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United StatesDepartment ofAgriculture
Forest Service
Technology &DevelopmentProgram
5700 AviationDecember 20000057-2868-MTDC
Drop TestingAirtankers
A Discussion of theCup-and-Grid Method
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Drop TestingAirtankersA Discussion of theCup-and-Grid Method
Ann Suter, Statistician
USDA Forest ServiceTechnology & Development ProgramMissoula, Montana
TE92P32—Technical Services, Aerial Delivery
December 2000
Introduction ______________________ 1
Mechanics of the Release ______________ 3
History of the Cup-and-Grid Method _______ 4
Alternative Methods of Analysis _________ 10
References _______________________________ 11
Glossary _______________________________ 12
About the Author ____________________ 14
Contents
—Photo courtesy of Scot Peryam
ii
1
Introduction
Figure 1—Airtanker establishing an anchor point.
AAn average of 15.8 million gallons
of fire retardant has been usedin firefighting each year. Most ofthis retardant is released from
the air. Aircraft are used to transportfirefighting chemicals at the appropriateheight and speed. The types of aircraftinclude:
❏ Fixed-wing multiengine airtankers.❏ Fixed-wing single-engine airtankers.❏ Helicopters with fixed tanks.❏ Helicopters with suspended
helibuckets.
The fire-retarding chemicals typicallyused in wildland firefighting are short-term and long-term retardants, foam,and water. The retardants may includea gum thickener.
The 10 principles for proper retardantapplication are:
1–Determine tactics, direct or indirect,based on fire characteristics andavailable resources.
2–Establish an anchor point and workfrom it (figure 1).
3–Use the proper drop height.
4–Apply the proper coverage levels.
5–Drop downhill and away from thesun.
6–Drop into the wind for the bestaccuracy.
7–Maintain an honest evaluation andeffective communication between theground and air.
8–Use direct attack when groundsupport is available or it is feasible toextinguish the fire.
9–Plan drops so that they can beextended or intersected effectively.
10–Monitor retardant effectivenessand adjust its use accordingly.
Knowing the characteristics of theground pattern obtained from a specificaircraft is an important component ofproper retardant application (principles1, 2, 4, and 9). Factors that influencethe ground pattern of a retardant dropinclude:
❏ Drop height and drop speed.
❏ Flow rate of the liquid as it exits thetank.
❏ Volume of the liquid released.
❏ Tank geometry and venting.
❏ Gating system (the tank doors andrelease mechanism installed in anaircraft to release retardant).
❏ Rheological properties of the firechemical (the flow characteristics ofa fluid).
❏ Wind speed and direction.
❏ Temperature and relative humidity.
❏ Fuel type.
❏ Topography.
❏ Safety concerns of aircraft andground personnel.
❏ Pilot proficiency.
Since the 1950’s the Forest Service hasused a procedure known as drop testingto quantify ground patterns. The proced-ure involves dropping fire chemicalsfrom an airtanker flying over open cupsarranged in a regularly spaced grid(figure 2). The cups are weighedbefore and after the drop to calculatethe amount of retardant deposited ingallons per hundred square feet (gpc).These values are plotted onto a mapof the grid. Points between cups areestimated, usually by an interpolationmethod that assumes uniform changebetween cups. Contour lines are made
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by connecting all points of equal cover-age level. The length of each contour,referred to as line length, is calculatedfrom observed and estimated data.
During a drop test, drops are made atvarying drop heights, drop speeds, flowrates, volumes, and with differentretardant materials to obtain a graphicaland numerical picture of the groundpatterns produced by the airtanker.Examining ground patterns providesinformation about the factors thatinfluence the distribution of the drop(figure 3).
Figure 2101%
515⁄16 x 23⁄8"515⁄16 x 23⁄8"
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Figure 2—The Dromader spray aircraft dropping water over a test grid.
Figure 3—Drop pattern characteristics for the Marsh Turbo Thrush with a drop speed of 80 knots and a drop height of 31 feet.
Introduction
drop is when door opening times arestaggered to produce a long stream ofretardant. An accurate set of groundpatterns from an airtanker provides datato predict the time between releasesneeded for a successful trail drop.
