312 



Canadian Forestry Journal, July, 1919 



CAN TIMBER LIMITS BE MAPPED BY AIRPLANE? 



By Rolph Thelen. 



The development of aerial photography as a 

 highly important and indispensable phase of 

 modern warfare has been one of the many won- 

 ders of the great war. Photographic reconnais- 

 sance was practically unthought of durmg the 

 early stages of the conflict, and may be said to 

 have been an outcome of trench warfare. At 

 the time the United States entered the war this 

 art had become of tremendous importance, and 

 in the final stages a complete detailed photo- 

 graphic map of each sector had to be made 

 daily. General Squier states that the British 

 army made 17,000 photographs before the 

 operations at St. Quentin in order that a relief 

 map of the whole sector might be prepared 

 before undertaking the drive. 



Military maps of this character are commonly 

 called mosaics, and are made as follows: An 

 aii'plane (other forms of aircraft could be used 

 under certain conditions) equipped with a 

 magazine camera flies over the area to be 

 mapped, maintaining as uniform an altitude as 

 possible, and exposures are taken at the proper 

 intervals to insure a sufficient over-lapping of 

 the resulting negatives. If the area is too wide 

 to be mapped in one flight ,a number of parallel 

 flights must be made, and the negatives of each 

 succeeding flight must overlap those of the pre- 

 vious flight. After the negatives have been de- 

 veloped, prints are all made to the same scale; 

 this is done by making them in an enlarging 

 camera instead of by contact. Distortion, 

 caused by obliquity of the plate at the instant of 

 exposure, can also be corrected in the enlarging 

 f '.mera if proper base points are available. The 

 cameras are usually rigidly attached to the 

 planes, and since it is impossible to fly con- 

 tinuously on an absolutely even keel, a certain 

 amount of distortion is bound to occur. After 

 the i^'rints are made, they are matched up, trim- 

 med and assembled into the finished mosaic. It 

 is obvious that in the case of flat terrain it is 

 possible by this means to product an accurate 

 scale map. However, in the case of mountain- 

 ous country, this is not possible, since the scale 

 will vary unevenly throughout the negative with 

 variations in elevation. Thus a peak will be 

 abnormally large in scale compared with a 

 valley appearing in the same photograph, since 

 it will be nearer the camera in elevation at the 

 instant of exposure. The summit will be large 

 scale, the valley small scale, and the slopes on 



various intervening scales. In spite of this un- 

 desirable feature this type of map answers mil- 

 itary requirements admirably, especially when 

 accurate scale maps of the region are available, 

 as was the case in France, and the main need 

 for the photographic maps is to show the activ- 

 ities of the enemy. Points on the photographs 

 can be tied in with the corresponding points on 

 the scale maps, and the desired amount of de- 

 tail filled in with almost any degree of accuracy. 

 The speed with which aerial mosaics can be 

 made is remarkable. The Division of Military 

 Aeronautics recently made a mosaic of the City 

 of Washington and surrounding country in a 

 total flying time of only 2'/4 hours. The area 

 covered was 27 square miles. 



The possibility of producing accurate topo- 

 graphic maps from photographs has been ap- 

 preciated for many years, and the camera has 

 been used for topographic surveys to a limited 

 extent in India, France, and Italy, and almost 

 exclusively in the Dominion of Canada. Cameras 

 used for this purpose are known as photo- 

 theodolites, and are equipped with suitable 

 cross-hairs leveling devices, horizontal scale, 

 and magnetic compass, as well as with a small 

 telescope for the reading of vertical angles. 



ARSENIC TO KILL USELESS TREES. 



"Of late years, the action of arsenic has 

 been introduced with marked success in hasten- 

 ing the killing by the ring-barking process, and 

 trees that ordinarily would take months to kill 

 by the old method, are now killed in a few 

 weeks, and frequently in a few days, byt the 

 application of arsenic." — Australian Forestry 

 Journal. 



St. Johns, Nfld., July 4. — Reports say that a 

 big forest fire is raging on the Exploits River, 

 and that dwellings near Exploits were in danger. 

 Minister of Mines and Agriculture Walsh re- 

 ceived the following message from Badger last 

 evening: "Fire has encircled Badger, but so 

 far only one house has been destroyed. An- 

 other has been badly damaged. The fire has 

 crossed to the south side of the Exploits River, 

 and the place is still in danger. Forest fires 

 are now raging within a radius of ten to twelve 

 miles north-east and spreading to south side of 

 river." 



