Instructions to topographers of the United States geological survey

Contents

Page.

Prefatory note 5

Horizontal control ; 7

Prefatory Note

The following instructions will eventually be incorporated in a handbook of instructions which is in preparation for topographers of the Geological Survey. They are printed in the present form for temporary use, and any criticisms or suggestions that would tend to improve them should be sent promptly to the chief geographer.

Approved :

Geo. Otis Smith, Director.

Washington, D. C, June 30, ign,

R. B. Marshall,

Chief Geographer.

U. S. GEOLOGICAL SURVEY

INSTRUCTIONS TO TOPOGRAPHERS PLATE I

7/feft

MARKS FOR HORIZONTAL OR VERTICAL CONTROL STATIONS.

A, Tablet used in cooperating States. The State name is inserted at G. A, C, and E, Tablets for stone or concrete structures. F, Iron post used where there is no rock.

Instructions to Topographers of the United States Geological Survey.

HORIZONTAL CONTROL.

GENERAL CONDITIONS.

The boundary lines of all regular United States Geological Survey maps are parallels of latitude and meridians of longitude. In order that these shall be properly located and that intermediate points shall be placed in correct positions according to scale, some system of horizontal control is required. The method to be adopted for linear control should be fixed by the character of the country, the requirement being that all control work shall be so accurate that no errors will be apparent in maps several times as large as those to be published. In mountainous regions or in hilly, partly timbered areas horizontal control is effected by a system of triangulation, the whole area being divided up into triangles whose apexes are represented by stations established on prominent points several miles apart. The angles between each station and all others visible from it are carefully measured with theodolites arranged to read angles as small as one second. One side of one of the triangles, called the base line, must be measured with great care with steel tapes, account being taken of slope of the line, elevation above sea, temperature of the tape, and other essential details, and for at least one station the exact latitude and longitude must be determined by astronomic observations.

In heavily timbered areas where it is difficult to see from any point more than a mile or two in any direction, horizontal control is best obtained from distances actually measured on the ground with 300-foot steel tapes and angles measured with a transit at each bend in the line. Such control, called primary traverse, must begin and end at points whose positions have been previously determined, and be carried around the edge of each quadrangle and once across its center east and west.

Because of the great expense involved in base-line measurements and the fixing of astronomic positions, it is generally necessary to connect tri angulation systems or traverse lines with positions previously determined, even though they may be some distance away. There are now but few localities in the United States that can not conveniently be connected with known positions and distances, and therefore, before horizontal control work is begun, the records of the Coast and Geodetic Survey, the Lake Survey, the United States Army Engineers, and other Government organizations should be examined in order to ascertain what positions in the area surveyed have been determined and are available for use in the work on hand.

The results of triangulation or primary traverse by the Geological Survey can always be obtained by anyone having occasion to use them by applying to the Director, United States Geological Survey, Washington, D, C.

PRIMARY TRIANGULATION.

FIELD WORK.

On the flyleaf of each field notebook is a blank in which shall be recorded all information necessary to identify the book. This blank should be filled so far as practicable on or before the first date of entry of field notes, aiid it must be completely filled before the book is forwarded to the Washington oifice. Any failure to fill in completely the blank on the flyleaf of a field notebook, should be reported by the computer to the geographer in charge of the division.

Personnel of party. — Each party usually consists of a triangulator and a recorder; also a cook and a teamster (or packer) in regions where camping is necessary. Additional men are required for heliotroping, one for each heliotrope station, and local laborers may be employed to clear timbered summits or to erect large signals.

Instruments, tools, books, etc. — The following instruments and books are used in primary triangulation :

One 8-inch theodolite, with leather carrying case and shoulder straps.

Two pairs field glasses.

One prismatic compass.

One protractor (6-inch celluloid, full circle).

One boxwood scale, graduated to inches and tenths.

One 50-foot steel tape.

One electric hand lamp.

One 6-foot steel tape,

Heliotropes.

One plumb bob.

Triangulation tablets or posts, according to requirements of country.

Cement, cans.

Signal notices, printed on cloth.

Climbing irons, for use in wooded regions.

Sun umbrellalFor use in regions where improvised sun and wind shelters

Wind screen j can not readily be built.

Triangulation field notes (9-912).

Computation of geodetic distances (9-901).

