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A Training Program for Men Measuring TreeHeights with Parallax Instruments
EVERT W. JOHNSON,
Department of Forestry, Agricultural Experiment Station,Alabama Polytechnic Institute, Auburn, Ala.
ABSTRACT: A program for the training of men in the use of parallaxinstruments for tree height measurement is described. This program iscontroUed by constant testing to establish when an operator has reachedhis natural level of competence, i.e. he is no longer able to systematicallyreduce his errors. The utility of several common statistical measures forcontrol of such a program is examined. The measures showing thegreatest promise are: the mean of the absolute errors, the mean of thealgebraic errors, and the standard deviation of the algebraic errors. Thelatter two are particularly valuable since they provide data for the evaluation of subsequent work. The mean algebraic error provides a measureof operator bias which can be used as a correction factor. The standarddeviation of the algebraic errors provides a measure of the residual random error which cannot be compensated without repeated measurements.
INTRODUCTION
T HE ability to control a training program is essential whenever a manual
skill is involved. In the organized buildingtrades this is accomplished by making eachbudding carpenter, plumber, or electricianundergo a relatively lengthy apprenticeship during which he learns procedures andgains skill by constant practice under closesupervision. In the case of individuals withgreat inherent ability this period of apprenticeship is excessively long, but, sinceno absolu te standards have been established in these fields, it is necessary tomake all candidates complete the full period of training in order to protect thereputation of the trade. This is uneconomicuse of manpower. I t would be more desirable to set up standards and to promote themen as soon as they are capable of meetingor exceeding these standards.
The problem of training in a specificskill was encountered in the course of a research project initiated in the Departmentof Forestry, Agricultural Experiment Station of the Alabama Polytechnic Institute.The project involved the measurement oftree heights on vertical aerial photographsusing parallax methods. Since skilled photo-interpreters were not available, it wasnecessary to train student volunteers tocarry out the work.
50
I n the initial planning phases of this project, it was felt that supervised practice fora specified number of hours would produceoperators with the required degree of skill.This idea corresponds with the apprenticeship plan used in the building trades. Subsequently, however, it became evident thatthe main project would consume a veryconsiderable amount of time and, if theteam was not to be broken up by graduation before the project was completed, thetraining program would have to be as shortas possible. As a result, a testing programwas developed that provided a runningcheck on the operators' progress, makingit possible to move the men to the mainstudy as soon as they gained a satisfactorylevel of proficiency. This program is described in the remainder of this paper.
REVIEW OF LITERATURE
A thorough search of the literaturefailed to yield much information thatwould help in setting up a training program. Sharp (4), in his discussion of education and training in photogrammetry, indicated that training in the use of instruments is usually received on the job orthrough short courses; he did not discusssuch training programs in detail. Custerand Mayer (1) stated that military photointerpretation courses provide 12 to 52
TRAINING PROGRAM FOR MEASURING TREE HEIGHTS 51
hours of training in photo metrics, bu t hedid not specify how this time is allocatedor give any details on the training.
Getchell a nd You ng (2) have reportedon an experiment designed to determinethe length of ti me needed to reach a satisfactory level of proficiency with a parallaxbar instrument and with the parallaxwedge. This report provides some usefulinformation. Apparently only one operator was used so that it is impossible todraw general conclusions safely from thisexperience. The training program used inthis experiment consisted of a short familiarization period with one of the instruments, and then 10 cycles of measurementson 20 trees of known height on a set ofvertical aerial photographs at a scale of1: 15,840. These 10 cycles required approximately 18 hours to complete. At the end ofthis period, the operator was considered tobe reasonably proficient in the use of theinstrument. He was then given a shortfamiliarization period on the second instrument and the 10 cycles were repeated. Thesecond 10 cycles required only 12 hours tocomplete. The operator's proficiency at theend of the training program was tested bythe measurement of 21 new trees. In theirreport Getchell and Young did not analyzethese measurements statistically. The following statistics can, however, be computed from the published data:
1) the mean error for the parallax bar instrument was -0.8 feet, while that for theparallax wedge was -0.7 feet;
2) the standard deviation of the errorswith the parallax bar instrument was±2.72 feet, and that with the parallaxwedge was ± 3.78 feet.
Getchell and You ng were probably correct in their assumption that the operatorwas sufficiently trained for most tree heightmeasuring operations. Tn personal correspondence wi th Getchell and Vou ng, theevidence resulting from this experimentwas confirmed from past experience by Dr.R. N. Colwell, of the School of Forestry ofthe University of California and formerlywith the U. S.. Jav)' Photo InterpretationCenter, and by K. E. Moessner of the nation-wide Forest Survey conducted by theU. S. Forest Service.