This report will discuss factors affectingthe ground pattern and give some his-torical background of the cup-and-gridmethod used to measure groundpatterns. Additionally, new statisticaltools will be presented that may improvethe current cup-and-grid method.
Figure 4—Computer simulation of a trail drop.
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Some factors in a drop can be con-trolled, such as height, speed, flow rate,tank and gating system, and rheological(flow) properties of the retardant. Windspeed, wind direction, temperature,humidity, fuel type, and topography areamong the factors affecting the groundpatterns that cannot be controlled(Newstead and Lieskovsky 1985). Droptests allow different tank and gatingsystems to be compared under similarconditions. Ground patterns can helpmanagers learn the capabilities of anairtanker by determining the intervalsbetween trail drops (figure 4). A trail
2
Mechanics of the Release
AAs an airtanker releases a load ofretardant (figure 5), the fluid isdistributed along the flight path.The characteristics of the drop
(length, width, and coverage level) area function of the height and speed ofthe aircraft, the flow rate and volume ofthe fluid exiting the tank, the rheologicalproperties of the fluid, and the meteor-ological conditions (Swanson andLuedecke 1978).
The design of the tank and gatingsystem directly affects the retardantflow rate. Relevant design elementsinclude the size and shape of the door,the speed with which the door opens,and the geometry of the tank vents,baffling, cylinders, torque tubes, andother items inside the tank (figure 6,George and Blakely 1973). We haverelied on the cup-and-grid method tounderstand how these factorsinfluence the ground pattern.
Figure 5—Distribution of fluid along the flight path.
Figure 6—View of a tank and gating system showing baffling and torque arms, which produceclutter effect.
Deformation
Taylor breakup
Surface erosion
Droplet impaction
3
4
History of the Cup-and-Grid Method
Figure 7—Paper cups being collected from the test grid. —Photo courtesy of California Department ofForestry and Fire Protection. The fact that the California Department of Forestry and Fire Protection provided aphotograph for this publication in no way implies the Department’s endorsement of any theories or positions takenwith this document.
❏ Drift increases with both height andspeed.
In 1959, Storey, Wendel, and Altobellisused the cup-and-grid method to invest-igate the retardant distribution andpenetration patterns from a TBM air-
tanker in pine timber with and withoutunderstory. Hodgson drop tested avariety of aircraft in 1967. The grid di-mensions were 480 by 172.5 feet, laterincreased to 570 by 202 feet. A totalof 935 paper cups sat in can holdersspaced at 15- by 7.5-foot intervals.
SSome of the first drop tests wereconducted between 1955 and1959 in California by the PacificSouthwest Forest and Range
Experiment Station and the CaliforniaAir Attack Coordinating Committee(Davis 1959). These tests were de-signed to answer six main questions.
❏ Does retardant viscosity affect dropbehavior?
❏ Does retardant weight affect dropbehavior?
❏ Do borate and bentonite have thesame drop characteristics?
❏ How do airtanker height and speedaffect the drop?
❏ How large should the gate be?
❏ Which is more important, airtankercarrying capacity or maneuverability?
The grid design for these tests con-sisted of 10-ounce paper cups sitting ina holder less than a foot off the ground(figure 7). Over a hundred cups werelaid out in a geometric grid pattern. AStearman biplane made the drops.Once the gpc was calculated, the valueswere plotted on graph paper. Contourplots were constructed by drawing linesbetween the points of equivalent gpc.Film was used to observe the airtanker’sflight path in relation to the target. Thefilm was also examined to determineretardant breakup, retardant drift, andthe time from release to impact.
These early tests determined that:
❏ Increased viscosity reduces drift.
❏ Increasing the drop height increasesthe coverage area.
❏ Fall speed and drift are not increasedby higher fluid weights.
❏ Borate and bentonite drop alike.
5
Figure 8—Diagram of Grigel’s test grid. —Courtesy of Joseph Grigel.
SCALELarge squares = 10 ftSmall squares = 7.5 ft
History of the Cup-and-Grid Method
Flight direction
Points were estimated by linear inter-polation. The report recommendedcloser cup spacing in the center of thegrid where the coverage level tendedto be heaviest.
