Mapping Little Ice Age glacier maximum in Jotunheimen, Norway, using aerial photography and LANDSAT imagery

State-of-the-art
Investigations on the glacier maximum extent during Little Ice Age (LIA) in Norway have, until recently, mainly been carried out as locally focussed studies on selected glaciers. These investigations include the application of lichenometry for moraine dating, and the mapping of selected glaciers using stereo photogrammetry. Previous studies in other regions have shown the potential of the use of LANDSAT imagery as efficient tool for mapping the maximum LIA-extent of glaciers on a regional scale.

Aim
The aim of this study is to reconstruct the glacier area during LIA-maximum on a regional scale using LANDSAT imagery and aerial photography. Additionally, the applicability of the LANDSAT sensors in different climatic settings within Norway is investigated. The data will be used to analyze the area change since LIA-maximum until present, and to detect spatial differentiation in glacier behavior.

Methodology
To generate LIA-glacier outlines, different data sources have been used: aerial photos in scale 1:40 000 taken in 1966, 1976 and 1981, a satellite image (Landsat 5 TM, 09/08/2003), and existing geomorphological-geochronological maps.
In the aerial photos, moraine ridges have been used to determine the maximum ex-tent of the glaciers during the LIA. This attempt has been successful due to fairly well detectable moraines on most glacier forelands in Jotunheimen, and due to the almost complete absence of pre-LIA moraines. In the LANDSAT image, a bands 5–4–3 composite has been applied to identify the area without vegetation cover around the recent glaciers and the bare rock on the glacier foreland, and this was digitized manually. The visibility of the former LIA-glacier extent area was adequate.
To evaluate the results obtained, a sensitivity analysis was made for a selection of glaciers. The derived three outlines for each glacier (from aerial photo – satellite image – geomorphological map) were compared in order to estimate the error between the sources. The mean standard deviation was 6.7% and, therefore, the resulting LIA-glacier outlines were judged to be sufficiently accurate.

Results
During the LIA-maximum (between AD 1750 and AD 1790), there have been 377 glaciers in Jotunheimen with a total area of 399 km². Due to variations in hydrological basins and identification codes of the glaciers at different times, only 95 glaciers have been chosen for this study. Since LIA-maximum this glacier area in Jotunheimen declined from 241 km² to 174 km² in the 1980s and to 163 km² in 2003. That means an areal reduction of 27% from 1750 to the 1980s and of 32% from 1750 to 2003. The comparison of the glacier area between West-, Central-, and East-Jotunheimen, shows a similar distribution for all three different dates (1750 -– 1980s -– 2003).

Conclusion and potential application
The application of LANDSAT imagery for mapping LIA-outlines in Jotunheimen was successful on a regional scale and showed a better accuracy than mapping with aerial photography. Fortunately, there are no debris-covered glaciers in this area that could have caused difficulties in detecting the recent glacier outlines. Regarding single glaciers, however, only 69% of the LANDSAT-LIA-glaciers lie within the 95%-confidence interval. Hence, an effort in increasing the precision of mapping has to be made.
By further improving of this mapping, the generation of LIA-glacier outlines using aerial photography and LANDSAT imagery could be extended to other glaciated areas in Norway.