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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.
Journal Staff
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