Also during 1967 in Alberta, Canada, aUniversity of Montana graduate studentconducted a cup and grid test as partof his master’s thesis. Joseph Grigelused a Snow Commander airtankerwith a 250-gallon tank to drop GelgardF, a short-term fire retardant, over anarray of cups laid out in a grid formaton the ground. Grigel went into moredetail regarding methods and design.The objectives of the study as outlinedby Grigel were to determine:
❏ The ground distribution patterns ofGelgard F retardant mixtures re-leased from the Snow Commanderairtanker onto an open area and amature, well-stocked lodgepole-pinestand.
❏ The effect of release height andmixture viscosity on ground distribu-tion patterns formed by Gelgard Fretardant.
❏ The effect of tree canopy on theground distribution patterns andrecovery rates or the estimatedamount of liquid that landed on theground.
Grigel concluded that groundpatterns were determined by height,speed, volume, gating system (as itcontributes to flow rate), retardantproperties, weather, and fuel orvegetation. A grid-within-a-grid layoutwas used. The inner grid consisted ofcups spaced 7.5 feet apart, covering a270- by 82.5-foot area (figure 8). Anouter grid was built around the innergrid with 10- by 10-foot spacing. Theouter grid was to catch any retardantthat missed the inner grid. The entiregrid, including the inner and outerportions, measured 350 by 142.5 feet(figure 9).
6
History of the Cup-and-Grid Method
Figure 9—Grigel’s test grid set up in an open field. —Photo courtesy of Joseph Grigel.
Figure 10—Diagram of George and Blakely’s test grid.
It was assumed that each cup on thegrid would represent one-half thedistance to the adjacent containers.Linear interpolation was used toestimate points. Grigel noted someprincipal sources of error:
❏ Evaporation of water in the cupbefore weighing.
❏ Variation in the cup lid weights.
❏ Cup and can missing or not mountedvertically during the drop.
❏ Splashing of drop material.
❏ Human error.
The cup-and-grid method was examinedmore closely in 1973 (figure 10) atthe Intermountain Forest and RangeExperiment Station’s Northern ForestFire Laboratory in Missoula, now theMissoula Fire Sciences Laboratory(George and Blakely 1973). Two mainquestions were addressed:
❏ What is the accuracy or variationfor a particular grid point?
❏ Is the overall grid spacing andsampling adequate?
In addition to an inner grid of closelyspaced cups and an outer grid of morewidely spaced cups, nine tables with
nine cups each were placed in theinner grid. The nine cups were withinan area of 20 by 20 inches. They wereused to provide a measure of variationaround a single sample point. Thisarrangement used the four surroundinggrid points to provide a value to com-pare to the predicted value for thepoint. In addition, holes were cut in thetops of the three tables and deep canswere positioned so their tops were thesame height as the cups (figure 11).These were used to determine if asignificant amount of retardant wassplashing out of the cups. Contour plotswere calculated “using a method oflinear proportioning.” A vertical distri-bution test was conducted to determineretardant’s penetration through thetree canopy and its retention on fuels.
The grid system was evaluated by plot-ting the observed mean concentrationin gpc from the inner grid area againstthe observed mean concentrations fromthe table data. The observations werefound to be highly correlated. Propor-tioning points for prediction purposesproduced an error of less than ±0.5
7
Figure 11—Table of nine cups for measuring variation around a single sample point.
Figure 12—An example of retardant reaching the ground before forward momentum has stopped.
History of the Cup-and-Grid Method
gpc. Variation was examined by plottingthe standard deviation for the tableconcentrations against the mean gpcfor each table. Levels up to 3 gpc wouldresult in a standard deviation of 0.2gpc or less.
D. H. Swanson and A. D. Luedecke usedground patterns in their 1978 paper,Tank Design Guide for Fire RetardantAircraft, to compare the effects ofdifferent factors on the drop. Groundpatterns obtained from the cup-and-grid method along with film showed thatthe most efficient drop occurs whenthe majority of the retardant’s forwardmomentum has stopped before theretardant reaches the ground. Thesefindings helped determine the properdrop height (figure 12).
In the 1990’s the Forest Service con-ducted an extensive series of drop testson a wide variety of retardant deliverysystems. The systems tested included anumber of helitankers and multiengineand single-engine fixed-wing airtankers.Drop-test data from each system wereassembled into drop-test data books.
