Inventory of Large Coal Piles*

WILLIAM S. MASSA,

Maps & Surveys Branch, Tennessee Valley Authority

ABSTRACT: One of the duties of the Maps and Surveys Division of. theT V A is to make frequent determination of the volume of large coal p~les.

A number of new techniques fOI' making inventory of coal p~les by Ph?to­grammetric methods have been developed. This paper descnbes a umqueapproach to this problem.

[ NTRODUCTION

T HIS subject finds a place on the Ameri­can Society program because photo­

grammetry plays an important part in oneof the current measurements. In this paperit is not my intention to dwell on the techni­cal photogrammetric details (actually theyare quite simple), but rather to point outsome of the advantages and selling pointsof the photogrammetric method, andproblems of inventory generally.

More and more the photogrammetricengineer will be asked to make an inven­tory. For such work he should know thedisadvantages and limitations, and par­ticularly should be able to recognize someof the fringe problem pitfalls.

The author does not know who first usedphotogrammetry in stock pile inventory.It is known that Robert A. Cummings, Jr.,and Aero Service were using it at least asearly as 1950. But there may be otherusers in much earlier dates. TVA first usedthe photogrammetric method in March1951 shortly after the appearance of anarticle in the December 18, 1950 issue ofElectrical World written by Biron Ganser,Manager, International Auditing Division,Philadelphia Electric Company and Ed­ward A. Schuch, then Chief Engineer,Aero Service Corporation, Philadelphia.This paper then is not the announcementof a new discovery, but is a report that theproced ure of photogrammetric stock pilein ven torv as a "new idea," has been"constru'c~ivelyabandoned" to productiveuse.

THE INYEI TORY

Good business demands periodic inyen-

WILLIAM S. MASSA

tory. The physical inventory of some com­modities can be accomplished by walkingthrough storerooms and warehou$es andsimply counting items. Some itemgl'can.beweighed. But did you ever try to weIghmountains that cover up to 45 acres andrise more than 80 feet? There are eigh tcoal piles of these general proportions inthe Tennessee Valley. Some contain morethan 600,000 tons. It would be simple todrive from Rogersville to Paducah onceeach year, count the piles and report eig~t

mountains of coal. But unfortunately thISis not an acceptable unit for an inventory.Also it will not give the order of accuracyrequired by the General Accounting Office.

Obviously these huge stock piles cannotbe reweighed. Historically the mathe­matical solution of volume X unit weighthas been relied upon.

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

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PHOTOGRAMMETRIC ENGINEERING

Numerous methods to determine andcombine these factors are available. Onephase of determining volume is accuratelydelineating the shape of the exposed surface.This is normally accomplished either by(1) cross-sectioning; (2) plane-table sur­vey; or (3) photogrammetry. Field surveysand measure men ts ha ve proved acceptal-lefor years, except maybe to the field men in­volved. (Pulverized coal down the neck andin the shoes is fairly disconcerting.) Also,the shape, particularly of the smallerpiles, can be changed materially betweenrod shots, by the rodman climbing andfalling on the steep slopes.

In both plane-table and photogram­metric methods. the piles are contouredwith a one foot interval on the slopes anda ! foot auxiliary on the tops. This isaccomplished photogrammetrically by thestereoscopic plotting of the contours of thepile, from aerial photography usually takenfrom a height above the ground of 1,200feet. The manuscript is drawn at scale1" = 40 feet. The TVA Kelsh Plotter has amagnification of 7!. This gives a C factorof 1,200 for a project where obviously ex­treme accuracy is the goal. This may bequestioned by some photogrammetrists.But it has recently been brought out thatC factor should not always be applied di­rectly to the stereoplotter, but rather tothe photogrammetric system and proce­dure of which the plotter is a part. It is be­lieved that is true for the TVA operation.Stock pile inventory is a special case inmany respects. For instance, there is novegetative cover to contend with.

After contouring, the area of each con­toured layer or slice is determined byplanimeter and the volume is computed.

I n choosing the method for the contour­ing, the shape of the pile, its location, theavailability of surveying personnel vs.photogrammetric equipment and even themethod of building the pile must all beconsidered. The photogrammetric methodis particularly adaptable where large per­manently located stock piles are widelyscattered and must be periodically re­peated. \iVhere survey parties are readilyavailable, and where the stock piles aresmoothed off and regularly shaped, plane­table methods continue to prove just assatisfactory and economical.

