Topographic maps and sketch mapping

Front matter

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Wmm Im

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Preface

The demand for instruction in map reading and sketch mapping, brought about by the Great War, has led to the development of a course on this subject in many of our colleges and technical schools which have assisted in preparing men for the various officers ' training camps. Outside of the purely military field our universities have in the past found it necessary to develop a course of this character for students in Geology and Physiography who are required to make extensive use of topographic maps. In recent years the development of the automobile and the camping habit has led to a more extensive use of maps by the general public, and a large number of people have " disco vered" our own United States Geological Survey Maps, but are still unable to understand them completely and to make the fullest possible use of them.

It will doubtless be necessary in the future for many educational institutions to continue instruction in this subject for students taking reserve officers' and other military training courses in connection with their regular college work. Furthermore it seems probable that in many of our institutions the demand and necessity for such a course will lead to its inclusion as a regular academic study, given perhaps as a part of the instruction in Geology or Geography, or as a separate course under Civil Engineering. It is with this idea in view that this volume has been prepared.

A large number of educated people have never learned to use topographic maps because it has been the general opinion that considerable mathematical knowledge was necessary. This is far from true. During the past few years it has been necessary to give instruction in this subject to a great number iv PREFACE

of men who never went beyond "plain" geometry, who passed this point many years ago and claimed to have forgotten all they ever knew of mathematics. They have picked up the subject quickly and are generally the most interested students and do the best work. This has been particularly true of work in sketch mapping and in several cases men have stated that they intended securing the simple Army sketching outfit and using it on summer outing and camping trips. Indeed there is no reason why work of this kind could not be made part of the instruction in the summer camps for boys. Interesting applications could easily be worked out and it would doubtless stimulate interest in the practical mathematical problems.

The author had the pleasure of cooperating with Dr. Charles P. Berkey, of the Department of Geology of Columbia University in giving the course on "War Topography," originated by Dr. Berkey for the Students' Army Training Corps. The notes prepared for this course have formed the basis for the present work. The author takes pleasure in acknowledging his indebtedness to Mr. F. K. Morris, of the same department, with whom he cooperated in giving a similar course, for many ideas of value. Mr. Morris has also contributed a Descriptive List of the Principal Topographic Maps of the World, which will be found in the appendix. This is believed to be the most complete list of this kind which has ever been published. A number of the questions following the first sections are based on the mimeographed notes prepared for infantry and field artillery candidates at Fort Sheridan, Major Cromwell Stacey, Senior Instructor.

J. K. F.

New York City, August, 1919.

Contents

PAGE

Introduction ix

Kinds of Maps. — Government Maps. — Map Reading. — Sketch Mapping. — Landscape Sketching.

CHAPTER I WHAT A TOPOGRAPHIC MAP SHOWS ItT

1. Topographic Maps. 1

Use. — Military Maps. — Essential Features.

2. Title and Marginal Information 3

Map Sheets. — Name. — Index Maps. — History.

Clearness and Detail. — Characteristic Sheets — Conventional Signs of U. S. G. S. — For Large Scale Maps.

4. Conventional Signs. — Continued 11

Ordnance Survey of British Isles. — Car.e de France.

Methods. — Hachures. — Spot Heights. — Contours. — Principles of Contours.

6. Relief. — Continued . 25

Contours on Maps. — Drainage and Ridge Lines. — Contours and Drainage.

7. Relief. — Continued 28

Estimating Heights from Contours. — Elevation of Points between Contours. — Underf eatures .

8. Relief. — Continued 31

Relief Models. — The Layer System. — Oroscopic Maps. — Shading.

9. Direction 36

Arrows or Pointers. — The Compass and Declination. — Meridian from the Sun. — From the North Star. — Compass Bearings. — Surveyors' Method. — Azimuth.

Representative Fraction. — Scale Equivalents. — Conversion of Scales. — Scales Used for Maps.

