Utilisation de la photogrammétrie aérienne pour les relevés traditionnels de fluctuations des langues glaciaires

Use of aerial photogrammetry for routine surveys of glacier snout fluctuations

¹ Division Nivologie du Centre Technique du Génie Rural des Eaux et Forêts
² Photogrammètre

Abstract

1 - Historical background

Some measurements of glacier snout fluctuations in France date back to a very long time ago, especially those carried out by the former Forestry and Waters Commission, which kept a close, regular check on the behaviour of some twenty Alpine glaciers over a period of 30 - 50 years, ending in 1965. This involved the following operations : - Measurement of glacier front movement with respect to carefully maintained benchmarks Surveys of cross-sections - Occasional measurements of annual glacier movement rates. A satisfactory overall picture of glacier snout fluctuations was obtained in this way. In addition, the survey methods were coherent and ensured continuity of the observed data, which was a considerable asset. Cooperation between various organisations concerned with glacier problems officially began at a meeting of the "Iceand Snow" group of the French Committee at the International Hydrological Decade on 21st April 1967, when the following precise operational programme was drawn up : 1) Aerical photography of all glacier are as in the French Alps and data-plotting to a scale of 1 :25,000. This work is being undertaken by the French National Geographical Institute, which is producing maps to the above scale. 2) Photography of glacier fronts and snouts and data-plotting to a scale of 1:10,000. This work is of special interest to a number of research and other organisations. Twenty-five glaciers were listed for "monitoring" under this programme. The Ministry of Agriculture finally decided in 1973 to resume the public service role it had held in this field until 1965, having realized the need to keep a close, constant watch on the behaviour of the country's glaciers. The Snow Research Division of CTGREF (Rural Engineering, Forestry and Waters Commission) was made responsible for preparing and implementing the survey programme and keeping records of the observed data.

2 - Programme of operations, and work completed to date

The Snow Research Division has concentrated on the twenty glaciers formerly surveyed by the Forestry and Waters Commission. To restore continuity of observed data - a most important requirement - the Division made it its first job to locate and recondition the pre-1965 benchmarks with the minimum of delay. This work was duly completed. As previously mentioned, the surveys are to be carried out by aerial photogrammetry, which involves the following operations : - Stereo-preparation of areas to be covered - Photography - Compilation of a file for each glacier, listing all control points, their Lambert coordinates and those of the pre-1965 benchmarks. Photography is to be completed in three years, by covering one group of glaciers each year. The following work has been completed to date : 1973 : A survey of Mont Blanc glacier snouts, by conventional methods as time was lacking for preparation of a photogrammetrie survey. 1974 : Stereo-preparation for the group of glaciers in the Savoy Alps. Owing to premature snowfall, however, no photographs could be taken. 1975 : Stero-preparation for the group of glaciers in the Oisans Alps, followed by photography of these and the glaciers in the Savoy Alps. The programme for 1976 provides for stereo-preparation and photography covering the Mont-Blanc glacier group.

3. Technical details

3.1. Stereo-preparation

A number of points to either side of the flight paths are plotted on a Lambert coordinate system. To allow traversing by aerotriangulation at the plotting stage, the flight paths run outside the glacier. The ground points are marked with 1.7 m x 1.7 m black plastic sheets with a 0.6 m x 0.6 m white square in the middle.

3.2. Aerial photography

The photographs, to an average scale of 1:15,000, are taken from an average height of 1,500 m, using a "Wild RC 7" 14 x 14 plate camera with an "Aviogon" wide·angle lens of 100 mm focal length. The camera is installed in a "Pilatus" survey aircraft and operates with 80 % overlap.

3.3. Use of the photographs

for glacier research The photographs can be used for either of the following purposes : 1) To produce plans to scales of 1:5,000 or 1 :2,000, longitudinal and cross-sections. 2) For direct numerical calculation of changes in volume between camera "takes", using a squaring method or calculating the volume from the longitudinal and cross-sections in much the same way as for calculating excavation volumes on building sites. Either method gives vertical and horizontal point positions to within 0.4 m. Programs are available for both methods.