For these tests, cups were mounted onstakes at 10- by 10-foot intervals forthe inner grid and up to 60- by 20-footintervals for the outer grid. Grid sizeranged from 250 by 600 feet up to 250by 3,000 feet, depending on the volumedropped (figure 13).
Aircraft drop height and drop speedwere measured using video analysis,a global positioning system (gps), anda radar altimeter. Video proved to bethe most accurate and reliable. Fourliquids were used: water, a 0.6-percentfoam solution, gum-thickened retard-ant, and water-like retardant (figures14 and 15). Flow-rate settings varieddepending on the system. For example,the Bambi helibuckets were tested withonly one flow-rate setting. Some air-tankers were tested with up to as manyas nine selectable flow-rate settings.
8
History of the Cup-and-Grid Method
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Figure 13—A 600-foot test grid from a 1999 drop test.
Figure 1482%
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Figure 14—Foam dropped on the test grid. Figure 15—Retardant dropped on the test grid.
9
Figure 16—Drop pattern characteristics for the Sims 2,000-gallon helibucket with a drop speed of 83 knots and a drop height of 119 feet.
Wind speed and direction were meas-ured using a portable weather stationadjacent to the drop site. Instrumentationwas mounted 20 feet above the ground.The weight of retardant in each cup andthe location of each cup within the gridwere recorded. Linear interpolation wasused to estimate unknown values (val-ues between cups). Contour patternswere made with computer software(figure 16).
As with previous drop tests, groundpatterns differed depending on aircraftheight and speed, type of retardantor suppressant, flow rate, and windconditions.
Drop testing over a regularly spacedarray of cups has allowed researchersto measure the line length, width, and
area, and to calculate the coveragelevel of the ground pattern. This hasimproved our knowledge of how height,speed, volume, tank geometry, flowrate, meteorological conditions, andretardant properties influence groundpatterns. Yet data regarding the accu-racy of the cup and grid method aresparse.
History of the Cup-and-Grid Method
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10
Alternative Methods of Analysis
AAside from the tests conducted by
George and Blakely, very littlework has been done to deter-mine appropriate cup spacing
and grid dimensions needed for anunbiased, efficient sampling method.
Linear interpolation, often in one direc-tion, has been frequently employed toestimate unknown values. Becauseground patterns are three dimensional,it may be more appropriate to treatthese data as spatial, and to use spatial
statistical tools for estimation,prediction, and
experimentaldesign.
Spatial statistical methods take intoaccount the location of an observationas well as the value of the observationto obtain more accurate estimates ofunknown values (Isaaks and Srivastava1989). This information is used to builda model for prediction and samplingimprovements. Spatial data such asthose obtained from drop tests areusually correlated. Modeling this corre-lation structure improves accuracy of
estimation and prediction. Spatialstatistics is a relatively new
field and has proven moreaccurate for estimating
spatial data. Usingthese methods for
drop testing maylead to a greaterunderstandingof our aerialcapabilitiesand improveour wildlandfirefighting.
11
Davis, James. 1959. Air drop tests:Willows, Santa Ana, Ramona 1955-1959. Res. Pap. 17382 4-60 2M SPO.U.S. Department of Agriculture, ForestService, Pacific Southwest Forest andRange Experiment Station and theCalifornia Air Attack CoordinatingCommittee. 22 p.
George, Charles; Blakely, Aylmar D.1973. An evaluation of the drop char-acteristics and ground distributionpatterns of forest fire retardants. Res.Pap. INT-134. Ogden, UT: U.S. Depart-ment of Agriculture, Forest Service,Intermountain Forest and RangeExperiment Station. 60 p.
Grigel, Joseph E. 1970. Drop testingwith the Snow Commander airtankerand Gelgard F fire retardant. Missoula,MT: University of Montana. 80 p. Thesis.
Hodgson, Brian S. 1967. A procedureto evaluate ground distribution patternsfor water dropping aircraft. Inf. Rep.FF-X-9. Ottawa, Canada: CanadianDepartment of Forestry and RuralDevelopment, Forest Fire ResearchInstitute. 41 p.
