448 TRANSACTIONS OF SECTION K. 



civilised world. Each sheet of the air-map will cover an area of 1° in longitude 

 by 1° in latitude. Twenty-four sheets of the same map will cover the same 

 area as one of the sheets of the international map on the scale of 1 to 1,000,000, 

 since each of the latter covers 6° in longitude by 4° in latitude. In order to 

 avoid the troublesome distinction between eastern and western longitudes, 

 northern and southern latitudes, with the inevitable errors caused by the change 

 of sign, the longitudes are to be reckoned from 0° to 360°, in an easterly direc- 

 tion, extending from longitude 180° E. or W. of Greenwich. In addition, there 

 will be given, instead of latitudes, polar distances, reckoned from 0° to 180°, 

 extending from the South Pole, in order that, in the northern hemisphere, where 

 lie most of the inhabited lands, the numbers may increase in the customary way 

 with increasing distance from the equator. Each sheet will be numbered by 

 the co-ordinates of its South-Western corner. In addition, marks, each of 

 which is the distinguishing figure of half a rectangle, with the small sides duly 

 set towards North, should be drawn on the roofs of convenient buildings or on 

 the ground itself. Each of these marks indicates the northern or southern half 

 respectively of the corresponding sheet of the aeronautical map. On each mark 

 a large dot will indicate the proper position occupied on the sheet itself by the 

 mark on the ground. Moreover, two large figures will be marked on each side of 

 the rectangle, set towards North ; the left one of which showing the number of 

 the units of degrees of the latitude of the mark, and the right one the number 

 of the units of degrees of the longitude. A mistake of 10°, say 400 miles in the 

 direction of the parallels or 700 miles in that of the meridians, being scarcely 

 probable, these two figures will suffice to define the number of the correspond- 

 ing sheet of the map and the rough co-ordinates of the mark. 



If the example thus given by France should be followed by other countries, 

 an international agreement would be necessary to fix positively the conventional 

 signs of the aeronautical map and other particulars. 



2. Aeronautical Maps. By Captain H. G. Lyons, F.R.S. 



Cartographers must now treat the material at their disposal so as to meet 

 the requirements of aerial navigation, as well as those of travellers by land and 

 by sea, and for this purpose they need the assistance of those who possess prac- 

 tical experience of this new means of transport. Considerations of economy 

 urge that existing scales should be utilised if practicable with such modifications 

 in the information included in the maps as may be desirable. National and 

 international committees are approaching a common basis of agreement on such 

 points as projection to be employed, scale to be adopted, and the identification 

 of localities. There remain these important groups of data which offer many 

 points for discussion by the cartographer and the aviator. 



(i) The relief of the land surface must be adequately represented, and show 

 both altitude and slope, so that they may be readily recognised. 



(ii) The detail forming the body of the map needs careful compilation, so 

 as to show all that is essential, while eliminating all that is of lesser importance 

 sufficiently to provide a map clearly and boldly drawn. Natural features, com- 

 munications, settlements, and prominent objects may need a treatment some- 

 what different from that in topographical maps to obtain the best results. 



(iii) Special information must be added which is important to aviators to 

 indicate localities where assistance may be obtained, or where especial dangers 

 or facilities are to be met with. The efficient assemblage of all such informa- 

 tion to the best advantage is not the work of a draughtsman, but calls for the 

 skilled co-operation of the technical cartographer and the experienced aviator. 



3. A Class of Map-projections — retro-azimuthal. 

 By J. I. Craig, M.A., F.R.S.E. 



Two conditions are necessary to determine a map-projection. In a wide and 

 useful class, one condition is that every point shall be in its true bearing from 

 a central point. The class of projection now proposed is such that a central 



