I 



NATURE 



481 



THE ART OF TOPOGRAPHY. 



Reclierches sur Ics Inst}~umcnts, les Methodes et le dessin 

 TopograpJdqties. Par le Colonel A. Laussedat. \o\. 

 I. Pp. xi + 449. (Paris: Gauthier-Villars, 1898.) 



IN his first volume on the art of topography Colonel A. 

 Laussedat gives us an excellent sketch of the history 

 and development of modern survey instruments, and an 

 epitome of the opinions expressed by the best continental 

 authorities on the subject of topography during the present 

 century. The object of his comprehensive work is ap- 

 parently to introduce to the scientific world the latest 

 developments in the application of photography to the 

 purposes of rapid delineation of topographical features, a 

 comparatively new art which demands the attention of 

 scientific surveyors in this country ; but the present 

 volume does not do more than touch this branch of his 

 subject slightly. 



The history of the evolution of the theodolite and other 

 modern instruments is especially interesting, for it shows 

 how very little the principles of construction have altered 

 during the last three centuries. We have made enormous 

 progress in the improvement of old instruments and in 

 the application of old methods to surveying purposes, but 

 the governing principles of triangulation, and of de- 

 lineating country by means of the planchette, or plane- 

 table, are so old as to be beyond even the historical 

 evidence collected in Colonel Laussedat's book. Any- 

 thing approaching to exact information only commences 

 with the Arab geographers of the middle ages. They 

 adopted the ancient instruments of the Alexandrian 

 school, and improved on them. They even went to war 

 with Greece in 829 A.D. because the Greek Emperor de- 

 clined to lend them the services of a savant to extend 

 their knowledge of mathematics. They borrowed their 

 system of enumeration from India, and the magnetic 

 compass from China ; they observed their latitudes with 

 the astrolabe (a very clear description of which instru- 

 ment is given by Colonel Laussedat), and their longitudes 

 by observations of the eclipse of the moon ; and with 

 these acquirements they made themselves masters of the 

 Eastern seas. It is certain that Vaso de Gama made 

 use of Arab pilots when he "discovered" the Cape 

 route to India. Many authors attribute the invention of 

 the plane-table to Pretorius, a Wittenburg professor, in 

 1537 ; but there is evidence of its existence in much 

 earlier days to be found under classical authority. So 

 obvious a method of map making, indeed, could hardly 

 have escaped the Roman engineers, who were quite 

 capable of turning out excellent plans of their cities in 

 days anterior to our era. 



The theodolite, which is only an adaptation of more 

 ancient instruments, was reduced to something like its 

 present form by Digges in 1571, and we find an alidade 

 fitted with a vertical circle in use in 1590 in connection 

 with the plane-table, so that, for three centuries, we have 

 made no radical change in our principles of geographical 

 surveying ; although the introduction of the system of 

 measurements by traverse and offset which are first 

 recorded in the field book and then plotted in the office, 

 NO. 1534, VOL. 59] 



has for many years superseded the more ancient use of 

 the plane-table by the Ordnance .Surveyors of England, 

 for cadastral purposes. The history of the discussion on 

 the relative value of these two methods of field work — 

 " I'antique lutte entre la methode du trace et du rapport 

 immediat des mesures sur le terrain, et cette des mesures 

 inscrites sur un carnet et rapportees dans le cabinet" — 

 is excellently well summed up by Colonel Laussedat. He 

 is, however, mistaken m supposing that the discussion 

 commenced in England with the lecture given by the 

 American surveyor, Mr. Pierce, at the Civil Engineers 

 Institute in 1888. Nearly ten years before that date, the 

 advantages of the planchette, or plane-table, system of 

 topography had been advocated at the R. U.S. Institute 

 by Colonel Holdich, and instruction in this system had 

 already been introduced into our military schools. Eng- 

 land, in fact, has adopted this system for all countries 

 and colonies with which she has surveying relations, out- 

 side England, but within her own borders the Ordnance 

 Survey still maintains its position against all other 

 civilised communities. But the use that is made of the 

 plane-table (universal as it has once more become) is not 

 quite the same all over the world ; and in this branch of 

 his researches Colonel Laussedat might, perhaps, have 

 extended his review with advantage. The difference in 

 its application to the various fields of continental or 

 colonial survey lies chiefly in the amount of independence 

 which is admitted of other and more exact methods of 

 triangulation. Russians and Americans, for instance, 

 by making use of a complicated plane-table, claim for 

 it nearly all the potentialities of a small theodolite. 

 English surveyors use it absolutely for topographical 

 delineation, but subordinate it to primary triangulation 

 with the theodolite. Its independence is not carried 

 further than the limits imposed by triangulated and 

 computed points or positions. What Colonel Laussedat 

 fails to appreciate, is the advantage of mathematical 

 proofs of the correctness of all that preliminary frame- 

 work of fixed positions which is attained by the use of 

 the theodolite. The correctness of such fixed positions is 

 capable of mathematical demonstration quite apart from 

 any survey process in the field. Within these limits no 

 accumulation of error can occur, and it may be said that 

 the demonstrable accuracy of the triangulated points is 

 the gauge of the general accuracy of all details in the 

 map. No position should be more inaccurate than the 

 points from which it is interpolated. Where this proof 

 supplied by computation is missing, there is no ready 

 test of accuracy available. The surveyor who uses his 

 plane-table only for graphic triangulation may accumulate 

 error indefinitely, and no absolute check is applicable. 



The net result of Colonel Laussedat's examination into 

 topographical methods is that he gives his approval to 

 the use of the plane-table in close alliance with the 

 theodolite ; and in this opinion he has the support of 

 nearly all continental and most English surveyors. 



When he comes to the consideration of the best 

 method of representing the inequalities of ground, i.e. 

 of giving proper value to the scales of shade indicating 

 greater or less slope to be employed in cartography, 

 there is very much in the book that is worth attentive 

 study. Doubtless the system of continuous contours at 

 equidistant vertical intervals is the most scientific and 



Y 



