Few authors of treatises and papers on engineering subjects have given adequate attention to topographic maps. The statement applies especially to mining engineering in all branches. Even those who have discussed such maps have treated the subject only in a general way. Therefore it is proposed to outline in this paper a few of the uncommon yet important relationships which topographic maps and their by-product, structure-contour maps, have to modern methods of mining investigation and development when the facts depicted on such maps are properly interpreted. Topography, as suggested by the etymology of the word, means a detailed description of particular places. Written descriptions have been found less effective than the pictorial representations, therefore attempts have been made in various ways to picture the surface features of places. Lines and shading have been used, hachures drawn, and, finally, the contour topographic map. To a large extent this style of map is the result of the demands of engineers. It is designed to meet their needs far more than those of the man untrained in engineering. In fact, the average man obtains a better idea of the topography of an area from the hachure system than from contour maps. Since the maps have been made chiefly for the use of the engineer, they ought to meet his demands on the one hand, and, on the other, should be used by him to the fullest extent. Such maps, when properly made, accurately portray a portion of the earth’s surface and should present the topography better than a personal examination of the area without a map could present it, because the person making such examination views only a limited portion of the surface at a time, and estimates distances only roughly with his eye. Let the engineer consider carefully the fact that in many instances the topographic map is superior to a personal examination of the surface features of a property because the map is based on careful instrumental measurements, whereas the eye gives imperfect data because it can be used merely for the estimation and not for the measurement of distances. Such estimation is frequently unreliable, since it depends upon the engineer’s physical condition. When weary in the evening, after a hard day’s tramp, he views a cliff or steep slope which he may have to climb in an entirely different way than he views it when he is fresh in the morning. The long mountain slope appears different in the ascent than during the descent. It is not uncommon for the traveler approaching the steep mountain front from the plains to feel that he is gradually descending, whereas he is actually going up a gentle slope. The experience is so common that, in irrigated regions along the mountains, the feeling is expressed in the conclusion that water runs up hill. These familiar experiences will no doubt suffice to demonstrate the statement just made, that, in some ways at least, topographic maps give better information than field examination.
That engineers rely upon topographic maps is emphasized by the following table, compiled under the direction of the Chief Geographer from the record of the U. S. Geological Survey to show the great number of maps purchased annually and also the growth of their use :
This table shows that more than half a million maps are now purchased annually for work in the mapped portion of the United States, which is 38 per cent of the total area. At this same rate, if all the country were mapped, the demand during the year would have been for 1,400,000 maps. Furthermore, it seems very conservative to consider that at least half of the maps purchased during the preceding year would have been used also. On this assumption—namely, that the additional maps might have been sold, and that half of those sold during the preceding year were used—we infer a potential demand for the use of 2,000,000 topographic maps. This conclusion does not seem unreasonable, because as people are educated to the use of topographic maps there is a greater demand, and as contiguous areas are completed the demand increases.
Both surface and subsurface features are now represented by contour maps, the former by the topographic maps and the latter by the somewhat less known structure-contour maps. Surface work is directly related to topographic maps and only indirectly related to structure-contour maps, whereas with underground work the reverse is true. Therefore, in this paper the two kinds of maps will be treated more or less independently. Furthermore, since most mining engineers are familiar with topographic maps and their uses, this paper discusses only certain data, presented on topographic maps but generally overlooked by the engineer because he fails either to read the map properly or to use the information which it contains.
One of the common problems in both the development and the operation of a mining property is to control the water supply which is to be used, and to protect the property from any excess that is not needed. In many plants the flood water of a nearby stream is a menace. It takes years of measurements to determine the high-water stage of a stream, but a close approximation may be made in another way. With the size of the drainage basin given and the daily rainfall known, the stream’s flood stage can be roughly estimated. In the United States the observations of the Weather Bureau have given us data on the rainfall for most parts of the country. The other factors, such as the size and characteristics of the drainage basin, are shown on topographic maps. In this way two of the important elements in the solution of the water problem can be obtained. But more data can be actually obtained by proper interpretation of the map, especially with regard to flood. If the slopes are steep the run-off occurs rapidly and the streams are quickly flooded, but if the slopes are gentle the flow of the water is more gradual and lower flood stages may be expected. These considerations are not theoretical, but actual. They were real to the engineers of the Burlington railroad a few years ago when tracks were washed out by floods and bridges were overflowed. The railway’s engineers attacked the problem just as indicated above. Rain-gauge measurements had shown the extremes of rainfall which were to be expected in a given area in southeastern Nebraska. They knew the character of the soil, forestation, and similar factors which control the run-off. Of course, the run-off as a result of the rainfall must pass under the bridges of the railroad. These bridges must be of a certain size in order to permit the water to flow beneath, so as to avoid washing out the grade. The engineers recognized that, with their general knowledge of conditions and with specific data on the amount of rainfall and on the size and topography of the basin to be drained, they could closely estimate the maximum amount of water which might be expected to flow under any given bridge. As previously stated, they knew the amount of rainfall, but they did not know the second factor—namely, the topography; hence the railroad went to the expense of making a topographic map of the basins which drain towards the railroad in order that they might know the second factor, and thus construct their bridges properly.