Computation of geodetic coordinates (9-902).

Computation book, blank (9-989)-

Nautical almanac (abridged).

Geographic tables and formulas.

Seven-place logarithm tables.

A good watch must be provided by the chief of party.

The following additional articles may be purchased in the field : Ax, hatchet, saw, nails, tacks, signal cloth, guy wire, stone drills (1^8-inch bit), drill hammer, post-hole digger, wire cutter, brace and bits.

Amount of control. — At least three serviceable stations must be established in each quadrangle and as many more as may be necessary to afford adequate control. In addition, a number of secondary points — such as church spires, windmills, water tanks, trees, and in high mountain regions some of the more prominent summits — must be located by intersection or by the ''three-point method." Where no such objects are available, at least two points should be flagged for intersection if practicable. These points are intended to afford supplementary control for the topographer and should be selected with special reference to their usefulness in that connection.

IO INSTRUCTIONS TO TOPOGRAPHERS.

The triangulator is also expected to locate, when practicable, either by direct measurement from his stations or by the three-point method, conspicuous objects, marks on State and county boundary lines, and township and section corners. Especial attention should be given to township and section corners because of their recognized value in the control of the land-line net.

Reconnaissance. — Stations should be selected and signals built before any observing is done, and to this end the triangulator and his assistant should make a reconnaissance over the area to be controlled. Such reconnaissance should disclose every practical scheme of tri angulation, the angles at each point selected being measured with a prismatic compass and platted with the protractor so that the size and proportions of the figures may be ascertained. All preparatory work, such as the setting of tablets and posts, the erecting of signals and scaffolds, and the clearing of lines of sight, should be completed during this reconnaissance, so that the final observing may be performed with economy and dispatch. The reconnaissance affords the triangulator opportunity to acquaint himself with the shortest routes of travel, with the best stopping places, with the available camp sites, water holes, pastures, and trails, and with the best routes for scaling each peak to be occupied, and it enables him to gain a familiarity with the special character of each station and its signal which will be invaluable to him in identifying the points when he sights them later on.

Figures-. — The most desirable groups of triangles consist of either quadrilaterals with both diagonals sighted or central point figures with four to seven sides. The triangles composing these figures should be well proportioned, angles measuring not less than 30 nor more than 120 each. The scheme should not be allowed to dwindle down to simple, unsupported triangles, and especial care should be taken to connect the work done with other work by means of well-proportioned triangles. Overlapping figures or an excess of observed lines beyond those necessary to insure a double determination of each length are undesirable, although an occasional diagonal through some figure may be valuable as a check. Additional lines of this kind only complicate the main scheme without materially adding to its strength, and the numerous observa-

INSTRUCTIONS TO TOPOGRAPHERS. II

tions made for them are discarded by the computers as superfluous . Judgment is to be used in this matter, however, for in many regions the atmospheric conditions are exceedingly uncertain and the topographer can not always count on being able to observe in both directions over every line that may be essential to the main scheme . In such regions it is well to err on the safe side and to obtain too many data rather than too few.

Angles should be read to all prominent points outside of the area for use in future expansion, even though they are without signals or are not sharply defined.

Secondary points. — In cutting in secondary points for topographic control it should be remembered that locations which depend on two sights only, even if the angles are of adequate size, are likely to be of doubtful value, because of the absence of any check on possible gross errors in observing or computing, or because of mistakes in the identification of the points. An endeavor should therefore be made to obtain at least three sights to every secondary point, even if the triangles are not of the best shape. Triangulators are especially cautioned not to slight the location of secondary points merely because they happen to be of no importance in their scheme of figures. The topographer may find it expedient to start his control from a secondary point, so that a blunder in the location of such a point may result in his starting with an erroneous base and having to make corrections at a great cost.

Consent of owner. — Before a site for a station on private land is selected, the written consent of the owner should be obtained, if practicable, for establishing a permanent station mark and erecting the required signal. If a summit must be cleared of timber, or if lines of sight must be cut, the value of the timber to be cut should be definitely fixed and agreed upon with the owner before cutting is begun. Payments on this account should be made and subvouchers taken before the station is left. A suggested form for wording these vouchers is as follows:

Received from the sum of in full payment for all damages incident to destruction of timber on hill (or mountain), in

County, State of , in connection with the establishment and occupation

of triangulation station [give name of station] [date].

