ALAN D. JONESThe University of New England

Armidale, N.S.W., Australia

Computers and the Teachingof Airphoto InterpretationThe technique described allows the computer to check thecorrectness of the student's interpretation.

INTRODUCTION

T HE ASSEMBLY of a series of practical exer­cises in airphoto interpretation is fre­

quently the result of trial and error over aperiod of years. Identifying and obtainingsets of photographs which illustrate not onlythe general principles of photo interpreta­tion, but also serve to introduce the studentto either a particular geographical area orfield of study, or both, takes time. Whileavailable workbooks and other publicationsprovide valuable material, one of the basicproblems remains, that of checking the stu­dent's results.

Frequently a student exercise begins"Identify the objects numbered one ton...." Alternatively, he may read "Com­pile a land use map from...." The formertask is useful in the initial stages of a coursein that it directs attention to a particularfeature and aids checking of interpretationwith large classes. The latter rubric may beopen-ended in which no guidance is givenas to a classification system. This has merit,but at a more advanced level. Where a par­ticular classification scheme is to be usedthe problem becomes one of checking. Be­cause no two students produce identicalthematic maps (under normal circum­stances), an overlay helps the checkingprocess which still remains laborious andopen to error. In discussing why a particu­lar interpretation is incorrect the problem ofthe positioning ofboundaries also arises, andwhile this too is an impOitant palt of thelearning process it divelts attention from theinterpretation.

It is possible to overcome some of theseproblems by drawing up a rigid specifica­tion. This solution might be quite accept­able for in-service training but would pre­sent problems in a more general educationalenvironment with the need to produce inter­preters who would be flexible and not con-

strained by a particular organizationalhamework.

The increasing trend towards caltographicdata banks and the widespread use of line­printers to produce thematic maps led theauthor to experiment with the use of a com­puter as a means of displaying thematicmaps based on air photo interpretation. Theuse of computers in this way allows studentsto be simultaneously introduced to threeaspects of data collection, manipulation, anddisplay in addition to the interpretation ofthe air photo. These aspects are-

• Collection of data from air photographsusing regular grid sampling,

• Creation and manipulation of cartographicdata banks, and

• Automated output of spatially distributeddata.

In addition the method also enables theinstructor to be relieved of the tedious taskof correcting students' work and makes hisassessment of that work more rigorous.

DATA COLLECTION

The data collection part of an exercise in­volves systematic sampling of the studyarea. Ideally, if the data are to be incor­porated into a data bank, the grid usedshould be a national sUlvey grid. In Australiathis would be the Australian Map Grid(AMG) which is based on the UniversalTransverse Mercator Grid.

For reasons of cost, simplicity, and readyavailability the most acceptable system forgenerating graphic output is the computerlineprinter. The use of a lineprinter, how­ever, presents problems in that the charac­ters printed are rectangular and not square.Using the national survey grid as the basisfor digitizing would produce output which,if uncorrected, would be stretched in onedirection. Thus, the output could not bedirectly superimposed on the source docu­ment. For instruction this would be an

1267PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING,

Vol. 44, No. 10, October 1978, pp. 1267-1272.

1268 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1978

impOitant feature; for this reason it was de­cided that the grid to be used for digitizingwould not be the national survey grid, but anarbitrary grid based on the character size ofthe lineprinter. \Vith a lineprinter operatingon ten characters per inch horizontally andsix characters per inch veltically, a grid wasestablished based on three characters hori­zontally and two characters veltically. Such agrid approximates reasonably closely to asquare without producing too large a gridinterval. Alternatively, a 5 by 3 set of char­acters is the smallest arrangement availableto produce a square grid.

A fUlther reason for not using the nationalsurvey grid is that the data are collecteddirectly from the air photographs on whicha grid is superimposed. The effects of tiltand ground relief would introduce anotherproblem into the exercise if the national sur­vey grid were to be used. While this is animportant aspect of data collection it is anadditional complication not generally wel­comed in the earlier stages of an airphotointerpretation course.

Initially, students were asked to constructtheir own grids on the air photographs. Itsoon became apparent that the accuracywith which the grids were constructedvaried considerably. Therefore, grids arenow premarked on the photographs. Other­wise, the instructor would have to deter­mine if a problem is due to incorrect locationof the grid or incorrect interpretation. At thepresent time the exercises operate withstereotriplets, the grid being marked on thecentral photograph.

To avoid problems which involve a sub­jective judgement, the use of the grid cellitself as the basic unit was abandoned infavor of using the grid intersection. It mightbe argued that by using the grid cell that themajor components of the area are recordedand this ought to be the method used. Thispaper does not attempt to review the argu­ments in favor of one system or another ingeneral terms. In the context of planningthis particular project the main objectivewas to eliminate as far as possible errorsother than those arising from incorrect inter­pretation. Two possible situations are shownin Figure 1. A decision as to which categoryeach square should be assigned will clearlybe subjective. For example, classes A and Bcover 50 percent of the area of each of thetop two grid cells, while in the two lowercells there is a three way split between A,B, and C. If each grid cell can only be re­corded as a single class, then there is nounique answer and assessment of the stu-

/ 1",/ ,

A/ ,

A/ ,/ ,

/ ,/ ,

/ B B,

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FIG. 1. Hypothetical land use map (dashed lines)with grid (solid lines).

dent's work is very subjective. The locationof the grid intersection, on the other hand,is objective and the student has only to inter­pret the feature at that point. This is not tosay one method is better than the other forgeneral use. The nature of the data beinginterpreted and their intended use will af­fect the choice of method.