Generally the photogrammetric methodis more accurate, approximately 25 percent less expensive, and certainly more

convenient, if proper conditions exist. Butobviously to tie up an airplane and thephotogram metric equipment necessary todetermine the volume of a small isolatedstock pile, would be folly (like employinga power shovel to plant a rose bush).Photogrammetric inventory becomes morefeasible economically when a number ofpiles can be photographed on the samemission. This fact has prompted photo­gram metric organizations around largeindustrial communities, to suggest co­operation between several customers inadopting common inventory dates, inorder to reduce cost of inventory to all ofthem.

In TVA, in addition to the eight largecoal piles there are several smaller onesand approximately 30 different raw ma­terial piles in connection with the chemicaland fertilizer industry, which must be in­ventoried. These outdoor piles are usuallyphotographed on the same day.

TVA uses fixed-wing aircraft which canbe throttled down to a comparatively slowspeed because the stock piles involved arewidely scattered in a triangular area some350 miles on two sides, from Paducah tonear Kingsport to Muscle Shoals. Insmaller areas, a helicopter might presentcertain advantages.

The actual number of photographic ex­posures taken represents a comparativelysmall proportion of the total cost. In theefforts to confine the contouring to onestereoscopic model for each pile, two orthree strips are flown across each pile.Flight maps are made from last year'sphotographs. This often gives a choice oflight, and if you are lucky the plume ofsmoke which invariably hovers over thepile on inventory day may shift off the pilebetween fligh ts. \\Then the piles are largeand it is anticipated that the model limitswill be close, rather than try to spot onestereo-pair, the camera is "turned loose"across the pile giving several exposureswith little forward gain. From these thebest centered stereo-pair can be chosen.

Photographing all of the material ownedby one organization on the same date en­ables the organization to adopt a commoninventory date. Then shape of the piles isrecorded on film and is available for com­pilation into topographic maps at any con­venient time. The photograph becomes apermanent record and the resulting inven­tory can be checked even by other indi-

INVENTORY OF LARGE COAL PILES 79

viduals years later, should questions ofaudit arise or if it otherwise becomes de­sirable to do so.

By recording the time that the planepasses over a given stock pile, and by cor­relating this time with the records of re­ceipt and use of coal during inventory,normal operations on the pile do not haveto be shut down as they often do for theduration of the conventional field survey.This is a definite advantage on very activepiles.

The smaller piles of known density andthe larger piles in so far as outside shape isconcerned do not require ground measure­ments after the first year.

Before the first photogrammetric surveyis made, both horizon tal and vertical con­trol of several permanent identifiable pointssurrounding and outside the stock pile isestablished. These points can be used forfuture inventories if their position is notdisturbed. However, if survey personnel isrequired at the site for other purposes,simple targets laid on the pile and spotelevations established furnish excellenttests on results.

The accuracy required varies from or­ganization to organization and almost in­variably has to be justified to laymen inthe photogrammetric field. Tennessee Val­ley Authority being a government-ownedcorporation is very closely accountable tothe General Accounting Office. The follow­ing is a portion of a statement by the As­sistant Director of the GAO early in 1951.

gone over in great detail the methods andprocedures, which TVA has adopted and,as photogrammetric laymen, some of themseem fascinated. Since then they have beenfurnished data for talks and papers advo­cating the use of the photogrammetricapproach to inventory.

Experience indicates the outside shapeof the larger piles can be determined andduplicated by photogrammetric methodswithin an accuracy that will affect volumeonly some 2 to 3 per cent.

The accuracy of the topography of thetop of two piles, of different height, bu tcovering the same area may be identical;bu t the percen tage of error of vol u mewould vary considerably. The higher thepile, the greater the probable accuracy inthe plateau-like stock pile.

Over 45 acres, two-tenths of a footrepresen ts approxi mately 14,000 tons ofcoal.

In the experimental days there was alittle "leg pulling" by the TVA Auditors.vVithout Maps and Surveys' knowledge,they chose a washed sand pile which hap­pened to be in the midst of several chemi­cal storage piles, and requested the usingdepartment, to neither take from nor addto this pile if they could avoid it. I t de­veloped that for three consecutive yearsan unchanged pile was inventoried by threedifferent methods unknowingly. The re­sults were gratifying to the victims of theleg-pulling.