When it is necessary to clear away timber and the owner or agent for the ground can not be reached without great delay, three residents of the locality should be asked to appraise the value of the timber cut and to sign a written statement regarding it. This statement should be forwarded to the office of the Survey for consideration should a claim for damage be filed.

Station marks. — Primary triangulation stations must be permanently marked by either standard iron bench-mark posts or by tablets, each tablet to be set in rock in place or in the top of a concrete or stone monument. (See second paragraph, p. 53, for instructions regarding the setting of tablets.) When practicable, bottles or other imperishable material should be left as a subsurface mark.

Two or more permanent reference marks should be established about each station mark. They may consist of holes drilled in rock in place, spikes in roots of trees, or large stones set solidly in the ground. The azimuth and the distance to each reference mark must be duly entered in the field record.

When old stations are revisited and any of the marks are found to be defective or to have been destroyed, new marks must be established in their place.

Signals. — Triangulation signals must be built with a view to their permanence as well as to their visibility. They may be of various forms, the form selected depending on the locality and the materials at hand. Thus, a signal on a bare mountain peak may be a rock cairn; one on a partly wooded summit may be a straight tree, the surrounding timber being cleared away; one on cleared land may be a tripod or quadripod.

Rock cairns should be not less than 8 feet high and should be well put together, so that they will withstand strong winds and heavy snows. A pole or a small green tree placed in the top is of advantage in sighting.

Signal trees are most satisfactory if stripped of their branches, except a tuft at the top. They form the best of targets when sighted against the sky, but if they are to be sighted against a dark background they should carry two triangular targets 3 to 6 feet on a side, placed at right angles to each other and covered with white cloth. Tripods or quadripods should be built of sawed lumber if such material is available. For the legs and center pole 2 -inch by 4-inch scantlings may be used, for the cross braces i-inch by 6-inch boards. The base of the pyramid should be large enough to permit a theodolite to be set up under the center pole. In order to increase its visibility, boards may be nailed across the sides about a foot apart and covered with signal cloth, and cross targets may be attached to the center pole above the apex of the pyramid. The best colors for this clothare white and black or white and red.

Most signals stand in exposed places and should be securely anchored to prevent their being blown over. The legs of tripods and quadripods should be planted in the ground at least 2 feet; each should be fastened to a "deadman" and the holes filled with thoroughly tamped earth or rocks, or else a stake 4 feet long should be driven into the ground at an angle with each leg and firmly spiked to it. If the ground is too rocky to permit the digging of holes, a 4-foot crosspiece should be nailed to each leg at right angles, flat on the ground, and weighted down with rocks.

Scaffolds. — If it becomes necessary to elevate the instrument a scaffold must be erected in the form of a tripod, capped with a thick board 12 inches square, to support the instrument. Around this scaffold, entirely independent of it, should be built another, in quadripod form, supporting a platform on which the observer is to stand. If very high, such a scaffold should be composed of successive bents, each 8 or 12 feet, with diagonal bracing. The outer scaffold, further, is to serve as a signal, and for that purpose should extend at least 6 feet above the observing platform and be surmounted by a mast bearing cross targets. Before fixing signals in position the direction in which sights are to be taken should be carefully ascertained, so that no woodwork will interfere with the observations.

The size of the timbers to be used necessarily depends on the height of the structure. The amount of lumber required may be determined by means of a rough drawing of the structure to scale.

Centering of signals. — Great care must be taken to insure perfect centering of signal and scaffold over the station mark, the plumb bob being used for this purpose. Signals should stand over station marks wherever possible, so as to avoid the necessity of computing swings for the angles, but if this is impracticable, as it is with a tree signal, then the distance and bearing of the signal to the station mark must be carefully measured and recorded.

The permanent mark, tablet, or post must be the station, and when observations are made for angles the theodolite should be set up over its center if possible. If it is impracticable to center the instrument over the station mark the distance between the point occupied and the station mark must be carefully measured and recorded. Also one or more sets of angles must be read between the station mark and the other stations, in order of azimuth, preferably with the.o° o / for the pointing to the station mark.

Heliotr oping. — The heliotrope outfit commonly used by the Survey is either the Steinheil heliotrope or a plane mirror with a screw hinged to the back to give it universal motion and improvised diaphragms of tin or wood with round apertures. The plane mirror is generally preferred to a heliotrope of the more elaborate form.

