Welsh Journals

A VEGETATION MAP OF SKOMER ISLAND FROM COLOUR AERIAL PHOTOGRAPHS A. D. JONES INTRODUCTION Skomer Island lies a little more than a kilometre off the southern ex- tremity of St. Bride's Bay in Pembrokeshire. Between the mainland and Skomer lies Midland Isle, separated from Skomer by Little Sound, ap- proximately 100 m wide at its narrowest point. Skomer itself is a little over 3.1 km long (in an east-west direction) and 2.1 km wide with a total area of 292.29 ha. Midland Isle in contrast is approximately 400 m across (in area 8.70 ha.) rising to an elevation of over 50m. The general elevation of the plateau surface of Skomer is 70m with a maximum elevation of just over 75m in the north of the island. AERIAL PHOTOGRAPHY Colour aerial photography of Skomer was taken by Fairey Surveys Ltd., on 6 March 1969 at two scales, 1:10,000 and 1:2,000. The photography was commissioned by the Geography Department at Aberystwyth and the Nature Conservancy, and West Wales Naturalists' Trust assisted in various parts of the survey. The purpose of the survey was to produce a detailed topographic and vegetation map of the island and these requirements governed the type of photography. Earlier work carried out, by the Geography Department, in the Dyfi Estuary (Jones 1969) had already shown the advantages to be gained by using colour photography for vegetation mapping and subsequent work (Jones 1971) has not changed that opinion. The increased costs of colour photography are difficult to generalise and the author's experience in- dicates increases of from 15 to 80 per cent over comparable panchromatic films. In general the increase will be smaller for more limited areas since the increased costs of colour materials and processing form only a part of the total costs of aerial photography. For instance, flying to and from an area is a fixed charge regardless of the size of area to be flown and will constitute a major part of the costs for small er areas. The photography was flown at two heights, 1500 and 3000 m. With a Wild RC 8 camera (principal distance 152 mm) this gave scales of 1:10,000 and 1:20,000. The larger scale was used for the topographic mapping at a scale of 1:5,000 and for the detailed vegetation mapping. The smaller scale was chosen so that the whole island could be covered by a single print which could be enlarged four times to give a scale of 1:5,000 for use with the topographic map. This enlargement, however, although not a map is very valuable for field studies as it shows far more detail than the conventional line map. This procedure proved to be more economic than producing a mosaic from the larger scale photographs. With equipment currently available* the 1:20,000 scale photography could also have been used to produce the 1:5,000 scale map, thus dispensing with the 1: 10,000 scale photographs. The detail of the vegetation map would not have been greatly affected. At the time the Department of Geography had a Kern PG 2 with a maximum enlargement of 2.5. Since then this has been replaced by the Kern PG 2L.