AERIAL SURVEYS

Ashburton Guardian, Volume 58, Issue 220, 29 June 1938, Page 3

AERIAL SURVEYS

RANGITATA FLOOD CONTROL.

INVALUABLE DATA AVAILABLE.

(Special to the “ Guardian.”)

TIMA R l

June 28

To supplement the ground surveys made for Hood remedial measures along the Rangitata River east of the Arundel traffic bridge to the sea coast, scores of remarkable aerial survey photographs of the •>() square miles involved are now in the hands of the resident engineer of the Public W ol 'k s Department at Temuka (Mr T. G. Beck). These form an important part in the solution of the problems piesented by the menace of Hoods at and about Rangitata Island, and will be used by the engineers in framing plans for effective protective works and for flood control. Made by the New Zealand Aerial Mapping Company, Hastings, the prints which were shown to a representative yesterday are tlie first ol tlie kind to he recorded in this part of the South Island, and are the ioreiunneis of others for the Downlands water supply scheme (350 square miles) and for the Mayfield-Hind s irrigation scheme (250 square miles). Actually the prints of these latter surveys are already being prepared at Hastings and should come to hand in the nea-i future. Only one hour was necessary for the photography at the Rangitata Island, hut naturally considerably more time was required for the development of the negatives and printing, as well as the preparation of a large map. of the area photographed, which is scaled at 14 chains to the inch. Aerial surveys have been carried out in South Canterbury before, but the photographs turned in from these have been of the pattern known as the mosaic type. The prints of the Rangitata River district are actually pairs of photopraplis of what is called the stereoscopic type and are a tremendous improvement on the old system. Each photograph measures 12 inches by 10 inches. At first glance the pictures look somewhat like an ordinary fiat map. but when viewed through the lens they present most extraordinary scenes wherein the impressive feature is that the third dimension is defined with almost dramatic clarity. Objects Clearly Defined. Each photograph is needle-sharp in focus and it is possible to distinguish even small animals on the ground m a picture taken from a height of 9200 feet. Objects such as houses, trees, and so forth seem to rise and stand straight above the ground, while a four foot gorse hedge shows clearly from one end of a paddock to the other. On each picture is automatically recorded the serial number of the print—this subsequently enables the surveyors to identify it's location on a largo scale map —the altitude from which the picture was taken, the time, date, and the name oi the area surveyed. From the photographs invaluable information and data is made available to the engineers and surveyors in the compilation of reports and maps. Ground surveys will always be necessary, and although these are in themselves often quite comprehensive, they, are made more complete when supplemented by the information gained from aerial photographs, as those provide great aids in deterininiing variations of land levels. Such photographs also enable the surveyors to record changes made by rivers in flood time, and there is the additional advantage that these records can be made even when floods are at their greatest heights provided that the weather is suitable. A further virtue of the aerial survey is the saving in both time and cost.