Settember 9, 1909] 



NA TURE 



In order that they may be usefiii in i:ase of war such 

 surveys must have beeit made before war breaks out. The 

 use, however, of topographical maps is not limited to 

 military purposes; on the contrary, they have invariably 

 proved of great value for civil requirements. In one 

 respect they are more useful for civil than for military 

 purposes, as a state of war occurs rarely, and hence 

 while the maps are only occasionally used in connection 

 with war, they are constantly used in connection with 

 ■civil administration and with public and private business 

 ■of all kinds. The topographical maps of the Ordnance 

 Survey, prepared originally solely for military requirements, 

 have proved extremely useful for civil purposes. Directly 

 •or indirectly all the numerous maps prepared by the trade 

 in Great Britain for civil use are based on them. 1 believe 

 .the experience of all other countries is similar to that of 

 the Ordnance Survey. In most countries in which land is 

 .of any value, a cadastral survey for land transfer purposes 

 is needed, as well as a topographical survey. In some 

 cases indeed, the need for a property survey has first made 

 itself felt ; thus in the Transvaal and in the Cape Colony, 

 neither of which yet has a topographical survey, there has 

 ior many years been a Government Survey Departmwt 

 for making property surveys. The question arises whether 

 there should be two separate surveys, one for topographical 

 and one for cadastral maps, or whether there should be 

 •only one survey, the topogiaphical maps being prepared by 

 reducing the cadastral survey. Incidentally the further 

 question arises whether, if two separate surveys are made, 

 they should be under one head. 



In most countries — the Ordnance Survey of the United 

 kingdom being an exception — not only are entirely separate 

 surveys made for these two classes of maps, but these 

 surveys are generally under different departments. In some 

 ■cases the cadastral surveys are isolated farm surveys, 

 :showing little detail except property boundaries. Such 

 surveys would, of course, not answer as a basis for topo- 

 ;graphical maps. In other cases, however, the cadastral 

 surveys show all necessary detail except ground forms, 

 which can be added by a separate survey. The only 

 •cadastral survey, so far as I know, which shows «round 

 forms is the Ordnance Survey, the 6-inch maps of which 

 :are contoured. 



A difficulty in the way of utilising the cadastral survey 

 for the smaller scale maps arises from the fact that a 

 •cadastral survey is, from its large size, much slower than 

 a topographical survey. It is often found advisable to take 

 ■up the survey of the former somewhat irregularly, while 

 it is important for the proper progress of the latter that 

 lit should be taken up regularly and methodically. The 

 Ordnance Survey i-inch map has, since 1824, not had a 

 separate survey of its own, but has been based on the 

 cadastral survey. Ordnance Survey experience has shown 

 that the delays in completing the topographical map, due 

 to this course, have been much greater than one would 

 have e.Kpected, and that there are grave disadvantages in 

 liaving the scale of survey very much larger than that of 

 the finished map. These objections do not apply, or can 

 be overcome, if the cadastral survey of any locality is 

 completed before the topographical map is taken up. This 

 is a condition not likely to be often fulfilled in the case 

 ■of future topographical surveys. I advocate therefore that, 

 'following the general practice, there should be entirely 

 separate topographical and cadastral surveys. I should 

 advocate this even where it is essential to keep the expense 

 :as low as possible. More economy would probably result 

 from the adoption of a fairly small scale for the topo- 

 graphical map, from curtailing the small detail to be shown 

 on it, and from showing on the cadastral maps only such 

 ■detail as is needed for property purposes, than would result 

 from making one survey do for both classes of maps. 



On the other hand I consider that, even when separate 

 surveys are made for the two classes of maps, it is 

 advantageous that both should be made under the same 

 Tiead. The more usual course is, however, to have the 

 two surveys independent, and in some cases local circum- 

 •stances may make the course I advocate inadvisable. 



T riangulation. 