An unreported training program hasbeen used by Professor John C. Sammi, ofthe College of Forestry, State Universityof New York, while teaching a course concerned with the application of photogram-
me try to forest mensuration (3). The author knows of this program from personalexperience. In this course a portion of thelaboratory work consists of making a timber inventory using Spurr's (5) "plot approach." Tn order to obtain reasonablysatisfactory resu I ts wi th th is proced ure,the photo-interpreter must be skilled intree height measurement. To gain a modicum of this skill Professor Sammi instructed his students to measure theheights of six to eight objects, located onthe Syracuse University campus, for whichheight data were available. These measurements were repeated five or more timeswithin a given "set." The standard deviation of the height measurements on eachobject was then computed. Standard deviations for the several objects were thenaveraged to obtain the mean standard deviation for the set. These mean standarddeviations were plotted over the nu mberof the cycle. Theoretically a trend shouldha ve developed that flattened ou t as theprogram developed, and finally showed amore or less constant value, indicating thatthe operator had reached his natural levelof proficiency. In practice several drawbacks to this plan developed. The mostcritical of these was that too few objectswere measured in a single cycle within aset. Several cycles could be completed in asingle laboratory period, and consequently memory bias became a very significantitem. However, this general method, ifproperly administered, has the great advantage that development of operator skillis under constant surveillance; and, whenthe operator reaches a natural level ofcompetence, he can be graduated to moreserious work.
PROCEDURE
Because it provided a guide to the development of operator skill, Professor Sammi's basic idea was used as the foundationof the training program. It was modified insome respects in an attempt to make itsomewhat more effective. In addition, itwas expanded in order to provide information concerning the relative merit of several common statistical measures as guidesto operator competence.
A group of six students began the program. These men met as a class at the beginning of the indoctrination phase. Instruction in properly setting up a stereoscopic pair of photographs and in operat-
52 PHOTOGRAMMETRIC ENGINEERING
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were used as a guide to the operator's development (see Figures 1 through 6). Thesetrials were repeated, with at least one daybetween successive trials, using the sameinstrument, until the operator was judgedto be ready for serious work. The strict sequence and the required time lapse between successive trials with the same instru men ts were specified in the programdesign in order to reduce, as much as possible, the effect of memory bias_
In order to keep the development ofskill with one instrument approximatelyparallel with the second, a trial was firstmade with the parallax wedge on a givenset of 10 trees, and then this was followedby a trial with the Contour Finder. Thenboth instruments were moved to a new setof trees. This, of course, meant that thesame trees would be measured twice inclose succession, but, since the instrumentswere so different in design and operation
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ing the instruments was provided. The instruments used were U. S. Forest Serviceparallax wedges, printed in red on a flexible film base, and an Abrams ContourFinder. Every technique known to the author concerning use of these instrumentsfor tree height measurement was demonstrated and explained. The operators wereurged to try these techniques and to usethose that seemed most effective. Thisfamiliarization period required approximately 3 hours and the time seemed to bewell-spent.
Three sets of vertical aerial photographswere provided for the experience-gainingphase of the program. These were standardU. S. Department of Agriculture photographs taken in late winter of Lee andMacon Counties, Alabama. The sets covered areas near Auburn, Opelika, andTuskegee. Ten trees, whose heights hadbeen measured with an Abney level, weremarked on each set. An attempt was madeto secure a wide range of species andheights. True average photographi.c scaleswere determined from measured distancesor from com pu ted distances betweenbench marks. From these data the truedifferential parallax for each tree was computed. These true differential parallaxesprovided the standard for subsequent testing of the operators.
I nstructions to the operators stated thatthe three sets of 10 trees each were to bemeasured one after the other in a strict sequence. A group of 10 tree height measurements made with one instrument from asingle set of photographs was calle~ a"trial" and the statistics for such tnals
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FIG. 3. Learning curves. Standard deviations ofthe absolute errors for the parallax wedge.
TRAINING PROGRAM FOR MEASURING TREE HEIGHTS 53
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FIG. 4. Learning curves. Standard deviations ofthe algebraic errors for the parallax wedge.
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mean absolute errors. As can be seen inFigure 2, trends for the individual operators became evident as the program progressed. Theoretically, as the operatorsgain experience, these trend Jines shouldapproach, then flatten out, and finallyparallel, the line of zero error. The meandifferences between the trend lines and thezero error line are measures of the operators' biases in making differential parallaxmeasuremen ts. If these differences aresubtracted algebraically from measurements made by the respective operatorsunder similar conditions, the effect of thebiases can be greatly reduced or possiblyeliminated. The residual errors would thenbe non-correctable variables best measuredby the standard deviation.
I n the same man ner as wi th the meanerrors, the standard deviations of the absolute and coded errors were computed andplotted over the number of the trial (seeFigures 3 and 4). Theoretically, the resulting curves should have trended downwardand then leveled off indicating that as theoperators became more experienced theybecame less erratic. I n general, the actualcurves agree with this theory. They showthe downward trend but relatively littleevidence of leveling off. A larger number oftrials undoubtedly would have provided abetter example of this phenomenon. Afteran operator attains stability and his curveof the standard deviation of coded (algebraic) errors remains relatively flat, themean difference between his trend line and
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and since they were graduated in differentunits, it was felt that memory bias wouldhave little effect on the readings.
In an attempt to further reduce the effect of memory bias, the operators were instructed to record the instrument readingsat the top and bottom of each tree, but notto do any computation. The computationoperations were all carried out by the author. The measured differential parallaxeswere compared with the computed values.If suggestions for improvement of technique could be made, they were passed 011
to the operators. I n no case, however, didthe operators see their actual results untilafter the testing program was complete. Inthis way development of tree height measurement ability could be studied with relatively few photographs.
