T HE speed with which modern develop­ment programmes can be executed today

calls for a corresponding speed in the prelimi­nary stages of survey and planning work. Tocarry out accurate surveys using groundmethods alone is a costly and lengthy proce­dure involving large amounts of men andequipment. The answer to this need forspeed in the preparation of maps and plans isthe use of aerial survey techniques.

Many people tend to regard aerial surveyas insufficiently accurate, particularly wherelarge-scale plans are called for. The fact is,however, that the precision of modern photo­grammetric equipment makes it possible to­day to set up aerial photographs in stereo­scopic plotting machines with such exactitudethat they may be observed and their detailsrecorded entirely free from any form of dis­tortion. The article on the opposi te pagelgives an indication of the precision of themodern survey camera-a precision which isfully maintained throughout every stage ofthe photogrammetric process.

In many cases photogrammetry has con­siderable advantages oyer ground surveymethods, since it can provide a far truer rep­resentation of the actual shape of the ground.The smallest detail can be closely followed inthe three-dimensional photographic "model"and contours can be observed at every pointalong their path. The accuracy of these con­tours can normally be guaranteed to withinone-half of the interval; thus a specificationcalling for one foot contours implies (if it doesnot actually state) that the tolerance is plusor minus six inches. In other words, no con­tour deviates from its true path by more thanhalf the interval. Ground surveyed contours,on the other hand, must be interpolations of agrid of spot levels, so that they cannot folio\\"the exact shape of the terrain.

Variations in elevation, even violent ones,presen t no problem to the aerial surveyor,

How Accurate Is Aerial Survey? *

H. G. DA WE, Manager,Survey and Production

Hunting Aerosurveys Ltd.,London, England

since differences in scale are automaticallyaccounted for. The relationship between de­tail, say, on top of a cliff and at the bottomis recorded wi th the same precision as if theywere on one horizontal plane. Furthermore,in the plotting instrument it is not necessaryto follow a circui tous path to go from onepoint to another; where access to a site is dif­fietdt or dangerous, across electric railwaylines, for example, the major part of the sur­vey may be completed in complete safety.

Broadly speaking, surface levels on clearground can be measured from aerial photo­graphs to an accuracy of between 1/5,000thand 1/8,000th of the height from which theywere taken. This means that photographstaken from an aircraft flying almost a mileabove the ground will yield measurements inthe plotting machine accurate to within onefoot. From lower flying heights the accura­cies will be proportionately increased, untilthe stage is reached where the photographicand photogrammetric aspects are no longerthe limiting factors.

At this point, although plotting instru­ments are capable of finer measurements,they can be used only to record what the op­erator can actually see. For example, in con­ditions that permit measurements to a± 2-3" accuracy, the subject may be a stub­bly field or an ill-kept front garden and ob­servations must of necessity be made on thetop of these features. In practice, an allow­ance may be made but under such conditionsit is not possible to guarantee all heights tothe accuracy of ± 2-3". Only on clear hardsurfaces can such an accuracy apply.

A further point concerns the scale at whichthe height data is presented. In a case whereheights are required along a given line indi­cated on, say, an Ordnance Survey 1/2,500plan, the ground surveyor can record heightsto 1" accuracy-precision which the air sur­veyor could not hope to achieve. However,the leveller's stave could easily be moved by

1 "Camera Calibration" by R. Hall, Research an amount not plottable at that scale to showOfficer, Hunting Aerosurveys Limited. differences in readings of as much as three to

* This paper was included ill Aerial Survey Review, 1959 No. 21, published by Hunting Aerosurveys,Ltd. -1-, Albemarle St., London \V.l, England. For the privilege of reprinting in PHOTOGRAMMETRICENGINEERING, thanks are extended and credit is given to the author and to the publication.

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760 PHOTOGRAMMETRIC ENGINEERING

six inches. In fact, then, a specification call­ing for -1-" accuracy at that scale becomesmeaningless.

Photogrammetry can always provide detailto within the plottable limits of the publica­tion scale and complexity in detail-shape canbe dealt with as easily as regularly laid outfeatures. In large-scale surveys exhaustivefield examination follows the photogrammet­ric plotting. Very often details which are re­quired to be shown on large-scale plans areobscured in the vertical view from the air andin some cases are too small to register on thephotographs. 1/480 railway plans, for exam­ple, include such details as signals, mile andgradient posts, telegraph poles, lamp posts,manhole covers, inspection pits, watercocksand notice boards, all of which are plotted toan accuracy of three inches. vVhere this typeof detail is obscured it can be readily suppliedon the ground by simple offset measurements.Normally, however, for 1/500 plans 80 to 85per cent of the detail is plottable by photo­gram metric methods.

I t is well known that aerial survey de­pends in the first instance upon a network ofground surveyed control points so that over­all rigidity of the survey is ensured by theprecision of the field control rather than byany photogrammetric process. In the case ofa strip road survey, for example, the accuracyof the total distance from end to end is a re­flection of the accuracy of the basic fieldwork. It has been found from experience thatdetail plotted from aerial photographs is ingeneral at least as good as can be represen tedby field measurement and in many casesmuch better for scales up to 1/500 or 40' toone inch.

It might be mentioned that there have beenseveral occasions when the plotting instru­ments have detected errors in ground controlfor, within fairly close limits, the instrumentswill not accept faulty data and soon locateany anomalies. The photogrammetric proc­ess is, therefore, a check in itself carried ou tin the terms of the scale specified.

