Triangulation with H elicopters*

KENDALL B. WOOD, K. B. Wood & Associates,

Inc., Portland, Ore.

By FAR the most severe problem of run­ning map control survey parties in

remote areas is the problem of transporta­tion. On triangulation problems it is notunusual to find the field party spending agood 80 to 90 per cent of its field time intraveling from one station to the next andin reconnaissance of areas to determinethe selection of stations.

The use of helicopters in transportingtriangulation crews of course is not new.The U. S. Geological Survey has used thismethod of transportation in Alaska, thesouthwest and in other places. Possiblyprivate agencies have used helicoptersfrom time to time. A description of suchuse by the K. B. Wood Company for asurvey problem in Death Valley last yearmay be interesting and helpful.

In this case triangulation was used asa means of checking the accuracy of ananalytical photogrammetric bridge. (1)To satisfy our company concerning theaccuracies achieved by photogrammetricmethods and (2) to make possible someexperimental studies by this company ofvarious methods of bridging adjustment.The object of the survey therefore was notto determine high order monumentedpositions but to develop a series of scalepoints tied to picture points on the ground,with an accuracy sufficient to warranttheir use for map checking purposes.

The project was located between SearlesLake and Death Valley, a distance ofsomething in excess of 40 miles. The areaconsisted of typical desert conditions withbarren rocky peaks and large flat basinareas devoid of heavy vegetation. Theground elevations varied from 200 feetbelow sea level to approximately 6,000feet above sea level. The only existingtransportation in the area was a very loworder road and this would permit passageby four-wheel-drive vehicles only underideal conditions. The nearest town of anysize was Trona; a round trip from this pointto the center of the area required a good 7hours by four-wheel-drive car.

The only alternative to using helicoptertransportation obviously would have beento set up a series of camps and to supplythese camps with water and groceries. Inaddition to the problem of housing a crewin the area was the hazard of going on footfrom mountain peak to valley bottom un­der desert conditions. Unquestionably thehelicopter was the method to be used forpurposes of transportation because of theease of transporting men from the moun­tain top to valley bottom and back toTrona every night.

The operating cost of using helicoptersmade mandatory a gr'eat deal of thoughtbeing given to increasing crew efficiency.The following plan of operation was there­fore inaugurated:

1. No targets would be set in advance oftriangulation. From a study of existingtopographic maps and the topographicmaps produced for the area, an approxi­mate location of stations to be occupiedwas determined.

2. Three Kern 1 inch theodolites wereused on the operation, with three theodo­lite operators. In this manner each theodo­lite was itself its own target and the anglesof any given triangle were turned simul­taneously.

3. One recorder was used for the entireoperation and the angles observed wereradioed to the recorder and were turnedcontinuously until closure of the trianglecame within allowable tolerances. No re­occupations were therefore necessary. Thehelicopter operated as a shuttle bus jump­ing one theodolite operator ahead as eachsubsequent triangle was closed, and thena new triangle would be observed.

The operation started from a baseformed by U. S. G. S. stations Brown andLayton. In the net U. S. G. S. station VTwas cut in, and final closure of the systemwas made on U. S. C. &. G. S. station BM5672. In the entire operation 18 stationswere occupied in a distance of approxi­mately 40 miles. The operation was carriedout within a period of four days, with a

* Paper read at the Society's Semiannual Convention and Trade Show, Statler Hotel, Los An­geles, Calif., Sept. 9, 1955.

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410 PHOTOGRAMMETRIC ENGINEERING

crew of three theodolite operators, onehelicopter pilot and one mechanic, utiliz­ing three Kern theodolites and one heli­copter.

Since standby time of helicopters is anexcessively high item of cost, this opera­tion had to be planned in such a manneras to take advantage of ideal weatherconditions. The occurrence of storms andrainfall in the Death Valley area duringthe spring months is not a great possibility;in other areas such weather conditions maybe the critical item in deciding whetherthis type of surveyor some other should beused. The greatest hazard in the DeathValley as regards weather was the dangerof high wind velocities; these occur fre­quently in this area. It was very quicklyfound that the limiting factor from thepoint of view of wind velocity was not thehelicopter itself but the actual theodoliteoperation. In other words the helicopterswere able to land and take off underconditions of wind which were unfavor­able for stable triangulation operation.

An added advantage discovered in thecourse of this operation was that picturepointing and the selection of occupiedstations became much easier when thestation was approached by air. As thehelicopter with one theodolite operatorproceeded to the next station, the othertwo theodolite men would continue signal­ing with a heliotrope, as the helicopterdescended to a picture point selected fromthe air. At the same time the photographswere studied for the best position from thepoint of view of picture pointing. When apoint was determined the helicopterwould descend, the operator continuing toobserve the other two stations until hewas on the ground. If either' one of themwent out of sight he would choose a newand higher elevation position. In thismanner very little time was lost either inthe selection of picture points or in findinga good position from which to observe theother two stations.

The ability of the helicopter pilot tomaneuver the aircraft into almost anyspot in this area was indeed miraculous.The only places where a competent pilotcannot safely land a helicopter is in narrowdeep dra ws where there is insu fficient roomto obtain forward speed in a take-off. Fortriangulation operations such locations arerare but this limitation on copter use mightbe important in other types of control op­erations.

Kern Copters, Inc., at Bakersfield pro­vided the helicopter operation for thisproject, furnishing a pilot, mechanic,helicopter and truck. The helicopter was

brought into the area on a trailer behindthe truck. The truck was used to haulgasoline and to spot it in areas close to theoperating area so to facilitate refuelingoperations. One of the weaknesses of pres­ent daylight helicopters is their shortrange; therefore a great amount of timeis saved by setting up refueling stationsclose to the project. Mr. Forrest Raebel ofthe K. B. Wood Company was in chargeof the operations at Trona and was directlyresponsible for the successful operation ofthis venture.

Even though the cost of operating heli­copters is at the present time extremelyhigh, the speed and efficiency of this opera­tion made the project economical. In photocontrol operations usually speed is asimportant as cost and the necessity forgetting through the area with preliminarycontrol rapidly is of paramount interest.We feel these objects were well met on thisoperation.

Second order closures were obtainedbetween all U. S. G. S. stations a third­order closure resulted however when tyingbetween U. S. G. S. stations and theU. S. C. & G. S. station at the eastern endof the project.

It was felt that this lessening in theclosing accuracy in tying in this last pointwas due to the two systems probably beingadjusted ~eparately and not tied into theoriginal work. Vertical angles were ob­served on all legs of all triangles in bothdirections, reading the angles to the near­est second. After being correctly reducedand projected throughout the area be­tween bench marks, a total vertical closingerror of 0.82 feet over the 40 mile stretchwas obtained. After adjustments throughthe system it was felt that this closingerror was sufficient for checking of 10 footcontour accuracy throughout the project.

In conclusion it can be safely statedthat the use of theodolites on simultaneoustriangulation with helicopter transporta­tion is a fast, efficient and economicaloperation provided its use is limited to thefollowing conditions.

1. Daytime triangulation operation.2. Areas where the terrain is fairly de­

void of vegetation making landings possi­ible throughout the project.

3. Where weather conditions will notcause delays and shut-downs in the opera­tion. Standby cost of the pilots and me­chanics on this operation would soon useup the advantages if weather shut-downswere frequent.

4. In areas where a good base of existingtriangulation accessible with helicopterscan be occupied.