The first computational step was to determine differences between the computedand measured differential parallaxes. Thenthe mean absolute difference, or error, foreach set of 10 trees, \\"as computed andplotted over the number of the trial (seeFigure 1). As the operators became moreexperienced their mean absolute errorstended to become smaller un til they stabilized at relatively constant values. \iVhenthis occurred it was felt that the operatorshad probably reached their natural levelsof competence and were considered "experienced."
The algebraic errors were then examined. First the negative error values wereeliminated by the addition of a constant(coding). The mean algebraic error for agiven trial was then obtained by subtracting this constant from the mean codederror for the trial. The mean algebraicerror was then plotted over the number ofthe trial, as was done in the case of the
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54 PHOTOGRAMMETRIC ENGINEERING
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FIG. 6. Learning curves. Per cent correlation(100r2) between computed and measured differential parallaxes.
pattern is revealed in Figure 6. It is interesting to note that the levelling off portionof the curves for all the operators completing the program lay between 60 and 70per cent. The correlation coefficient, whileinteresting, is, like the coefficient of variation, quite sensitive to minor changes inthe error pattern and shows considerableinstability. As a result it was not used inthe evaluation of operator competence.
DISCUSSION
The con trolled program of trall1l11g asdeveloped for this project contained faultsand weaknesses, but it is believed thatthese were not due to the fundamental design of the program. Instead, they weredue to the rather peculiar circumstancesunder which the program was carried out.The operators were senior students in theDepartment of Forestry of the AlabamaPolytechnic I nstitute. These men hadheavy academic loads and had to fit thephotogrammetry training into their schedules where it would disrupt their normalschool activities as little as possible. Thiscaused the training to be more erratic thanshould have been the case. This also madeit necessary to terminate the training before the leveling out of the training curveswas clearly defined, in order to provide sufficient time for the operators to finish themain study before they graduated.
A time record was kept for wage purposes. This record substantiated Getchelland Young's (2) statement that it takesfrom 12 to 18 hours to become reasonablyproficient in the use of a parallax measuring instrument. In this study, however,the operators trained on both instrumentsat the same time, rather than completingthe training on one and then shifting tothe other. The three men completing thetraining program required a total of 21 to26 hours to attain reasonable proficiencywi th both instru men ts.
In summary, it is evident that a trainingprogram such as this would be very valuable to any organization utilizing parallax instruments for measuring objectheights. With it a constant check is available on the operator's development, andhis ultimate ability can be assessed. If heoperates with a constant bias, it is revealed, and compensating factors can beapplied. If a man requires an unusuallylong period of training to hpConme proficient, or if a man shows no talent for the
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the zero error line would provide a measureof the random variability in measurementsto be expected from that operator, underthe specific conditions existing when thedata were obtained.
I n most cases, when the coefficien ts ofvariation of the absolute errors wereplotted over the trial numbers (see Figure5), the scatter of points was so great as toprevent detection of any but the crudestof trends. Most such trends appeared to beessentially horizontal indicating that, asexperience grew, the reduction in scatter oferrors tended to keep in step with the meanerror. There were, of course, exceptions tothis rule. For example, see Operator IIIon Figure 5. This man reduced his meanabsolute error quite rapidly compared tothe rate of reduction of his variability.This resulted in an upward trend of the coefficient of variation. Actually this statistic is of minor importance when comparedwith the mean error and the standard deviation of the errors, for purpose of evaluating operator competence. It is unnecessarily sensitive to minor changes and doesnot reveal much information not alreadyavailable. For these reasons it was not usedas a judging tool.
The last statistic considered as a possiblegauge of operator competence was the correlation between the computed and measured differen tial parallaxes. This was expressed as the correlation coefficient. Theoretically, when the correlation coefficientsare plotted over the numbers of the corresponding trials, there should be an upward trend and then a levelling off as theoperator becomes more experienced. This
TRAINING PROGRAM FOR MEASURING TREE HEIGHTS ss
job, it becomes evident as the program develops. Indeed, it appears to be a very useful tool for the training of personnel engaged in the application of photogrammetry to forestry.
LITERATURE CITED
1. Custer, S. A. and Mayer, S. R. 1955. "AComparative Analysis of Curricula andTechniques used in the Training of Photographic Interpreters." Boston Univ. OpticalResearch Lab. Tech. Note No. 119.
2. Getchel, W. A. and Young, H. E. 1953.
"Length of Time Necessary to Attain Proficiency with Height Finders on Air Photos."Univ. of Maine Forestry Dept. Tech. NoteNo. 23.
3. Sammi, J. C. 1957. Personal correspondence. Professor of Forest Mensuration, College of Forestry, State University of NewYork, Syracuse, N. Y.
4. Sharp, H. O. 1952. Education and training.MANUAL 01' PHOTOGRAMMETRY, 2nd. Ed.Amer. Soc. Photograml1letry, Washington25, D.C.
5. Spurr, S. H. 1948. "Aerial Photographs inForestry." Ronald Press Co., New York,N. Y.
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