Chapter XIX: Appendix: A

SURVEY METHODS EMPLOYED IN THE CONSTRUCTION OF THE SEVEN ACCOMPANYING TOPOGRAPHIC SHEETS

BY KAI HENDRIKSEN, TOPOGRAPHER

The main part of the topographical outfit consisted of (1) a 4-inch theodolite, Buff and Buff, the upper part detachable, (2) an 18 x 24 inch plane-table with Johnson tripod and micro-meteralidade. These instruments were courteously loaned the expedition by the U. S. Coast and Geodetic Survey and the U. S. Geological Survey respectively.

The method of survey planned was a combination of graphic triangulation and traverse with the micro-meteralidade. All directions were plotted on the plane-table which was oriented by backsight; distances were determined by the micro-meteralidade or triangulation, or both combined; and elevations were obtained by vertical angles. Finally, astronomical observations, usually to the sun, were taken at intervals of about 60 miles for latitude and azimuth to check the triangulation. No observations were made for differences in longitude because this would probably not have given any reliable result, considering the time and instruments at our disposal. Because the survey was to follow very closely the seventy-third meridian west of Greenwich, directions and distances, checked by latitude and azimuth observations, undoubtedly afforded far better means of determining the longitude than time observations. In other words, the time observations made in connection with azimuth observations were not used for computing longitudinal differences. Absolute longitude was taken from existing observations of principal places.

Principal topographical points were located by from two to four intersections from the triangulation and plane-table stations; and elevations were determined by vertical angle measurements. Whenever practicable, the contours were sketched in the field; the details of the topography otherwise depend upon a great number of photographs taken by Professor Bowman from critical stations or other points which it was possible to locate on the maps.

CROSS-SECTION MAP FROM ABANCAY TO CAMANÁ AT THE PACIFIC OCEAN

Seven sheets. Scale, 1:125,000; contour interval, 200 feet. Datum is mean sea level. Astronomical control: 5 latitude and 5 azimuth observations as indicated on the accompanying topographic sheets.

On September 10th, returning from a reconnaissance survey of the Pampaconas River, I joined Professor Bowman’s party, Dr. Erving acting as my assistant. We crossed the Cordillera Vilcapampa and the Canyon of the Apurimac and after a week’s rest at Abancay started the topographic work near Hacienda San Gabriel south of Abancay. Working up the deep valley of Lambrama, observations for latitude and azimuth were made midway between Hacienda Matara and Caypi.

On October 4th we made our camp in newly fallen snow surrounded by beautiful glacial scenery. The next day on the high plateau, we passed sharp-crested glaciated peaks; a heavy thunder and hail storm broke out while I occupied the station at the pass, the storm continuing all the afternoon--a frequent occurrence. The camp was made 6 miles farther on, and the next morning I returned to finish the latter station. I succeeded in sketching the detailed topography just south of the pass, but shortly after noon, a furious storm arose similar to the one the day before, and made further topographic work impossible; to get connection farther on I patiently kept my eye to the eye-piece for more than an hour after the storm had started, and was fortunate to catch the station ahead in a single glimpse. I had a similar experience some days later at station 16,079, Antabamba Quadrangle, on the rim of the high-level puna, the storm preventing all topographic work and barely allowing a single moment in which to catch a dim sight of the signals ahead while I kept my eye steadily at the telescope to be ready for a favorable break in the heavy clouds and hail.

At Antabamba we got a new set of Indian carriers, who had orders to accompany us to Cotahuasi, the next sub-prefectura. Raimondi’s map indicates the distance between the two cities to be 35 miles, but although nothing definite was stated, we found out in Antabamba that the distance was considerably longer, and moreover that the entire route lay at a high altitude.

From the second day out of Antabamba until Huaynacotas was in sight in the Cotahuasi Canyon, a distance of 50 miles, the route lay at an altitude of from 16,000 to 17,630 feet, taking in 5 successive camps at an altitude from 15,500 to 17,000 feet; 12 successive stations had the following altitudes:

16,379 feet
16,852 "
17,104 "
17,559 "
17,675 " --highest station occupied.
17,608 "
17,633 "
16,305 "
17,630 "
17,128 "
16,794 "
16,260 "

The occupation of these high stations necessitated a great deal of climbing, doubly hard in this rarefied air, and often on volcanoes with a surface consisting of bowlders and ash and in the face of violent hailstorms that made extremely difficult the task of connecting up observations at successive stations.

At Cotahuasi a new pack-train was organized, and on October 25th I ventured to return alone to the high altitudes in order to continue the topography at the station at 17,633 feet on the summit of the Maritime Cordillera. Dr. Erving was obliged to leave on October 18th and Professor Bowman left a week later in order to carry out his plans for a physiographic study of the coast between Camaná and Mollendo. Philippi Angulo, a native of Taurisma, a town above Cotahuasi, acted as majordomo on this journey. Knowing the trail and the camp sites, I was able to pick out the stations ahead myself, and made good progress, returning to Cotahuasi on October 29th, three or four days earlier than planned. From Cotahuasi to the coast I had the assistance of Mr. Watkins. The most trying part of the last section of high altitude country was the great Pampa Colorada, crowned by the snow-capped peaks of Solimana and Coropuna, reaching heights of 20,730 and 21,703 feet respectively. The passing of this pampa took seven days and we arrived at Chuquibamba on November 9th. Two circumstances made the work on this stretch peculiarly difficult--the scarcity of camping places and the high temperature in the middle of the day, which heated the rarefied air to a degree that made long-distance shots very strenuous work for the eyes. Although our base signals were stone piles higher than a man, I was often forced to keep my eye to the telescope for hours to catch a glimpse of the signals; lack of time did not allow me to stop the telescope work in the hottest part of the day.