A knowledge of the topography of a drainage basin is applicable in another way. Snow slides have wiped out more than one mining camp. With the fragrance of the pine enveloping him in the balmy days of summer, when claims are usually examined, the engineer does not fully appreciate the dangers from a winter’s accumulation of ice and snow above the property in question. Possibly, as stated above, his point of view cannot give him the proper relations of slopes, which might warn him of the danger Yet many of us have seen those streaks on the mountain side bared of their normal verdant cover by the avalanche and know that slides are a menace. We have seen too that, as the avalanche plunged into the narrow valley below, the width of the area devastated was less, but the destructive power increased. Do all geologists and engineers, however, know that the topographic maps present data for the proper location of a camp to prevent its destruction by snow slides ? The snow must accumulate somewhere, and it can slide only on steep slopes. The size and slope of the catchment basins and the angle of slopes over which the snow will move are shown on topographic maps. Reliable results can be determined from these data.
Commonly, topographic maps are used as a general guide and accurate surveys are made subsequently as a basis for detailed work; but in many cases the second and more expensive survey is unnecessary, because the accurate location of points can be determined from data shown on topographic maps. In this work the map is used as a base and needed refinements can be added to it. It may be that the map available is drawn with 50-ft. contours, whereas a 10-ft. interval is required for the work in hand. If so, the map may be accepted and supplemental contours and other refinements may be added. Ordinarily, the geologist takes his topographic map as a good guide, and a standard map is deserving of this confidence, but even good things have their limitations. On a 25-ft. contour map the elevation between the two lines is generally estimated. In this case the error cannot be more than 25 ft. Likewise, horizontal positions are more or less a matter of estimation within certain limits, but the engineer is constantly demanding accurate measurements, not approximations. Good topographic maps like those of the U. S. Geological Survey furnish a basis for this accurate work.
To obtain these precise data it is necessary for the engineer to repeat some of the operations of the topographer at the points where such data are desired. To do this work it has been found convenient to place the topographic sheet on a suitable-sized plane-table board, and use it as an original sheet; or, better still, to have a photolithograph printed on good drawing paper on a scale suitable for the work in hand. The engineer proceeds to the field equipped with his sheets, telescopic alidade, and stadia rod. The bench marks and triangulation stations, shown on the sheet, give him points from which he can establish his locations. This is an important feature. Here are useful data presented on topographic sheets and seldom considered, yet they furnish a basis for accurate work. They comprise some of the best data on the sheet, such as triangulation stations, houses, water towers, oil derricks, etc. Some points are shown only by bench marks, and signals should be re-established. If this is done for the engineer when he enters the field, he proceeds to the point whose location and elevation are desired and orients his table by the magnetic or three-point method, locates his point by intersections, and determines his elevation by vertical angles. After he has determined the location and elevation of his station he records the observations about the outcrop of the vein or bed. So far, the engineer has worked alone and he can continue to do so if necessary, but if he is accompanied by an assistant who can serve as a rodman, or, better still, who can operate the plane table, the engineer, serving as a rodman, traces the outcrop and holds the rod at points to be located. The plane-table operator sights to each stadia station and determines the location and elevation accurately. With these data he can plot the outcrop of a vein, if that is what he is tracing, though it is exposed intermittently, and determine its position with regard to land lines which are also shown on the sheet. Furthermore, he can obtain data from which he can determine the pitch of the vein. For the sake of illustration, a simple problem is assumed by supposing that the engineer has found three exposures of a vein, two at about equal elevations on opposite sides of a valley, and a third in the bottom of the valley, 500 ft. lower. If the three fall in a line the vein is vertical, but if the valley point falls to either side of the line joining the other two the vein dips in the direction of that point in an amount equal to the distance of the point from a line joining the two points on the sides of the valley. In the case assumed above, if this distance of the point from the line is equivalent to 100 ft. the vein dips 100 ft. in 500, or 20 ft. to the hundred.