1950 Cross-sec- 26,148 2.9tioned

1951 Plane-table 25,405 2.71952 Aerial sur- 26,098 2.7 0.2

vey

The widest variance was 2.9% betweenthe plane-table and cross-section methodand the smallest is 0.2% between the aerialsurvey and cross-section method.

Summarizing, the photogrammetric methodhas several advantages:

1. It is more accurate, economical, andconvenient.

2. The cut-off time of all inventory can

"One of the primary purposes of taking physi­cal inventories is to establish that the quanti­ties shown by the accoun ti ng records are actu­ally in existence. The taking of physical inven­tories of bulk materials which are stored inpiles has always presented a problem inasmuchas it is impractical to determine the quantitiesby actual weight. The only other recourse,which is accepted practice, is to determine thevolume of the pile and convert the volumeinto a measure of weight. The methods usedin determining the volume must be such as toassure a reasonably accurate determination.

"If it can be shown that the photo methodresults in a reasonably accurate determinationwhen compared with the results obtained by theground-survey method, we cannot foresee, atthis time, any objection to the use of the photomethod."

The lack of objection in 1951 has nowapparently approached the point of en­thusiasm. Representatives of GAO have

Year MethodRe­

sults,Tons

% Diff. % Diff.between betweenPlane- Cross­Table Section

Method Method

80 PHOTOGRAMMETRIC ENGINEERING

be set for one date-all pictures canbe secured in one day.

3. Ground-control need be establishedonly once.

4. The method provides a permanen trecord of the size of the pile at thetime the picture was taken, and vol­ume can be checked at any futuretime if any question arises as to theaccuracy of the record.

5. No bulldozing or pile dressing is re­quired as is usually done for the cross­section method.

There a're also some disadvantages:

1. The difficulty of catching weather ona predetermined day and coordina­tion with many superintendents atseveral plants (but these are just forone day as compared to weatherproblems for many days for field par­ties. Anticipating the photographicweather problem, usually one day isdesignated in advance as the pre­ferred inventory date with a few daystolerance if weather requires).

2. A lapse between photography and thebeginning of final volume determina­tion (when plane-table survey isfinished, volume study can be started;in the photogrammetric method, filmmust be developed, plates made andtopography drawn before volumestudy can be started. However, thisdifference can be offset by shift worksince these are inside operations).

3. The color of coal and problems oflights and shadows (in one area TVAhas adjacent piles of coal and phos­phate).

So much for the photogrammetricmethod, and the most obvious problem ofthe determination of outside shape. Thereare two other problems in inventory, thesolution to which is more elusive. Theseproblems continue to require ground workon the bigger piles at the time of photo­gram metric survey. One is the determina­tion of base subsidence, the other of unitweight. These are not photogrammetricproblems, in fact the customer may wishto furnish or be responsible for these items;but they must not be overlooked.

The coal stock piles at the large steamplants represent a specific type of pile, asthe type affects the problems of inventory.They are plateau-like in shape and arebuilt in layers, in a sense. At the more

modern stations the coal is crushed andstock piled by means of both rubber-tiredand crawler-type tractors hauling rubber­tired scrapers. Operators are instructed tospread the coal in as thin a layer as practi­cable and to accomplish compaction of thecoal pile by successive passages of tractorsand scrapers over the newly depositedcoal. The coal involved in our area isbituminous and under these stockingmethods, it is surprising how completelythe voids are filled with fines and hownearly the coal piles so built resemble coalin its solid state, even to the point of glis­tening in the sun in the wakeof a bulldozerbeing used to reclaim the coal.

Modern stocking and reclaiming meth­ods practically demand that the base oflarge piles be a plane surface with onlysufficient slope for drainage. To developsuch an area in the type terrain of moststeam plants often requires heavy grading.

Storage areas are constructed to definiteplans and grades, and detailed surveysmade of them after completion.

But what happens to these areas, par­ticularly the fill areas, when a half-millionor more tons of coal are placed on top of it?

And at the time'of inventory, with thecoal in place, how does one determine theshape of the bottom of the pile, and whateffect does distortion of this base have onvolume?