Isaaks, E. H.; Srivastava, R. M. 1989.An introduction to applied geostatistics.New York: Oxford University Press.561 p.
References
Newstead, R. G.; Lieskovsky, R. J. 1985.Air tanker and fire retardant droppatterns. Inf. Rep. NOR-X-273. Alberta,Canada: Canadian Forestry Service,Northern Forestry Centre. 31 p.
Storey, Theodore G.; Wendel, GeorgeW.; Altobellis, Anthony T. 1959. Testingthe TBM aerial tanker in the Southeast.Station Pap. 101. Asheville, NC: U.S.Department of Agriculture, ForestService, Southeastern ForestryExperiment Research Station. 25 p.
Swanson, D. H.; Luedecke, A. D. 1978.Tank design guide for fire retardantaircraft. Hopkins, MN: Honeywell, Inc.,and Missoula, MT: U.S. Departmentof Agriculture, Forest Service, NorthernForest Fire Laboratory. 41 p.
12
Glossary
Airtanker—Fixed-wing aircraft certifiedby the Federal Aviation Administrationas being capable of transporting anddelivering fire retardant solutions.
Anchor Point—An advantageous lo-cation, usually a barrier to fire spread,to start constructing a fireline. Theanchor point is used to minimize thechance of being flanked by the firewhile the line is being constructed.
Baffling—Partitions placed in tanks toreduce shifting of the water load andto give the tank greater structuralintegrity.
Bambi bucket—A collapsible helibucketslung below a helicopter, typically usedto dip water from a variety of watersources for fire suppression.
Bentonite—A clay slurry, short-termfire retardant whose effectivenessdepends primarily on the water it holds.
Breakup—The dispersion of retardant,foam, or water in the air after it isreleased from a tank or bucket.
Borate—One of the first long-term fireretardants. Its use was discontinuedin the 1960’s.
Constant-flow airtankers—Airtankersthat use doors to control the flow rate.
Contour plots—A graphical picture onwhich the characteristics of a surfaceare shown by contour lines. In droptesting, the contour line is a line thatjoins points of equal coverage levelon a surface.
Conventional airtankers—Airtankersthat divide the retardant load intoisolated compartments. The releasesystem is designed to open the doorsof these compartments in sequence,producing a line of retardant.
Correlated—When two or more thingsare mutually related. In statistics,correlation refers to interdependencebetween two or more variables.
Coverage level—A recommendedamount of retardant (in gallons) appliedto a specific area (100 square feet) ofsurface. Coverage level 2 represents2 gallons of retardant per hundredsquare feet of surface.
Cup-and-grid method—A procedureincorporating containers set in aregular, defined pattern to measuredeposition patterns created by theaerial release of fire-retardingchemicals.
Direct attack—Any treatment applieddirectly to burning fuel to stop com-bustion.
Drift—The effect of wind on retardantor suppressant drops.
Drop test—A test of a fire chemicaldelivery system flying over a cup-and-grid matrix to determine the coveragelevel producted for each drop type.
Fire retardant—Any substance, exceptplain water, that by chemical or physicalaction reduces the flammability of fuelsor slows their rate of combustion.
Fire suppressant—An agent that isdirectly applied to burning fuel toextinguish the flaming and glowingphases of combustion.
Flight path—Track of an aircraft overthe earth’s surface.
Flow rate—The rate at which theretardant exits a tank or bucket, usuallyexpressed in gallons per second.
Foam—The aerated bubble structurecreated by forcing air into a water solu-tion containing a foam concentrate.Foam may be produced by suitablydesigned equipment or by cascadinga water solution containing a foamsolution at high velocity through the air.
Gate—Refers to the door area andrelease mechanism of a tank.
13
Glossary
Gelgard F—A firefighting gel. Gelsmodify the physical properties of water,especially its ability to cling to fuel.Gels rely on the moisture they containto reduce or inhibit combustion.
Ground pattern—The characteristicsof a ground pattern of fire-retardingchemicals, including length, width,area, and coverage level expressedin gallons per hundred square feet.
Gum-thickened retardant—Any fire-fighting product containing a thickener.The thickener increases the product’selasticity.
