The Montgomery advertiser., 1903-12-27

Boston, Dec. 2s.-;Did you ever see the mysterious letters D. M. permanently inc
 scribed on tablet, post or doorstep some where within the circle of your daily wanderings? Probably not, unless you
 are surveyor, and even. then not unless you had some special reason to look for
 them. although Bench Marks. for which
 the letters are an equivalent, are widely
 scattered all over the United States and
 are of first importance whenever a sur
 veyor wishes to locate the exact altitude of any given spot either above sea level, I or. in many inland eases, abova that of
 the nearest important body of water. A
 Knowledge of these attitudes Is of so little consequence to the average citizen that he may even have bench mark on his own doorstep without knowing it, but the | modest bench mark is an important meIiit
 bre of society whenever the growth of his l city demands a new road. bridge or rail-
 road in it's immediate vicinity, to say nothing of the solution of many other engineering problems. In laying out 6 new road, for example, the engineers | must first know the elevation of the die
 ferent adjoining parts of the county | which 1t Is to traverse and this can oDs viously be done most easily if one can draw on paper a small but accurate pic- | | geography along the upper edge of which
 the proposed road is to facilitate travel. To obtain this perpendieuiaF slice of lo- l cal geography is one of the most integest- | ing problems of the modern surveyor.eas we'll as the very beginning of nearly every I form of modern real estate improvement, and 1t Is also one of the problems that the student of surveying must learn by ! actual practice. Boston, for example, ! ha Mu Afm V.aa fkaskM nF
 nology hardly a stone's throw from the Public Library. has probably been more thoroughly examined by these useful stu- dent suFveys than any other city in the country and whoever obtains permission
 to accompany a party of students in their I field work obtains also Q very clear ir- precision of many of the first steps nec- l essary to the laying out of public works which we all see so constantly In various stages of completion. Making a contour map ,as 1t is eailed-a map that reveals the altitude of every spot included in 1t just as an ordinary map shows Us the distances in straight line from enc place to another-is an especially inter- esting process for it not only necessitates the use Of delicate instruments but re- quires something of the same skill in
 sxetching as is possessed by the landscape artist. s To make one of these maps demands first of all an established starting point In a populous city like Boston these starting points. the bench marks already referred to, are numerous, having been definitely settled by many previous sur veys. sometimes by the city surveyors. sometimes in connection with the making of a road or the development of a system of sewage disposal, and sometimes by the National Government in the series Of oVA dakt hat hox included the entire United States sea- board. The directions by which these marks may be located in the records of the surveyors ofilce are to the uninitiatedS almost as hieroglyphic as those by which the pirate of old romance indicated the burial spot of his hidden treasure. "R. O.C.L.S.S." followed by the number of house in the fashionable district of the city becoTnes translated "Right outer cor ner lower stone step" the corner of the step being chosen 1t is seldom if ever stepped upon and therefore remains per manent unce its exact height above sea level has been accurately determined.
 A contour map. taking hill as an ex- ample. has as base the outline that would be produced If the hill were sur rounded by water at half tide. or to be more technically accurate at mean half tide. for the mean half tide is a fixed
 
 of the extremes of low and high water varies at different seasons and under different conditions. On opposite sides of the Isthmus of Panama, for example. there Is a difference of several feet In the level of high water. but the mean half tides are identical. Now if the base of our hill were surrounded by water at this mean half tide level. our hill would obviously become an island, It's shore pre- senting a distinct outline. If the tide rose ten feet our hill would be smaller is- land with a different outline. every point of which, however, would be exactly ten feet higher than the level at which we started and so ,on until. the advancing flood covering the top of it. it vanished completely and we with 1t.
 The first step in reducing this imaginary condition to the terms of a map is the establishment of the exact altitude of several different points. using as start ing point the single bench mark already established, the surveyor climbing the hill with an instrument that enables him to set up his successive marks by sduinting throui"i 1t at tan pole held upright at the initial bench mark. These points are arbitrarily established and then comes into reauisition a peculiar sort of drawing board with a small telescope and ruler on top. One surveyor stands at this draw ing board which Is thus equivalent. rough ly speaking. to the unruffled surface of the ocean If 1t extended to that part of the earth's surface. and another at some point not more than mile distant hold ing a tall pole evenly divided into alter. nate black and white spaces. As viewed through the little telescope that swings at any necessary angle above the distant drawing board these black and white spaces have a plain mathematical nean- ing and the young man who sauints through the telescope knows the exact