This objectivity is important when check­ing students' work because the computer isable to calculate a percentage correct figurewhich refers to the correctness of the stu­dent's interpretation of the air photographand does not incorporate variables such asgrid location or a dubious decision as towhich feature covers more than 50 percentof a grid cell.

DATA PROCESSING

Even with clear instructions regarding thecollection and presentation of the data,errors continue to be made. To reduce thefrequency of these errors a separate programwas written to operate on a minicomputer(PDP-llT34) to which students have directaccess on a cafeteria system. This allowsstudents to input their data and produce asimple lineprinter map. If the data set doesnot have the correct dimensions (e.g., a rowmissing), the computer rejects the data andthe student needs to look for the source ofthe problem.

When the data have been checked, thecard decks are handed to the instructor whoruns them on a larger computer. His pro­gram checks each student's data matrix withthe correct data matrix, and computes thepercentage correct and the percentage ofeach land use category (or whatever is beinginterpreted). In addition, an error matrixindicating the ways in which each point hasbeen interpreted or misinterpreted is pro-

COMPUTERS & THE TEACHING OF AIRPHOTO INTERPRETATION 1269

duced. While each exercise is beingchecked, a running total of the error matrixis also calculated. This allows the instructorto check if a palticular error in interpreta­tion is common to the whole class or only toa few individuals. The program * is writtenin FORTRAN as a series of subroutines whichcan be called up in varying combinationsto produce single map outputs, comparechanges in land use, and so on.

DATA OUTPUT

As the system is currently arranged eachstudent receives three maps:

(1) A map showing his interpretation andwhere he has made a mistake in interpre­tation (Figure 2). This is done by puttingXXX in the top row of the appropriategrid cell and the student's interpretationbeneath. Beneath this map is an indica­tion of the percentage area covered by

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* Copies of the program may be obtained fromthe author who makes no claims to being anythingother than an amateur in this field.

1270 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1978

each land use and a statement of the per­centage of data points correctly inter­preted. The percentage land use figuresare based on all the grid cells and includemistakes.

(2) The second map (Figure 3) shows the stu­dent's uncorrected version as it wouldnormally appear together with the key.

(3) The third map (Figure 4) shows the in­structor's version and the key togetherwith an error matrix indicating where thestudent has gone wrong.

By using the maps and data the studentcan directly relate the maps to the air photoand see his errors of interpretation. If he

consistently misinterprets a particular landuse type and usually assigns it to another,then there is a fundamental error in interpre­tation being made. On the other hand, a ran­dom distribution of errors normally indi­cates the usual problems faced by a studentin learning air photo interpretation. The stu­dent and instructor are thus able to directtheir attention to specific problems of inter­pretation without the need to worry if theyare looking at different points.

CONCLUSIONS

Although the method described fails to

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1271

1272 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1978

use a standardized grid system, this is morethan countered by the advantages of beingable to directly relate back to a single photo­graph. It has been operating for three yearsand it is hoped that most of the problemshave been encountered. The transition tothe problems of data input based on nationalsurvey grids, at a later stage, is made easier,if this is required. FUlther refinements are

clearly possible, but the aim here is to pro­duce a simple system which allows thechecking of air photo interpretation per sewithout all the other problems usually pre­sented in land use mapping and the like.

(Received December 20, 1977; revised and ac­cepted May 18, 1978)

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AUTOCHART: Compatible with the Automap systems in hardware and system software,and based on the same modular approach, Autochart was developed to interface with, andenhance the current procedures for the compilation of, published charts for coastal andocean navigation and marine development.

AUTOMAP II: Similar to the Automap system for topographic mapping, Automap IIprovides for extensions and enhancements, such as area or polygon coding and linkage, textprocessing, and user land code editing, data file structures and conversions within a widerange of fOl1nats for geographic data bases.

AUTOPLOT: This system offers users of outdated analytical stereoplotter systems thecapability to modernize their facilities and equipment to take full advantage of moderncomputer technology, closed-loop servo system designs, and user-oriented software mod­ules that incorporate state-of-the-art techniques. Autoplot is designed to be compatible withAutomap, Autochart, and Automap II, thereby allowing for interactive graphics on-line withan analytical stereoplotter, yielding the highest production capabilities of any other type ofphotogrammetric compilation technique.

Systemhouse, Ltd. has offices throughout Canada and is represented internationally. Forfurther information regarding products, services, and international agents contact William R.Detwiler, SHL, 8625 Plymouth Road, Alexandria, VA, or Brian J. Giles, Systemhouse, Ltd.,560 Rochester Street, Ottawa, Ontario, Canada, KIS 4M2.