A heliotrope is usually set up by mounting the mirror on a stake or board immediately over the center of the station and the diaphragm on another stake, 10 or 20 feet away, which is carefully lined in with the distant station. The operator must constantly watch the reflected image of the mirror and keep it symmetrically over the aperture. If the sun is back of the observer a second mirror placed at a distance of a foot or two from the first may be used to reflect the light into the first.

To the observer the flash should appear as a clearly defined point of light; if of appreciable size it will be necessary to bisect it, and an error is thus likely to be introduced. A good rule to follow is to make the diameter of the opening in inches equal to one-fiftieth of the distance in miles for work in the West, and twice this size for work in the East, with a minimum opening of one-quarter inch.

Time of observing. — As a rule the best time for observing is the three hours before sunset; the atmosphere is then steadiest and shows no " boiling." The early morning hours are occasionally good but are likely to be less satisfactory. Many cloudy or overcast days are favorable. As a last resort observations at night may sometimes be necessary, but these require special night signals and assistants to operate them, and because of the additional cost involved are seldom warranted.

Preparation for observing. — Whenever practicable the theodolite must be set over the station mark for reading angles, to obviate reduction to center, In setting up the tripod the head bolt thumbscrews must be left loose until the legs are firmly placed and then tightened.

The instrument must be sheltered from both wind and sun. If the region affords no material" that is readily available for constructing wind screens and sun shelters a folding wind screen and a sun umbrella must be carried as a part of the regular outfit.

Before observations are begun at a station all adjustments of the theodolite must be tested and such as are found in error must be corrected, special attention being paid to the micrometers to eliminate errors of run. The stations to be sighted must next be carefully identified by means of the directions shown on the plat or by means of angles previously taken with a prismatic compass. If any of the distant stations can not be seen with the unaided eye some object in line with each which can be found quickly must be selected, or, if necessary, the direction to each may be marked by some object near by, so no time shall be lost in making the pointings when the angles are being read.

Method of observing. — With micrometer theodolites either single angles may be measured or the method of circle readings (directions) may be adopted. In using the latter method select for the initial point some station that is especially distinct and easily sighted, and use it as the initial point for all sets of readings. The telescope being set on the initial point, read both micrometers, then sight the other stations in succession in the order of their azimuths (clockwise rotations), closing on the initial point. Then reverse telescope, set on initial point, and sight the stations in reverse order. This completes one set of readings with telescope direct and reversed. Now shift the circle about 36 (examine the plate bubbles after this shift and relevel if necessary) and commence another set. When pressed for time it is advisable to shift the circle when telescope is reversed. No angle should be considered as well determined that has not been measured on at least 5 different parts of the circle or 10 times in all, 5 with telescope direct and 5 with telescope reversed. When the telescope is reversed each end of its a£is will rest in the same Y as before. Reversals are of especial importance when there is an appreciable difference in the elevations of the points sighted.

If the observations are made in the afternoon it is advisable to take all secondary pointings before commencing the observations to stations, and there should be at least two sets of such pointings: the remaining time for observing can then be devoted to the accurate measurement of the important angles while conditions are the most favorable.

The graduated circle should never be placed so that when pointing at any particular station the micrometers will be set to even degreer except, as before noted, while data are being obtained for ''reduction to center."

Field record. — The field record is to be kept in book 9-912. It must be written in a plain, neat hand, with a No. 4 pencil, or with ink and no part of it must on any account ever be erased. A single line should be drawn through erroneous records, the corrected figures being written above. If deemed necessary an explanation should be written in the column for remarks. The memory should not be trusted for data of any kind; the record must be faithfully kept in all particulars and be made so complete that it can be understood by another person at any time.

The flyleaf of each notebook must be properly filled in when the book is first used and one of the blank flyleaves must contain an index of the contents.

The date, name of station, time of observing, and names of observer and recorder should be systematically entered at the head of each page.

The position of the instrument with respect to the center of the station must be clearly defined and if it is set up off the center a full statement must be given of the distance and the angles measured.

On the page immediately preceding the record of angles should be written a minute and complete description of the station occupied, the station marks, character of signal, nearest camping or other stopping places, roads, and trails, also a statement regarding the ownership of the land and such other information as will be helpful to the topographer. The description must be written before the recorder leaves the station and should be accompanied by a rough diagram showing directions to other stations and plan indicating location of instrument if it was not centered on the station.