There are many additional applications of the data other than contours, but this discussion is not prolonged to include them.
The remaining part of this paper relates to structure-contour maps, or maps that might well be called strata-form maps because they portray the form of the strata. Their discussion is introduced in this paper because they are very closely related to surface-contour maps in their construction, interpretations, and uses. Such maps are in demand in coal fields, and especially in oil and artesian water fields, but are also applicable to some metalliferous areas. The methods of construction of such a map vary with conditions which are receiving consideration in some geologic magazines; therefore this paper begins with the assumption that the maps have been made.
These maps, illustrated in Fig. 1, show the structure of some distinctive stratum, such as a coal bed, oil sand, or any persistent bed. Contours are drawn on equal elevations of the bed just as surface contours are drawn through points of equal elevation of the surface. Synclines are depressions just as valleys are depressions, and anticlines are ridges in the strata just as mountains and hills are land ridges. To illustrate the relation of structure contours to topographic contours, let us suppose that we have strata folded into a syncline and an adjacent anticline, and that a definite unbroken stratum is the surface rock over both. Then the structure and surface contours are coincident if the same datum plane is used for both. Sea-level is generally used in the construction of both kinds of maps. The structure lines and topographic contour lines agree exactly. Now, if a portion of the anticline is eroded and the debris partly fills the valley without attendant crustal movement, the structure contours remain the same, but the surface contours are shifted. On the crest of the anticline the surface contours show lower elevations than the structure contours, whereas the reverse is shown in the valleys. In the construction of the map, however, the portion of the structure contour map representing the crest of the anticline is generally shown by broken lines to indicate that the bed does not actually remain and that its position has been assumed. In the adjustment of the assumed eroded surface the contours are shifted from their former position of coincidence to new ones, depending on the amount of erosion or deposition. If the syncline and anticline trend north-
south, the structure contours extend in the same direction; but if the syncline is filled so that the surface slopes to the north or south, the topographic contours extend east-west across the structure contours. The difference in elevation of any two lines at the point of intersection is the distance of the contoured strata from the surface. The depth at points between contour lines can be determined by interpolation. Thus, if a point lies midway between two surface contours, 2,100 and 2,150 elevation, its altitude is assumed to be 2,125, and if the same point is two-fifths the distance from the 1,150-ft. structure contour to the 1,200-ft. contour its elevation is assumed to be 1,170, and the depth of the bed below the surface is the difference between 1,170 and 2,125, or 955 ft. The uses of such maps are apparent and their accuracy is surprising. One such map, made by M. R. Campbell, of the U. S. Geological Survey, is published in the Masontown- Uniontown Folio, No. 82. The structure lines are drawn on 50-ft. contour intervals to indicate the position of the Pittsburg coal west of Laurel ridge, and the Upper Freeport coal east of that ridge. Diamond drilling and mining subsequent to the construction of the map have shown that the maximum error is only a few feet. Of course, it is unnecessary to point out the value of such a map to the mining engineer in studying a mining problem. Supposing he has both surface and structure-contour maps printed on one plate, as they usually are, then from the one sheet he can select the possible locations for the surface works, can infer the depth to the coal if it is a coal mine, the lowest point for drainage purposes, the slope for haulage, etc.
The construction of maps of this kind has often given surprisingly useful results. It is therefore urged that, whenever possible, the engineer dealing with such problems construct corroborative maps both for use and to reassure himself that he is correct. Two examples in which the engineer’s judgment may be erroneous are given in Fig. 2 to illustrate this necessity. Supposing an oil well is to be sunk to reach the highest point in the oil sand of an anticline. If the anticline is symmetrical, as in A, Fig. 2, the well should be sunk upon the crest of the surface part of the anticline. If, however, the anticline is unsymmetrical, a location upon the previous supposition would be unsatisfactory, as shown at B. A structure map on oil sand would show the location of the highest part of the oil sand. In the construction of one such map the engineer found the structure in the immediate vicinity of possible oil location to indicate that the beds formed a fairly symmetrical anticline, but consideration of the data obtained in the surrounding area showed conclusively that the crest of the contoured bed had been forced to one side at a depth of several hundred feet below the surface and was not directly under the crest, as the surface indications along the anticline would have led him to infer (C, Fig. 2). From these considerations the writer concludes that structure-contour maps are valuable to an engineer if constructed for him, but if forced to add details for himself he will receive great value from them both in making the proper field observations and in his subsequent interpretations. Of course, such work is hopeless without a topographic map; therefore, the preparation of topographic maps is advocated because of their usefulness in assisting the engineer to obtain a correct conception of the surface and also as a basis for the study of subsurface structural relations.