11. Scale. — Continued 46

Graphical or Reading Scales. — Scaling Distances. — Construction of Graphical Scale. — Accuracy of Scaled Distances.

v vi CONTENTS

Chapter II How To Get Certain Information From A Map

ART. PAGE

12. Sections and Profiles 51

Sections — Construction of Profiles. — Scales. — Profile Paper.

13. Slopes 55

Slope Angles. — Slope Fraction or Ratio. — Grade Per Cent. — Conversion. — Practicable Slopes for Various Operations.

14. Slopes. — Continued 59

The Slope Scale. — Construction of Slope Scale. — Use.

15. Intervisibility 62

Topographic and Military Crests. — Dead Ground. — Intervisibility of Points. — Curvature of Earth.

16. Visibility of Areas 66

Use.- — Method of Plotting Dead Ground. — Landscape Sketch from Map.

Chapter III Use Of Topographic Maps In The Field

17. Coordinate and Grid Systems 72

Descriptions. — The English Grid System. — The French Coordinate Method.

18. Use of Maps in the Field % 76

Orienting the Map. — Location of Observer's Position. — Resection,

PART II. SKETCH MAPPING Introduction 81

Use of Sketch Mapping. — Instruments. — Practice.

Chapter I Topographic Drafting

19. Freehand Lettering 83

Form of Letters. — Guide Lines. — Hints. — Spacing.

Conventional Signs. — Signs for Sketch Mapping. — Relation of Size and Scale. .

21. Enlargement and Reduction 87

Use. — Methods. — The Method of Squares.

Chapter II

22. Surveying and Mapping 90

Mapping Methods. — Mapping by Distance. — Traversing. — Triangulation.

23. Pacing and the Scale of Paces 93

Methods of Pacing. — Pacing on Slopes. — The Scale of Paces.

CONTENTS^ vii

ART. PAGE

24. The .Sketch Case and Traversing 96

The Sketch Board. — Use of Alidade. — Methods. — Traversing with Sketch Board. — Error of Closure.

25. Location of Details 103

Radiation Method. — Intersections. — Offsets. — Field Procedure.

26. Position Sketching 106

Triangulation. — Base Line. — Development of Triangulation Scheme. — The Three Point Problem.

Chapter III

27. Contour Interpolation Ill

Drainage Lines. — Controlling Points. — Interpolation of Contours.

28. Determination of Elevation 115

Difference in Elevation. — Leveling. — The Slope Board. — Use. — Reduction Diagram.

General Plan. — Hints. — Field Procedure.

PART III. LANDSCAPE SKETCHING Introduction , 125

Use. — A Map in a Vertical Plane. — Steps Necessary in Making Sketches.

30. Delineations and Perspective 127

Delineations and Outlines. — Effect of Distance. — Perspective. — Vanishing Point.

31. The Sketching Screen 131

Construction. — Use. — Guide Points. — Steps in Working Up a Sketch.

32. The Sketch Pad 133

Description. — Method of Using.

83. Descriptions and the Mil Scale 136

Direction. — Target, Range and Deflection. — The Mil Scald

Appendix

1. A Descriptive List of the Principal Topographic Maps of the World.

2. Suggestions for a Course in Map Reading and Sketch Mapping 166

Introduction

A map is a conventional picture of a portion of the earth's surface as seen from directly above, showing more or less completely the various features of the country represented. Thus a land map may show only the boundaries of a certain piece of property and would consist simply of a series of lines forming a closed figure with the lengths and directions determined by the surveyor — in short a conventional outline of the property in question. On the other hand, a complete "topographic' ' map, such as would be used in planning a landscape design for a park or estate, for example, would show every detail of the property — houses, roads, streams, and even, in some cases, individual trees as well as the relief, or ups and downs of the land surface which form its hills and valleys. Between these two extremes are all sorts and kinds of maps used for various purposes.