Perhaps the main difference betweenground and air survey techniques-and onethat on occasion has led to misunderstanding-is that the latter is normally designed rightfrom the start to fit a given specification. Theaerial surveyor must take into account thepurpose for which the map is required and theaccuracy specified, so arranging each stage ofthe survey-the flight altitude, type of cam­era and plotting process-to ensure the bestpossible results.

Planned for presentation at a specific scale,

and based on a framework of sound groundcontrol, there is no reason why aerial surveyshould not attain any normal standard of ac­curacy required. Each stage of the survey iscarried out by extremely precise instrumentsand supplementary field checks provide anydetail which may be in doubt. \iVhere con­tours are required there is no doubt that it issuperior to ground methods and in additionit has the considerable advantages of speedand economy of manpower.

The three basic advantages can be summa­rised as:

SPEED-Once the photography 1:S acquiredplotting can proceed rapidly, utilising a num­ber of machines if necessary.

ACCURACY-Maps prepared from aerialphotographs can be as accurate as those pre­pared entirely by ground methods, and su­perior when true contour positioning is essen­tial.

ECONOMY-The need for large teams of sur­veyors is eliminated. Costs are comparablewith ground surveys and can on occasion beless.

"How ACCURATE Is AERIAL SURVEY?"

AN EXPLANATION2

This article was originally published in theAerial Survey Review earlier this year. Its pur­pose was to emphasise the accuracy of mod­ern photogrammetric techniques and it wasin no way intended as a reflection on theclassical methods of surveying. To avoid anypossible misunderstanding we would like toamplify some of the author's remarks:

(1) Mapping from air photographs is stilldependent (and is likely to remain so for avery long time to come) on the control frame­work supplied by the field surveyor on theground.

(2) For mapping at scales of say 1/5,000and smaller there can be very few cases in­deed where modern photogrammetric meth­ods have not entirely supplanted those of theconventional "topographer." One exceptionto this rule is the use of ground levellingrather than photogrammetric contouring forareas of "flat" ground such as in an irrigation

2 In a letter of June 15, Lord Pentland of Hunt­ing Technical Services, Ltd. stated that a groundsurvey organization had complained that Mr.Dawe's paper was unfair to ground survey tech­niques and said that they felt very strongly aboutthis. Lord Pentland stated that it was not Mr.Dawe's intention to make any direct comparisonwith ground techniques. However in view of thecomplaint it was requested that the herein-givenexplanation follow the reprint of the original paper.PHOTOGRAMMETRIC ENGINEERING is very pleasedto take the suggested action-ED.

HOW ACCURATE lS AERlAL SURVEY? 761

survey where the amount of levelling requiredto control the photogrammetric survey is al­most equal to that required to provide a gridof spot levels dense enough to permit the in­terpolation of contours by eye.

(3) Again ground survey is essential forsmall-scale mapping of dense areas of forestor bush where trails, drainage, and even smallvillages may often be screened entirely fromthe aerial view and can only be supplied byrunning a traverse on the ground.

(4) As the scale of the plan becomes largeso the work of the ground surveyor becomesincreasingly important. At very large scales,say in excess of 30 ft. to the inch, it is usuallyadvisable to do the whole survey on theground, for the sake of both accuracy andeconomy.

(5) Ground survey is also preferred for thepreparation of large-scale plans of very small

areas where the cost of aerial photography isdisproportionately high to the size of the job.The undisputed field of the ground surveyormust lie however in practically all work whichinvolves setting out, including the running oflines of precise engineering levels.

(6) As regards the reference to t inch ac­curacy becoming "meaningless" this doesnot, of course, refer to the many cases wherelevels are referred to precise ground marks,but to level information read off a plan andof which the accuracy is dependent on thescale of the plan. For instance, the nearestone can re-establish a given point on theground from measurements taken on a1/2,500 plan is ±3-4 feet. Within this circleof location on uneven or sloping ground it ispossible for the level to vary by an amountfar in excess of the precision with which it issurveyed and its value marked on the plan.

Considerations tor the Designof a Projection Plotter*

J. A. EDEN,

Directorate of Overseas Surveys,Kingston Bye-Pass,

Tolworth, Surbiton, Surrey, England

T HE advantage of the projection plotter with regard to model clarity and definition,compared with the more complicated and obviates the necessity for spectacles.

binocular instruments is its extreme simplic- (2) The instrument should he nominallyity. distortion free and utilise full size 9/fX9/f dia-

Having examined a number of projection positives. Optimum projection should be be-plotters of different makes, I am convinced tween X 5 and X 6. The working table shouldthat a cheap projection plotter could be built be large enough to accommonate the whole offor plotting individual models with better a X 6 projection. Allowing margins at frontperformance and facilities than any existing and back to accommodate the tracing tableprojection plotters and with a working ac- supports when working close to the modelcuracy as good, if not better than any existing edge, the table width would need to be 62first-order plotter, except of course, the in- inches. This is not too large to be convenient-strument would not measure machine coordi- Iy worked from both sines.nates or be convenient for air triangulation. (3) The projectors should be mounted so

The design should feature the following that the diapositives are held in an approxi-points: mately vertical plane, and prisms should be

(1) Rotating shutter viewing should be provided in front of each lens to reflect theemployed. Compared with anaglyph viewing projections down vertically onto the horizon-this system presents enormous advantages tal working table. The advantage of this

* This paper was included in The Photogrammetric Record (Vol. llI. o. 13. April 1959) publishedby the Photogrammetric Society. London, England. Permission to reprint in PHOTOGRAMMETRIC ENGI­NEERING was requested provided credit be given to the JOURNAL, the Society and the author. This per­mission was given. This publication extends thanks and gives credit as above stated-ED.