Inasmuch as station names are to be published, effort should be made to select names that have local significance.

Reading and recording of Angles. — When the micrometer wires &e set for a reading with the Geological Survey theodolites it is very important that the last movement of the wires be tow r ard the right. The readings on the graduated head are then decreasing and the spring attached to the slide which holds the wires is being compressed. If the cross wires are moved the least bit too far to the right they must not be turned backward to the setting but must be turned backward at least a halfturn of the screw, then brought forward slowly to a correct setting. When the setting is properly made a division on the graduated plate will appear exactly midway between the two movable cross wires and an equal amount of white space will show on each side of it. A part at least of the micrometer adjustment errors can be eliminated by making the settings with less than five turns of the screw; this can always be done if the right-hand part of the comb scale is sometimes used for comb scale and micrometer head readings, the io-minute space being taken from the left.

For all precision instruments where a tangent screw and spring are used together, the setting should be made while the spring is being compressed, otherwise the "slack" of the screw may cause an error.

The recorder should not only take down the readings called off by the observer but should without delay compute the angles between successive stations and also the mean readings. The following form is to be used for recording angles by the method of directions: 8221 — 11 2 i8

INSTRUCTIONS TO TOPOGRAPHERS.

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Opposite each angle record any necessary information as to visibility of signals or atmospheric conditions.

Field computations. — Angles at each station should be reduced to center in the field in order to test the triangle closures, which for a primary scheme should not exceed five seconds.

Arbitrary adjustments and preliminary computations of positions should also be made in the field. Book 9-889 is to be used for summary of angles and for miscellaneous computations. Computations for distances should be entered in book 9-901 and for coordinates in book 9-902. For field computations of coordinates where the lines are short five or six place logarithms will give sufficient accuracy and the computations may be shortened by omitting some of the minor corrections, carrying results to tenths of seconds of latitude and longitude only.

Triangulation plot. — A careful plot of the work should be kept on the scale of 10 miles to an inch, and each month a reduced copy, on which angles measured are indicated by the usual sign, should be sent in on the monthly-report blank. The plot, if carefully made, will prove invaluable for finding directions to distant stations. Place the protractor on the plot with o° in line with a station that can be seen clearly, then read in turn the angle to each other station, thus obtaining an observing list.

Azimuth observations . — There must be not less than two azimuth stations in each triangulation scheme, but if the azimuth of any line in a scheme can be computed from former observations then only one azimuth station need be established for each square degree controlled.

The azimuth mark should be placed at least half a mile from the station. It should consist of a vertical slit one-fourth to one-half inch wide and 6 inches long, cut in a small box containing a candle or lantern. To illuminate the cross wires of the instrument and to read the angles, an electric hand lamp is to be preferred.

The observations should consist of not fewer than five direct and five reversed measurements between the star and mark. As the star is at a much higher angle of elevation than the mark it is important that the horizontal axis of the theodolite be adjusted with care and leveled. The ends of the striding level bubble must be read at each setting on the star and a level correction computed if there is an appreciable difference between them, as shown in the example attached.

INSTRUCTIONS TO TOPOGRAPHERS.

Observations on Polaris should be made immediately preceding and following elongation, as any error in the time of observation has then the least effect on the resulting azimuth. The time of setting the cross wires on the star must be recorded to the nearest second. The watch error must be known and to this end the triangulator should compare his watch frequently with telegraphic time, which is sent over Western Union lines once a day, usually at noon Washington time.

Example of record of azimuth observations.

Station: Canada, Ky. 8-inch theodolite No. 434. One division of micrometer= 2". One division of level=2" of arc. June 11, 1910. Watch o m 23 s60 slow.

Telescope direct.

Object.

Time a.m.

I^evel.

Micrometer.

Mean.

Angle.

West.

East.

A.

B.

Mark

H. m. s.

Div.

Div.

172 54 00 108 35 29

/ it

Polaris

2 30 50

11. 9.0

10. 12.

64 18 31

Telescope reversed.

Mark

352 52 13 288 33 27

352 52 47 288 34 07

Polaris

2 36 48 2 40 26

11. 9.0

10. 10.

10. 12.

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