Discussion
F. A. Linforth, Butte, Mont.:—I have not had an opportunity of reading this paper thoroughly, but I would like to call attention to the possible use of the contours on fault planes. It is obvious that if the contours of fault surfaces can be determined, we will be able to determine the axes of the hollows in the fault surface. It seems probable, and there are examples to point to the accuracy of the assumption, that the axes of these hollows may be the direction of movements on the faults, and if that is true it will aid materially in locating faulted ore bodies or the shoots of ore on the opposite sides of faults. A few such contour maps of faults have been attempted on some of the faults of the Butte district, and those faults which are believed to have the nearly normal movement exhibit for the axes of these hollows a line nearly up and down in the plane of the fault, but in those cases where the throw is believed to be nearly horizontal we find that the axes of the hollows are almost horizontal lines. In other words, the movements on faults, I believe, would be more nearly indicated by the larger grooves in the fault surfaces than by the small slickensides and minor hollows that are seen, so that contour maps might be valuable in geology in that way.
C. W. Goodale, Butte, Mont.:—I received a letter from Mr. Woodruff asking me to take part in the discussion of his paper, but I have not had time to give the paper any study. I can certainly add my opinion to that of all the engineers here that the topographic maps of the government are of great aid to the engineer. When I visit a new district, the first thing I do is to see if that district has been covered by the U. S. Geological Survey and if topographic map has been issued. And I find that some of the younger engineers frequently come into my office to see if I have such maps. I know in some cases the reports made out by engineers contain copies of these topographic maps, or clippings from them. I am sure that the topographic maps furnished by the government are of very great value to our engineers.
E. P. Mathewson, Anaconda, Mont.:—I would like to express my appreciation of these maps. Referring to a matter that is not strictly one to come before this audience : I had occasion recently to assist in getting out a tour book for automobilists for the State of Montana. Montana is a big State; a great part of it is not surveyed, and we have not county maps for all the counties. The consequence is that many of our principal roads in the State of Montana are not marked on the maps, so we were at a loss to connect up some of the main highways on our tour maps, and the thought was suggested that possibly the topographic maps of the U. S. Geological Survey would help us. We immediately sought out topographic maps covering the districts in question, and to our great delight we found in most instances the county roads clearly marked, and they were entered on our tour books.
E. W. Parker, Washington, D. C.:—I am not so familiar with the Topographic Branch of the Survey as I am with others, but we have with us to-day Mr. Manning, who is the Assistant Director of the Bureau of Mines, and I think he can explain something in regard to that if you can get him on his feet.
Van. H. Manning, Washington, D. C.:—I do not hesitate to get on my feet, but I know so much about the work of the Topographic Branch of the Geological Survey that I hardly know where to begin. I can, therefore, qualify to you as to my ability to speak authoritatively on this subject by saying I was a member of the Topographic Branch of the Geological Survey for over 20 years. However, I will start by replying to the inquiry as to why the U. S. Geological Survey has not been more liberal in the topographic mapping of the United States. The reason for this is because Congress has not made sufficient appropriations to survey a larger area.
I left the Geological Survey in 1910 and at that time there was about 33 per cent, of the total area of the United States surveyed. However, some of the topographic surveys covering this area were not up to the present standard of efficiency. Commenting upon the statement that has been made here of the accuracy of those topographic maps photographed up to a larger scale, I want to say that it is not always safe to make this enlargement by photography or by any other means, for such enlargements accentuate any error which in not appreciable on the smaller scale on which the map has been made. The maps should be revised about every 10 years, not because the topography will materially change, but for the purpose of adding the culture, such as roads, houses, etc., from time to time.
The Geological Survey is making better topographic maps to-day than it ever has before and the cost of making these maps is increasing. In the early history of the Survey the cost of making these maps amounted to from $2 to $3 per square mile, and to-day it costs from $20 upward for mining districts, the cost per square mile depending on the scale of the map and the character of the topography.