 The first preliminary to any survey should be a triangula- 

 tion. It is the most satisfactory course, and the best 



NO. 2080, VOL. 81I 



economy in the long run, to carry out with the greatest 

 accurac'v possible the primary triangulation on which the 

 survey is to be based. Such a triangulation will remam 

 good for a very long period. For example, the primary 

 triangulation of the Ordnance Survey was commenced in 

 1791 ; while some doubts have been expressed whether it 

 is accurate enough to combine with other more recent work 

 for the purpose of investigating the figure of the earth, 

 no one has questioned that even the earliest part of this 

 triangulation is amply accurate enough for map-makmg 

 purposes. 



On the other hand I do not advocate carrymg out ^ 

 primary triangulation until arrangements have been made 

 for basing a survey on it. In South .\frica an excellent 

 and very accurate primary triangulation has been carried 

 out. This triangulation was undertaken largely no doubt 

 for scientific purposes. While answering its purpose m 

 that respect it has so far had no surveys of any great 

 extent based on it. An accurate triangulation is now a 

 much quicker and less expensive operation than it used to 

 be. The introduction of Invar tapes and wires has largely 

 expedited and simplified the accurate measurement of base 

 lines, while the improvements effected in theodolites enable 

 equal or greater accuracy to be obtained with the com- 

 paratively small and handv instruments now made than 

 could be got formerly with large and cumbrous instru- 

 ments such as the 3b-inch theodolites, with which most 

 of the primary triangulation of Great Britain and Ireland 

 was carried out. Unless observations are rendered difficult 

 by numerous buildings, by trees or by a hazy or smoky 

 atmosphere a good primary triangulation should not now 

 be very expensive. It is usual to base on the primary 

 triangulation a minor triangulation of several orders, the 

 object being to have an accurate framework of trigono- 

 metrical points on which to base the survey. If it is 

 important to keep the expense low, the trigonometrical 

 points may be rather far apart, intermediate points being 

 fixed by plane table ; but it should be remembered that_ it 

 is the truest economy to make the best triangulation which 

 funds admit of. In forests or in wooded and rather flat 

 country, where triangulation would be very expensive, lines 

 of traverse made with every possible accuracy, and start- 

 ing and closing on trigonometrical points, may be used 

 instead of minor triangulation. 



Detail Survey. 



Provided the detail survey is based on triangulation, it 

 may be made by any recognised method. Plane tabling is 

 now almost universally resorted to, and is probably _ as 

 cheap and convenient as any other method. The vertical 

 heights of the trigonometrical points will have been fixed 

 by ''vertical angles with reference to some datum. The 

 heic<ht of intermediate points can be fi.xed by clinometer 

 lines, especially down spurs and valleys, and even by 

 aneroid, and from these heights the contour lines can be 

 sketched in. Altitudes can be more accurately fixed by 

 spirit-levelling, but this is an expensive method not likely 

 to be much used in the case of topographical surveys. _ It 

 is possible that in exceptional cases photographic surveying 

 may be resorted to with advantage, and undoubtedly pnoto- 

 ffraphic methods sometimes enable work to be done which 

 would not otherwise be feasible. The photographic method 

 suggested by Captain F. V. Thompson, R.E., is an advance 

 on previous methods. In Canada, I understand that a 

 good deal of photographic surveying has been done, and 

 presumably the conditions in Canada have been found 

 suitable for this method. It has been little used elsewhere. 



Scale of Map. 



The next point for consideration is the scale on which 

 the map is to be published, and it is an important one. 

 Speaking generally, the cost increases with the scale, and 

 cost is therefore one of the main determining considera- 

 tions. The physical and artificial character of the country, 

 the amount of detail it may be decided to show on the 

 map, the method adopted for representing hills and other 

 detail, and the method of reproduction to be used, all affect 

 the question. 



Clearness and legibility are among the first essentials of 

 a good map, and it is desirable that the scale should be 

 such that all detail it may be decided to show on the map 



