74 PHOTOGRAMMETRIC ENGINEERING

ject at the Annual and Semi-Annual meet­ings, and finally to publish a manual onPhoto r nterpretation.

r n the cartographic field the compilerengages in photo interpretation peculiar tohis needs. Reference to currently availablekeys generally requires the laborious use ofseveral publications, none of which meetour specific needs. For several years ACrChas produced a publication showing allcartographic symbols which appear on anyACrC chart regardless of scale. \~rhat

ACrC needs is a photo in terpretation guideto supplement the symbol book. TheCenter is preparing such a guide whichdescribes each feature in the symbol bookand illustrates it with stereograms, ifavailable, and with single photos otherwise.I t is believed that this material will makea definite contribution to photogrammetriccompilation as practiced at this Center.

This paper shows that ACrC is en­deavoring to develop procedures and tech­niques not only for the sake of increasedeconomy, accuracy and speed in produc­tion. but also to meet operational problemsresulting from the gifts of unconventionalphotographs we receive as source material.As in any development effort, many frus­trations are experienced. For instance, inour search for a suitable stretchable me­dium for elastic prints, we found a vinylfilm four mils thick which might be satis­factory if it could be obtained in two milthickness. However, upon inquiry to themanufacturer we found that in order toget two 10 X 10 inch sheets for testing, wewould have had to agree to take the entireoutput from a machine load of ingredi­ients or the equivalent of a roll one footwide and 30 miles long, without stretching.Needless to say our search continues.

A Comparison of the Kelsh and Balplex Plottersfor Large-Scale Mapping*

JOSEPH P. BURNS,

Vice President. Mark Hurd Aerial Surveys, Inc.,111inneapolis, Minnesota

ABSTRACT: This paper outlines the motivation behind the purchase ofthe Balplex plotter, discusses whether anticipated benefits were realizedand compares the Kelsh and Balplex plotters with respect to ease ofoperation, maintenance and accuracy of final results.

Y EARS ago it was axiomatic in aerialmapping that you were not consid­

ered a photogrammetrist until you haddevised and published a new method fordetermining tilt. Today with the boominghighway program, unless you have formedat least one new mapping partnership, youmay be considered a failure to your pro­fession.

Many of these new firms have inquiredregarding the relative merits of the Kelshand Balplex plotters for large-scale com­mercial mapping. This paper will attemptto furnish an answer by summarizing thepast two years experience with bothplotters.

There are, of course, several ways ofapproaching such a comparison.

Several years ago, for example, theGeological Survey initiated a research

project which had for its goal the determi­nation of practical C-factors for the Kelshand Multiplex stereoplotters. During thecourse of this study over one hundredquadrangles throughout the country werethoroughly tested. Following this theresults were analyzed statistically. It wassoon apparent that differences in individ­ual operator skill, differences in terrain,vegetation, soil and crop characteristics­these and other factors caused wide vari­ations in the end results. However, fromthe sheer weight of data submitted a pat­tern gradually emerged from which definiteconclusions could be safely drawn.

Compared with this epic undertakingthe report submitted in this paper offersno such broad and penetrating look. Theopinions and conclusions cited herein arebased on the use of the two plotters on

* Presented at the Society's Semi-Annual Convention, St. Louis, Mo., Oct. 4, 1957.

COMPARISON OF KELSH AND BALPLEX PLOTTERS 7S

large-scale mapping projects in a restrictedarea embracing chiefly the states of theUpper Mid west. I n this region the smooth,stone-free fields and black soil, while ex­cellent for farming, offer unique mappingproblems not encountered in other parts ofour land.

This paper will outline the reasons forpurchasing the Balplex plotter, will discusswhether this reasoning proved valid, andfinally will compare the two plotters fromthe viewpoint of ease of operation, mainte­nance and accuracy of results.

It should be stated at this point that thedecision to purchase Balplex units wasbased in no way on dissatisfaction withresults obtained using the Kelsh plotter.Field tests had previously proved beyonddoubt the accuracy and efficiency of thissturdy, simple instrument for large-scalemapping.

ADVANTAGES SOUGHT IN BALPLEX

In purchasing the new plotter, theprincipal motivation was a desire to widenour arsenal of photogrammetric weapons,to take advantage of certain features whichseemed to offer promise in the new instru­ment. Specifically it was hoped to obtainthe following:

1. An instrument designed to operateefficiently with both vertical and 1011'­

oblique photography.2. An improved lighting system cou­

pled with more positive means of distor­tion com pensa tion.

3. An instrument which could bemounted in series for horizontal andlimited vertical bridging.

4. An instrument better suited tomapping areas of high relief because ofsmaller model scale.

5. Finally a small financial saving wasanticipated due to lower first cost andless floor space required.

The question boils down to whetherthese obj ecti ves have been realized,whether new advantages have been dis­covered or unforeseen difficulties havebeen encountered.

First, regarding oblique use, the Balplexworks very weIll with 20-degree convergentand transverse photography. The well­distributed light, the compact model area,and the ease with which the lens may becanted make model orientation relativelyeasy.