The top of Coropuna was intersected from the four stations: 16,344, 15,545, 16,168, and 16,664 feet elevation, the intersections giving a very small triangular error. The elevation of Mount Coropuna’s high peak as computed from these 4 stations is:

21,696 feet
21,746 "
21,714 "
21,657 "
------
Mean elevation 21,703 feet above sea level.

The elevation of Coropuna as derived from these four stations has thus a mean error of 18 feet (method of least squares) while the elevation of each of the four stations as carried up from mean sea level through 25 stations--vertical angles being observed in both directions--has an estimated mean error of 30 feet. The result of this is a mean error of 35 feet in Coropuna’s elevation above mean sea level.

The latitude is 15° 31′ 00″ S.; the longitude is 72° 42′ 40″ W. of Greenwich, the checking of these two determinations giving a result unexpectedly close.

On November 11th azimuth and latitude observations were taken at Chuquibamba and two days later we arrived at Aplao in the bottom of the splendid Majes Valley. In the northern part of this valley I was prevented from doing any plane-table work in the afternoons of four successive days. A strong gale set in each noon raising a regular sandstorm, that made seeing almost impossible, and blowing with such a velocity that it was impossible to set up the plane-table.

From Hacienda Cantas to Camaná we had to pass the western desert for a distance of 45 miles. We were told that on the entire distance there was only one camping place. This was at Jaguey de Majes, where there was a brook with just enough water for the animals but no fodder. Thus we faced the necessity of carrying water for ten men and fodder for 14 animals in excess of the usual cargo; and we were unable to foretell how many days the topography over the hot desert would require.

Although plane-table work in the desert was impossible at all except in the earliest and latest hours of the day, we made regular progress. We camped three nights at Jaguey and arrived on the fourth day at Las Lomas.

The next morning, on November 23rd, at an elevation of 2178 feet near the crest of the Coast Range, we were repaid for two months of laborious work by a glorious view of the Pacific Ocean and of the city of Camaná with her olive gardens in the midst of the desert sand.

The next day I observed latitude and azimuth at Camaná and in the night my companion and assistant Mr. Watkins and I returned across the desert to the railroad at Vitor.

CONCLUSIONS

The planned methods were followed very closely. In two cases only the plane-table had to be oriented by the magnetic needle, the backsights not being obtainable because of the impossibility of locating the last station, passing Indians having removed the signals.

In one case only the distance between two stations had to be determined by graphic triangulation exclusively, the base signals having been destroyed. Otherwise graphic triangulation was used as a check on distances.

Vertical angles were always measured in both directions with the exception of the above-mentioned cases.

Observations for azimuth were always taken to the sun before and after noon. The direction used in the azimuth observation was also taken with the prismatic compass. The mean of the magnetic declination thus found is: East 8° 30′ plus.

Observations for latitude were taken to the sun by the method of circum-meridian altitudes, except at the town of Vilcabamba where star observations were taken.

As a matter of course, observations to the sun are not so exact as star observations, especially in low latitudes where one can expect to observe the near zenith. However, working in high altitudes for long periods, moving camp every day and often arriving at camp 2 to 4 hours after sunset, I found it essential to have undisturbed rest at night. It was beyond my capacity to spend an hour or two of the night in finding the meridian and in making the observation. Furthermore, the astronomic observations were to check the topography mainly, the latter being the most exact method with the outfit at hand.

The following table contains the comparisons between the latitude stations as located on the map and by observation:

Map Observation
Camaná Quadrangle S 16° 37′ 34″ 16° 37′ 34″[66]
Coropuna, station 9,691S 15° 48′ 30″ (15° 51′ 44″)
Cotahuasi, " 12,588S 15° 11′ 40″ 15° 12′ 30″
La Cumbre, " 16,852S 14° 28′ 10″ 14° 29′ 46″
Lambrama, " 8,341S 13° 43′ 18″ 13° 43′ 14″

The other observations, with the exception of the one on the Coropuna Quadrangle, check probably as well as can be expected with the small and light outfit which we used, and under the exceptionally hard conditions of work. The observation on the Coropuna Quadrangle just south of Chuquibamba is, however, too much out. An explanation for this is that the meridian zenith distance was 1° 23′ 12″ only (in this case the exact formula was used in computing). Of course, an error or an accumulation of errors might have been made in the distances taken by the micrometer-alidade, but the first cause of error mentioned is the more probable, and this is indicated also by the fact that the location on the top of Mount Coropuna checks closely with the one determined in an entirely independent way by the railroad engineers.

For the cross-section map from Abancay to Camaná, the following statistics are desirable:

Micrometer traverse and graphic triangulation, with contours, field scale 1:90,000.

Total time required, days 40.5
Average distance per days in miles 7.5
Average number of plane-table stations occupied per day 1.5
Average area per day in square miles 38.
Located points per square mile 0.25
Approximate elevations in excess of above, per square mile 0.25
Highest station occupied, feet above sea level 17,675.
Highest point located, feet above sea level 21,703.