The average storage yard base may de­press under certain conditions up to a footor more, under the weight of several feetof coal, and it may rebound some whencoal is removed. TVA has actual readingsof 1.8 feet of displacement.

The importance of this condition is beingrecognized more and more with experience.Twenty to 30,000 tons of coal might "hide"underneath such a pile, if only the theoreti­cal base contours are used.

In the more recently established fuel pilesof TVA, 6'X6' concrete slabs have beenplaced at selected and coordinated points,and their accurate elevations determinedprior to building the pile. Settlement canthen be checked by probing through thecoal to the slab.

Recently it has been determined that tinch steel plates laid on top of the groundserve better than the concrete slabs. Thesteel will likely deteriorate through chemi­cal action over a period of years, but theplates should last long enough to establishan adequate map of the base in its prob-

INVE TORY OF LARGE COAL PILES 81

able permanent position.This problem points up the need for

careful planning and for site preparationof the storage yard. For the more valuableores, one is often justified in constructinga rigidly paved base.

In a conical or pyramid shaped pile, anerror of 0.2 foot or 0.3 foot in elevation ofthe base can be more serious than 2 or 3feet error in the height of the pile.

In inventorying shallow piles over largeareas, it is often the practice to dig throughthe coal to base contact and to measurethat coal thickness.

After the yards have been in use for afew years and subjected to great loads, itis planned that as sections of the variouspiles become empty a resurvey of the basewill be made.

I t has been stated that the outside shapecan be determined with 2 to 3 per centaccuracy, and although there is the prob­lem of base subsidence, there are ways itcan be determined and measured.

If value is finally dependent on weight,what good is 2 to 3 per cent accuracy ofvolume-if the unit weight used should bein error maybe as much as 25 per cent. Thefinal accuracy of such inventories willlikely be more adversely affected by lackof knowledge, or inability to determine theaverage weight per cubic foot of the ma­terial in storage than in the determinationof the volume itself.

The average weight of coal in storagevaries considerably, depending upon thetype of equipment used in spreading andcompacting the coal, and the degree offineness to which the coal has been pulver­ized. The coal in certain of these piles(placed or reclaimed with dragline) hasweighed as low as 56 pounds per cubic footand in the more recent piles as high as 72pounds per cubic foot.

However, as the operators develop skillin building the pile, the density becomesmore uniform, and there is being foundconsiderable equality in the densities ofthe piles at the several plants independentof the sources of coal. For these new pilesa densi ty of abou t 71 pou nds per cu bicfoot has been fou nd to be a good average.

The best answer comes from the largestsample. Probably the most satisfactory

method for determining density occurs ifmeasurements can be started at the timethe stock pile is begun, and after a largeamou nt of coal has been placed; thenbefore any is used, make an aerial surveyof its volume. The average density of thepile can then be established by dividing theactual shipping weights by the measuredvolume.

In the hard packed piles, it is not toodifficult to obtain fairly accurate samplesnear the top of the pile by driving a twocubic foot cylinder, and such equipment asthe undisturbed soil sampler can take rea­sonable, although small, samples from deepin the piles.

But difficulty is still experienced in ob­taining adequate samples from the moreloosely placed piles.

These last two items are brought outmerely to indicate, that the survey of theexposed portion of the pile is only one ofthree major problems in accomplishingsuch an inventory.

I n the past, most organizations doingbulk inventory have accomplished the finalvolume determinations using conventionalcalculating equipment. In recent months,research and experimentation has been ac­complished in the use of electronic com­puters for volume determination. It is re­ported that several organizations haveadopted electronic computer methods. Sofar, however, photogrammetric cross-sec­tioning seems to lend itself to electroniccomputation better than the contour sys­tem.

These reports indicate that results fromelevations read in the stereoscopic modelson a 10 foot grid and compu ted electroni­cally are quite satisfactory.

Further research is now underway atMIT and elsewhere looking toward thescanning of profiles. of cross-sectionsthrough the pile and ·at selected' pointsalong the profile, taking off the three spacecoordinates of a given point in the form ofdigital data, which can later be fed directlyinto the electronic computer. t!!'!!

For those organizations which have boththe photogrammetric and electronic equip­ment required, this scheme will undoubt­edly prove feasible within the very nearfuture.