) and (14) By using cartographic reduc­tion processes, the original maps can be re­duced to a smaller scale. Some of these re­ductions require a new set of symbols.Therefore, a certain amount of additionaldrafting and editing cannot be avoided. Onthe other hand, this additional work can bekept to a minimum, if symbols are carefullyselected for the original maps, and if a certaincompromise is accepted. The goal is aspeedy and economic production of urgentlyneeded, technically acceptable maps. Oncethis basic need is satisfied there will beample opportunity to look into a higher de­gree of sophistication of the product, includ­ing its esthetic value.

(15) General land inventory maps are as­sembled in the form of overlays at uniformscale. For this purpose the original stereo­orthophotos must be reduced to the desira­ble scale already in phase 4.

(16) Computers are used for fast retrievaland manipulation of land inventory data. Atthis time, it would appear more practical tooperate with a number of specialized datafiles, than to try to establish and operate onecentral data bank that includes all the avail­able information. Therefore in phase 16 wehave in mind only the well-selected and lim­ited information that is essential in centralplanning, and in making decisions concern­ing the administration and development ofthe country. Whether these data banksshould be centrally or regionally located de­pends upon the political and cultural charac­ter of the country.

Before embarking on the establishment ofa nation-wide, integrated mapping systemsuch as is outlined above, several basic deci­sions must be made. The most important ofthem concern accuracy specifications, scales

1518 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

of original mapping, legal character of in­tended cadastre, the supporting legislation,and the time period within which the systemshould be completed. It should be clear thatthere is a close correlation between thesebasic features, and also between the charac­teristics of the intended system, its cost, andthe speed with which it can be established.

We cannot discuss the matter in much de­tail within the scope of this paper. However,some comments may help to understand thebasic philosophy of our approach better. Thekey to a successful solution is, in my opinion,a technically and economically justifiableaccuracy specification in the cadastral work.The accuracy must be set not according towhat is technically possible but what makessense and what can be justified economi­cally.

Society as such is not specifically in­terested in a meticulous definition of indi­vidual boundaries but is satisfied by theknowledge of who owns what, with an accu­racy sufficient for levying taxes and carryingout every-day planning, development, andadministrative work. On the other hand,property owners like to see their propertyrights guaranteed and protected to an accu­racy which is too costly to be acceptableeconomically. This natural divergence of in­terest can be solved by a simple com­promise, reflected in a known rule whichsets various accuracy zones depending uponthe value of the land. For example, onecould envisage the following zones:

Zone I-Towns and citiesZone II-Suburban regions and villagesZone III-Agricultural areasZone IV-Remaining lands

Accuracy specification for each zone is ac­companied by a suitable map scale, whichcould be selected from Table l.

The established accuracy of orthophotos of±O.lO to ±O.15 mm refers to well-definedpoints. Since any targeting of boundarypoints must be ruled out as impractical, asignificant, additional error when using any

photogrammetric technique must be ac­cepted. In open country and under generallyfavorable conditions one could probably staywithin twice the figures representing the or­thophoto accuracy. However, when bushesand trees obscure boundary points, quad­ruple and larger values must be expected.But even if the standard error at orthophotoscale should reach a value of ± 1 mm, theusefulness of these cadastral orthophotomaps would not be significantly affected.The cadastral system under discussion canstill guarantee the ownership of real estateand provide sufficiently precise data for fis­cal and administrative purposes, leaving themore precise definition of boundaries toother, legally recognized devices, overwhich the property owner could retain amore direct control. One such simple deviceis the placing of permanent monumentsalong the boundaries. This method,sanctioned by long-standing tradition in allcountries, obviously has its shortcomingsand limitations, but it is the only absolutemethod of defining the boundaries, and islimited in accuracy solely by the definitionof the monuments themselves, and their tinymovements due to the processes occurringin the soil and crustal movements. Themonuments do not have to be expensive ordifficult to obtain; one could use bottlesturned upside down and buried under­ground at a suitable depth, with ordinaryfield stones placed over them and protrudingover the surface of the ground.