Maps may be divided into two main classes — flat maps and maps showing relief. The former class embraces most of the maps with which we are familiar. They may show the principal features such as cities, towns, railroads, roads, rivers, streams, etc., but make no attempt to show the mountains, hills and valleys. In some maps the main mountain ranges are shown by a form of shading and, as the scale or size of the map in relation to the ground represented becomes larger, it is possible to give with increasing accuracy the relief, and to show more and more of the details of the country. Thus it would not be possible to show on a map of the United States, say twelve inches long, much more than the boundaries of the States, the main rivers and large mountain ranges, and to indicate by small circles their principal cities. If this same size, twelve inches, is used to represent a few acres of country it is practicable to show nearly all the details mentioned for the landscape map above with the relief so clearly shown that small x INTRODUCTION

inequalities in the land surface of a few feet in height would be accurately indicated.

Practically all the nations of the world have either made or are now making topographic maps of their land possessions. These maps are generally published in sections and show a portion of the land surface drawn so that one inch on the map represents about a mile of land. In Europe the making of these maps has been largely inspired by military considerations. In the United States the main object has been to furnish maps suitable for the economic development of the country, the study of its geological structure and resources and the planning of engineering projects. For this reason our maps, published by the U. S. Geological Survey, are not good maps for military purposes, but they do show very clearly the relief which is of great importance in engineering work. Such European maps as the French and German show every detail of military importance, but the relief has been shown largely by line shading which is easy to understand but not very definite or accurate. Greater care in showing relief was thought to be unnecessary and these maps were well suited to the tactics of open warfare which prevailed up to 1914. Modern warfare, however, tends to settle down to intensive action along almost fixed lines with a great deal of engineering construction in the form of trenches, mines, etc., and any advance must be preceded not only by the accumulation of vast quantities of stores and construction equipment behind the lines, but also by the complete planning of the engineering features of the construction to be done as the troops advance. For this work neither the size nor method of showing relief in vogue in Europe before the war was suited, so we have seen, since the war began, the issue of new, larger and more detailed maps of much of France in which the accurate American contour method of showing relief is used.

The ability to understand a topographic map and make use of the data shown in answering various questions in regard to the country represented is required of every officer and even the non-commissioned officers in the United States Army. The study of maps with this end in view is generally termed "Map Reading" and its importance m warfare will

INTRODUCTION xi

T)e readily appreciated when we think that every tactical movement, varying in size from the movement of an entire army down to a trench raid, is not only planned on a map but orders are issued with reference to a certain map or maps. Map reading may be divided into two parts. A. A study of the method of representing features on a map and B. How to make use of the map to obtain certain information.

Map making is primarily the field of the engineer, and in modern warfare they are usually prepared by a special engineering force trained and equipped for this purpose and assisted by aeroplane observers. Accurate up-to-date maps were found to be so important on the western front that specialists were developed in all the branches of the work from the purely engineering features of surveying to the interpretation of aerial photographs and the printing of the final map. New maps of active sections of the front were printed right behind the lines with field equipment and issued almost daily. In many cases large size maps were not available and officers had to make their own sketch maps to serve until better maps could be made and issued. This was particularly true in the recent activities on the Mexican border and in the campaigns in the Far East where, good maps had never been made. It has thus come about that officers in our army must not only be able to use maps, but to make them, and for this purpose the Army Sketch Case has been developed. A similar simple outfit is used by many geologists for field work in new country which has never been fully mapped. Such work, while not highly accurate, furnishes valuable information for various purposes, and is covered by the subject of Sketch Mapping given in Part II. It is part of the required training for all infantry officers. Artillery officers have to go much deeper into the subject and, while sketch mapping will furnish an excellent start, it will be found advisable to follow it up with a thorough course in surveying and the use of the surveyor's instruments.