Helicopter buckets—A containersuspended below a helicopter, usuallyused for dipping water for firefighting.
Helitanker—A helicopter equipped witha fixed-tank or a bucket-type containerthat is used for aerial delivery of wateror retardants.
Line building—Constructing a fireline.Aircraft drop retardant or suppressantin overlapping lines to help groundcrews build fireline.
Line length—The length, usuallyexpressed in feet, of a ground pattern.Line length is used to relate the lengthof different coverage levels within aground pattern.
Linear interpolation—Estimation of avalue of a variable between two knownvalues that assumes uniform changebetween the two known values.
Recovery rates—The estimatedamount of retardant recovered fromthe cup and grid method comparedto the amount of retardant dropped,expressed as a percentage of totalgallons dropped.
Retardant—see fire retardant.
Retardant mass—The total volume ofretardant released from the aircraft.
Rheological properties—Those prop-erties, including viscosity and elasticity,affecting the flow characteristics of afluid. These properties affect the be-havior of the retardant as it is droppedfrom an airtanker.
Short-term fire retardant—A formu-lation added to water to modify itsphysical properties, improving its dropcharacteristics or its ability to clingto fuel. The retardant relies on themoisture it contains to reduce or inhibitcombustion. It is ineffective once itsmoisture has evaporated.
Spatial statistics—Statistical methodsfor spatial data. Typically, these datafall into three categories: lattice data,geostatistical data, and spatial pointpatterns.
Tank and gating system—Tank, doors,and release mechanism installed inaircraft to release a drop.
Torque tubes—A hydraulically or pneu-matically powered, rotating shaft thatopens and closes the tank’s doors.
Tree canopy—The layer containingthe crowns of the tallest vegetationpresent (living or dead), usually above20 feet.
Understory—Low-growing vegetationunder a stand of trees. Also the portionof trees in forest stands below theforest canopy.
Venting—The openings in the tankvents used to allow air into the tank.
Viscosity—A measure of a fluid’sresistance to flow. With foam, an indi-cation of the foam’s ability to spreadover and cling to fuels and to itself.
Water-like retardant—Unthickenedproducts or thickened products thatdo not have elasticity.
Wildland fire—A fire in wildland.
14
About the Author
Ann Suter is a statistician for theMissoula Technology and DevelopmentCenter, Wildland Fire Chemical Systems.She joined the Forest Service in 1997
after serving 2 years as a Peace CorpsVolunteer in Jamaica, where she workedon reforestation and soil erosion control.
She holds a master’s degree in inter-national development from theAmerican University.
Additional single copies of this document may be ordered from:
USDA FS, Missoula Technology & Development CenterBuilding 1, Fort MissoulaMissoula, MT 59804-7294Phone: 406–329–3978Fax: 406–329–3719E-mail: [email protected]
Electronic copies of MTDC’s documents are available on the ForestService’s FSWeb intranet at:
http://fsweb.mtdc.wo.fs.fed.us
For further technical information, contact Ann Suter at:
USDA Forest Service, MTDC (Wildland Fire Chemical Systems)5775 Highway 10 West, P.O. Box 8089Missoula, MT 59807Phone: 406–329–4815Fax: 406–329–4811Lotus Notes: Ann Suter/WO/USDAFSE-mail: [email protected]
Library Card
Suter, Ann. 2000. Drop testing airtankers: a discussion of the cup-and-grid method.Tech. Rep. 0057-2868-MTDC. Missoula, MT: U.S. Department of Agriculture,Forest Service, Missoula Technology and Development Center. 14 p.
Presents the development of drop tests from the 1950’s through the 1990’s as amethod of quantifying ground patterns from aerial-retardant drops. During a droptest, drops are made at varying drop heights, drop speeds, flow rates, and volumes,and with different retardant materials to obtain a graphical and numerical pictureof the ground patterns produced by the airtanker. Examining ground patterns pro-vides information about the factors that influence the distribution of the drop. Thereport discusses the methods used to measure and describe ground patternsand proposes a spatial statistical method for increased efficiency and accuracy.Includes a glossary of technical terms.
Keywords: airtankers, coverage levels, drop tests, ground pattern, history, linelength, rheology, spatial statistics, tank and gating systems