On the Kelsh plotter, this operation had

JOSEPH P. BURNS

previously been found possible but ald:­ward to the point of impracticability.This was due mainly to the sheer extent ofthe stereomodels plus difficulties caused bythe weight of the projectors.

Lighting of the Balplex models appearscomparable to the Kelsh and little dif­ference in brilliance of the platen imagehas been noted.

Distortion compensation by correctionplate in the Balplex printer works ade­quately and requires no maintenance asopposed to the necessity for periodiccleaning of the cams and lens mountbearings in the Kelsh.

For bridging purposes, the Balplex unitsmay be readily used on the conventionalmultiplex 14-foot bar with minor modifi­cations, thus offering the possibilities ofstereotriangulation. 'v\lith the bulk ofpresent-day single-strip highway maps, thespeed and somewhat better accuracy ofstereotriangulation over sterotemplets isad van tageous.

The three-projector Balplex units offerfurther advantages in that adjoiningstereomodels may be tied together in ques­tionable areas before contours are drawn.

While it is true that the smaller modelscale of the Balplex is advantageous inareas of high relief, by the same token thelarger Kelsh scale is undoubtedly a strongadvantage in the flatter areas. The twoinstruments supplement each other in thisrespect.

The financial advantage in the lowerinitial cost of the Balplex is of course,complicated by the number of units pur-

76 PHOTOGRAMMETRIC ENGINEERING

chased, the type of printer to be used foreach instru men t and other considerationsbeyond the scope of this paper. If a largenumber of units are purchased, and ifautomatic dodging is required in diaposi­tive preparation, the cost advantage defi­nitely belongs to the Balplex.

Summing up this portion, in general theBalplex plotter has lived up to its advancepromises, particularly with respect to itsuse with oblique photography and forbriding purposes.

DETAILED COMPARISON OF PLOTTERS

Now for a point-by-point comparison ofthe plotters with respect to ease of opera­tion, maintenance and most important ofall, accuracy of final resul ts.

First, the Balplex plotter, by the internalnature of its construction, offers certainproblems, particularly in inexperiencedhands.

Calibration is more difficult as mechani­cal tolerances are smaller than those ofKelsh, due to smaller instru men t size.Compared with the open construction ofthe Kelsh, adjustments are harder to reach.'v"hile this feature may offer little diffi­culty in large organizations where special­ists are available, ease of calibration isimportant to small firms.

Despite an efficient blower system theheat generated by the Balplex bulb causesan index change when the plotter is firstturned on. This, of course, can be over­come by proper precautions.

Because of the detail inevitably lost inreduction, Balplex diapositives of goodquality are much more difficult to make,particularly where negative quality isborderline. This difficulty may resolve it­self when and if au tomatic dodging be­comes available for the Balplex.

On the other hand, the mechanicalmotions for clearing parallax are excellen tand the model is smaller and may bereached from one side of the instru men twith ease.

Generally the Balplex may be consideredas a "hotter," somewhat trickier instru­ment which performs well in expert handsand requires somewhat more know-how.

Regarding maintenance, this has so faroffered few problems on either instrument.

I t is possible that the higher operatingtemperatures within the Balplex mayeventually cause the lenses and mirrors todeteriorate. But this is mere speculation.So far no difficulty has been encountered

with lenses, mirrors or with maintainingcalibration of the Balplex.

The Kelsh plotters are still in excellentcondition after several years of continuoustwo-shift operations. Part of the wiring hasbeen replaced bu t the lenses, cams andother essen tial parts show Ii ttle wear.

It appears from limited experience thatthere is little to choose between plotterswith respect to maintenance.

Before going into the question of com­parative accuracy, it might be mentionedthat many areas in the Upper Midwest areextremely difficult to photograph for topo­graphic mapping, particularly for two­foot contour mapping. The fields are largeand very smooth; the soil is dark and tendsto remain discolored for several days aftereach rain. Furthermore the farmers have avicious habit of either burning the fields inthe spring or, equally as bad, disking thestubble into the soil, rendering the areaa homogenous black mass with little or nocontrast or texture.

In these areas the Kelsh plotter, usingdiaposi tives made on the LogEtronicprinter, offers the greater accuracy in thatany contrast between fine detail availablein the negatives, however slight, is cap­tured in the plates. These areas are practi­cally hopeless on the Balplex unless elec­tronically dodged plates are made byundergoing two extra steps in the printingprocess.

I n areas wi th a reasonable amou nt ofrelief and normal contrast in ground detaili.e. (the average "good" model), very littledifference in accuracy has been noticed.For two-foot contour mapping we nor­mally photograph at the same elevationfor either plotter, but try to work the ex­tremely flat or low contrast areas on theKelsh because of the improved modelsresulting for automatic dodging of thediapositives. When this feature is availablefor both plotters there should be little tochoose between them, in our opinion.

CONCLUSION

I n conclusion, it is felt that these plotterstend to supplement each other rather thanbeing purely competitive. An organizationequipped with both types is well preparedto take advantage of the strong points ofeach, using either vertical or oblique pho­tography, using stereotemplets or stereo­triangulation in areas of low or high relief.with a fairly wide range of direct plotterscales.