The subject of Landscape or Panoramic Sketching discussed in Part III is primarily of military importance. Simple sketching of this kind, however, is a very interesting pastime and will be found of value in illustrating geological and other reports and as an aid in visualizing topographic maps-

TOPOGRAPHIC MAPS AND SKETCH MAPPING

Chapter I

Art. 1. Topographic Maps

As has already been pointed out a topographic map is a conventionalized picture of a section of the earth's surface as seen from directly above. In fact a good topographic map shows the surface features of the country represented so perfectly that it is possible to make an exact model of the country, called a relief map, from it. Topographic maps have important uses both for civil and military purposes. Every road, railroad, canal, dam or bridge is first planned and laid out on a topographic map. On these maps the engineer studies out his problems in construction, determines the best location for his railroad, the best location and spans of his bridges, even the location of his construction plant and camp. The geologist also uses topographic maps in studying physiography and working out the relations between surface and underground structure. Indeed it is easily possible to form a very clear idea of the age of the land forms and the character of the soil, behavior of streams and sub-surface waters and the underground structure of any section of country from a topographic map. This subject, which is properly a part of the study of Geology and Physiography, is known as Map Interpretation.*

For those who enjoy the open and wish to study and know the country traversed in auto trips or on camping excursions a topographic map is the best possible guide.

In military work the necessity of complete, up-to-date military maps has been noted. These military maps are simply topographic maps to which additional information of a military nature has been added. They guide the operation of parties in the field, give positive information about the country in advance, and serve not only as a basis for planning all military movements, but also for directing artillery fire. In connection with the latter it should be noted that in modern warfare the target is seldom visible to the gunner and the direction and range which are required in aiming the gun are very often taken directly from a topographic map.

There are five essential features or divisions of a topographic map.

(a) The Title, showing the location of the country

represented — when the map was^ made — who made it, etc.

(b) The Conventional Signs, by means of which the

various features, such as streams, towns, roads, etc., are represented.

(c) The Pointer or Arrow, by means of which the map

may be properly oriented and directions determined.

(d) The Scale, which shows the relation between lengths,

or distances, as they appear on the map and the actual distances on the ground.

(e) The Relief, or the method of showing the shape,

slope, and height of the hills and valleys.

* See "The Interpretation of Topographic Maps" by Salisbury and Atwood. Prof. Paper No. 60 U. S. G. S. also "Military Geology and Topography," Edited by Prof. Gregory. Yale Press.

TITLE AND MARGINAL INFORMATION 3

Art. 2. Title and Marginal Information

In former times it was the custom to make the title of the map a very elaborate affair which occupied quite a portion of the sheet and was highly decorated. With the adoption of rectangular sections for maps this practice could no longer be followed and the name, characteristics and history of the map, which make up the title, was, of necessity, placed around the margin of the map.

Extensive topographic maps are made, as has already been noted, in sheets varying usually from 15 to 20 inches in height to 20 to 30 inches in width. When these sheets are bounded by meridians and parallels of latitude they are only approximately rectangular, due to the fact that they represent a portion of the curved surface of the earth which is shown on a plane. Most systems of projection,* or methods adopted to show on a plane surface the meridians or parallels of the globe, require that the meridians at the top of the map be closer together than at the bottom for maps of the northern hemisphere and the reverse for the southern. Such maps cannot, therefore, be fitted exactly edge to edge so as to lie perfectly flat, but may be made to do so by leaving a small space between the vertical edges. In other cases the edges of the maps are not based on latitude and longitude lines but are perfectly rectangular. The meridians and parallels of latitude when shown will, therefore, not be parallel to the edges of such maps. The International Mapsf and our own U. S. G. S. maps are examples of the former system and the English, French and German of the latter. In connection with longitude it should be noted that the origin is v seldom stated. The meridians on English and American maps are based on Greenwich as the origin while the French use Paris, Italian Rome, arid German Ferro.

Each of the sheets of a given country is given a name or number, or both, which is printed on the margin of the sheet. The name given is either that of the principal town or topo-

* See "A Little Book on Map Projection" by Mary Adams. London 1914. George Philip & Son.

f See appendix for description of principal government topographic maps.