When referring to a nation-wide cadastresystem in Latin American countries, it is ob­vious that only photogrammetry is capable ofproviding some solution. However, since themajor errors to be expected in photogram­metric determination of boundaries iscaused by a difficulty in identifying the loca­tion of boundaries on aerial photographswhen trees and bushes obscure the point,and not by the inherent inaccuracy of photo­grammetric techniques, the use of very pre­cise procedures such as analytical photo­grammetry, would be a little help. This is

TABLE 1. MAP SCALES AND CORRESPONDING ACCURACIES.

Zone IZone IIZone IIIZone IV

Zone Scale

scale 1:1,000scale 1:5000scale 1:10,000scale 1:25,000

or 1:50,000

Corresponding accuracy assumingmx = my = 0.1 - 0.15 mmat the scale of orthophoto

0.10 m-O.15 m0.50 m-O.75 m1.00 m-L50 m2.50 m-3.75 ill5.00 m-7.50 m

THE STEREO-ORTHOPHOTO TECHNIQUE 1519

also why the use of stero-orthophotos in thefield work is of such impOltance: not onlycan it speed up the work but it can also sig­nificantly improve its reliability and accu­racy. These factors are of basic importance.

The situation in city areas is different andshould be approached differently. The highaccuracy requirements in urban areas arewell justified by various technical andeconomic considerations. In spite of the dif­ficulties caused by buildings, conventionalphotogrammetric procedure based on low­altitude photographs can provide an accept­able solution for general, technical mapping.

In view of the intricate ownership situa­tion in urban areas, a high accuracy incadastral work pays off and is recommended.In large cities, only direct field surveyingcan satisfy these multiple requirements.However, we have already noted that produc­tion of surveying data for those Americancities and towns lacking maps, photogram­metry should be used and, for technicalreasons, preference should be given to theconventional stereo-photogrammetricmethods. This, of course, does not excludethe extensive use of orthophoto productswhich also present an attractive tool in a var­iety of studies and projects in urban areas.

The unique value of the orthophototechnique in establishing an integratedmapping system based on cadastre has beenconvincingly demonstrated, for the firsttime, by the country of El Salvador. After var­ious attempts by this country to follow amore conventional approach had failed, thewhole national territory was covered by1:5,000 and 1:10,000 orthophoto cadastral

maps, and a corresponding cadastral registerwithin a time span of a few years. The readercan find in Garcias a detailed account of thispioneering work. Another more recent andmore sophisticated example of a similar solu­tion can be found in the Canadian Atlanticprovinces6 . By developing the stereo­orthophoto concept, the author hopes to con­tribute to a further improvement of the over­all efficiency and quality of the orthophototechnique and to its extension into thoseareas beyond the limitations of single or­thophotos.

REFERENCES

1. T. J. Blachut, Mapping and Photointerpreta­tion System Based on Stereo-Orthophotos,

ational Research Council of Canada, Otta­wa,1971.

2. A. Garcia Amaro, "Aplicaciones de los paresortoestereoscopicos en los levantamientos to­pograficos y en el inventario de 10 recursosnaturales", Revista Cartograjica No. 18, AnoXVIlI, 1969.

3. D. D. Routin, "Estereo-ortofotografias", Re­vista CIAF 1975, Bogota.

4. T. J. Blachut & associates, "Stereocompiler",Canadian Patent 172475, 19 January, 1970.

5. Jose Alberto Gonzalez Garcia, "The Use ofOrthophotographic Techniques in a ModernCadastre Including a Data Bank", NationalResearch Council of Canada publicationCadastre edited by T. J. Blachut and J. Al­berto Villasana, Ottawa, 1974.

6. Willis F. Roberts, "Legal Measures as a Pre­requisite of a Modernized Land Data Sys­tem," Proceedings oj the North AmericanConference on Modernization of Land DataSystems (A Multi-Purpose Approach),Washington, D.C., U.S.A., April, 1975.

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