Mr. Mathewson :—Is it not true that the Survey has given particular attention to the large mineral districts ?
Mr. Manning:—Yes sir, and I may add, for the information of the mining engineers, the government has done away with the old contract system of land or rectangular surveys. Under the present system of having the work done by employees of the government working upon an annual salary basis these surveys are accurate and all fraudulent errors are thereby eliminated.
During my connection with the Geological Survey, I worked for a great many years in the public land States, and the locating of section corners set by contracts was in many areas a difficult thing to do. It was not always that I could find that a mile was a mile square. We of the Geological Survey tried to help you all we could in locating these corners accurately on the topographic maps, but we could not always locate the corners. Sometimes we thought we had them in their relative position, but the corners had either been moved by somebody, or they had been improperly set by the surveyor. The policy of the government is to give you a good survey, and a proper co-ordination of the topographic surveys by the Geological Survey and the rectangular surveys by the United States Land Office will bring about an improved condition both in accuracy and in economy.
John Gillie, Butte, Mont.:—I would like to commend the Geological Survey for the great assistance it has rendered the mining engineers in the West. Having been located here for more than 33 years, I have seen the start of it, and have been acquainted with many of the men who actually made the surveys. Also, I found the Director of the Survey has always been anxious to co-operate with the engineers in the localities and accept their suggestions as to the importance of carrying on any particular piece of work first. For that reason we have been enabled in Montana to secure quite a lot of topographic work. The districts like Butte, Helena, Philipsburg, Boulder, and others have been quite important in a mining way at different times, and upon our joint request we have always found the Survey courteous, and they have carried on good work. Mr. Mathewson and Mr. Goodale are familiar with an instance that happened in my own private practice a number of years ago in regard to the lost corners that Mr. Manning spoke of. A homestead locator, about 12 miles east of Butte, in Elk Park district, was desirous of knowing the location of his farm corners—his 160 acres. The public surveys in this locality were quite limited at that time, and it was often necessary, if you did not know the location of the corners, to go a long distance to start from in order to identify the particular section. Upon applying at the office we made an appointment, and I suggested that he look up a corner, the nearest one that he could find, that we could start from. He said: “ I have always known that would be useful, and I have a corner carefully established.” So, in a few days I went out there by appointment, and upon unhitching the team I asked if he could show me the corner he had located, and he said “Yes,” and he took me up to his cabin, and pulled down a 4 by 4 post which he had carefully stowed away in the rafters of his cabin and properly marked. I said, “ I don’t want the actual corner; where was the actual corner?” and he said it was down in the meadow there somewhere; that he did not know exactly where. That is the idea some of them have. A few days ago I was out in the Swan River country, in which the General Land Office is extending its surveys, and on the trail I passed about 10 or 15 pack horses loaded with the present type of corner that the government is now establishing in the heavy timber, where it is likely to be destroyed by fire. These are metal posts with turned ends, and after seeing them planted I am certainly very much in favor of the government carrying on its own survey work.
Mortimer A. Sears, Colo, (communication to the Secretary):—Mr. “Woodruff’s contribution calls to mind the excellent paper on The Application of Descriptive Geometry to Mining Problems, by Joseph W. Roe. The latter paper shows in detail and by specific problems how topographic and structural contour maps may be used in practice.
The writer is in almost constant use of topographic sheets published by the U. S. Geological Survey, and has found them of the greatest value. However, these maps should be used with caution, owing to the fact that formerly they were referred to an astronomic base and not sectionalized. Therefore, because of the notoriously bad condition of the public land surveys, it is difficult to correlate a definite point on the map with a certain definite point on the ground, the tendency being to assume a regularity of network which is far from correct. At the present time the topographic sheets are based upon a survey no less correct, but the government corners are tied in as they are found, so that the maps are much more satisfactory for quick reference. Also, in finding one’s way about in an unfamiliar region it is well to bear in mind that some of the maps were published so long ago that many of the roads shown have now fallen into disuse or have been washed away.
In figuring drainage area the topographic sheets furnish an accurate and speedy basis for computations, especially when a planimeter is used, and Mr. Woodruff has very properly laid considerable stress on this point.
In attempting to determine the line of outcrop where there are few exposures and where one must frequently rely upon the records of deep wells a vast amount of work may be eliminated through a proper use of the maps.
