Glacier Area Mapping in Norway using Optical Satellite Images

Solveig H. Winsvold1 and Liss M. Andreassen2 Norwegian Water Resources and Energy Directorate (NVE), Oslo, Norway. 1sohw@nve.no 2lma@nve.no

Introduction Glaciers are important climate change indicators. Optical satellites have been used to map glacier extent in many regions in the world. During the last decades glaciers in mainland Norway have retreated. A new updated survey of the Norwegian ice masses was required to get an overview of the present state of the glaciers and the changes of them. Within the CryoClim project NVE has mapped the glaciers in Norway for two periods, 1988-1997 and 1999-2006, by using satellite imagery from the complete time series of Landsat TM 4, 5 and 7 ETM+. Here we show examples of glacier area change assessments based on Landsat, and we compare the specifications of Landsat and Sentinel-2. Landsat TM/ETM+ imagery used for mapping A major challenge for glacier mapping in Norway was to find suitable Landsat scenes, as few appropriate scenes are available due to seasonal snow and especially cloud cover. After careful inspection 12 Landsat scenes for the period 1999-2006 were selected (Figure 1a). In addition, 9 earlier Landsat satellite scenes were selected for the 1988-1997 period. The images for the earlier period do not fully cover the glacier area in Norway as images with satisfying conditions were not available (Figure 1b). Example of assessing glacier area change The semi-automatic band-ratio method was used for the glacier mapping. A thresholded multi-spectral band ratio uses Landsat bands located in the RED and short wave infrared (SWIR) portion of the electromagnetic spectrum, and an additional BLUE band (TM 1) was used to improve glacier mapping in shadow (Figure 2). Glacier changes were derived by comparing Landsat images from different years, as an example we show changes of Nordmannsjøkelen (N) 1990-2006 (Figure 3), and Hardangerjøkulen (H) 1988-2003 (Figure 4). Both glaciers shrank in area.

Comparison of Landsat and Sentinel-2 The Landsat imagery available for glacier mapping in Norway made it impossible to make a proper change assessment for all of Norway. Mapping years varied from region to region and some regions could only be mapped once with satisfying conditions. Additional data from Sentinel-2 will give better quality of the glacier outlines, due to better time resolution and coverage of glacier mapping compared to Landsat. Spectral resolution The spectral resolution of Sentinel-2 and Landsat have only minor differences. The corresponding bands for Sentinel-2 will be band 4 (RED) / band 11 (SWIR), and band 2 (BLUE) (Figure 5, Table 1). Tests on deriving glacier outlines must be done on available band specifications of Sentinel-2. Temporal resolution The quality of the semi-automatic method is highly dependent on low amount of seasonal snow. The geometrical revisit time of Landsat is 16 days. Still, the number of available images is much less due to cloud cover which often makes imagery useless. When the two Sentinel-2 satellites are operational, the geometrical revisit time will be every 5 days (with one satellite every 10 days), and the effective revisit time will be improved (satellite images with clouds below a specific threshold value). Spatial resolution When using data from Sentinel-2 to derive glacier area outlines, the SWIR band 11 (20 m) has to be resampled to the spatial resolution of the red band 4 (10 m). The spatial resolution of the Sentinel-2 bands are presented in Table 1. Sentinel-2 will have a higher spatial resolution compared to Landsat (30 m). Radiometric resolution The radiometric resolution of Sentinel-2 (the bit depth of the sensor) is expressed as 12 bit, 4096 digital numbers (DN). Compared to Landsat (8-bit, 256 DN), Sentinel-2 has a more accurate separation between the reflected data from the earth retrieved by the satellite. Swath width The swath width of Sentinel-2 (290 km x 270 km) will map larger areas compared to Landsat (185 km x 180 km). This is an advantage, because fewer satellite scenes will then be needed to cover all glaciers in mainland Norway. The processing time needed for orthorectification and quality check can therefore be reduced. New applications using Sentinel-2 in mainland Norway The enhanced temporal resolution of Sentinel-2 can open possibilities for annual mapping of the distribution of snow, firn and glacier ice surfaces. This data are useful for extending mass balance measurements to unmeasured glaciers on a regional scale. Using the late summer snow line (end of ablation), the equilibrium line altitude (ELA) can be derived and the mass balance on glaciers can be estimated. Annual images during several years are needed for obtaining mass balance estimates with this method.

Conclusions Glacier change assessment using Landsat TM/ETM+ imagery is challenging in Norway due to lack of proper time series. Improved spectral, temporal, spatial, and radiometric resolution of Sentinel-2 compared to present optical satellites will give the opportunity for better and more frequent mapping of glaciers.

Figure 2: Automatic classification of snow and ice for a subset of glaciers in western Finnmark using a Landsat TM scene from 26 August 2006. The subset contains Seilandsjøkelen and Nordmannsjøkelen using a selected set of threshold values

Figure 1:a) A total of 12 Landsat images were selected for deriving glacier area outline for 1999-2006, b) 9 Landsat scenes covering glaciers in mainland Norway from 1988-1997.

Figure 3: -14.6% decrease in glacier area from 1990 to 2006 at Nordmannsjøkelen (16 years).

Figure 5: Spectral and spatial resolution of Sentinel-2. Band 4 (RED), 11 (SWIR) and 2 (BLUE)are the corresponding bands to Landsat when the multispectral band ratio is applied for segmentation of glacier outlines. Source: ESA.

Table 1: Wavelength of the Sentinel-2 bands, and corresponding Landsat bands.

Band 4

Band 11

Band 2

Figure 4: -4.9% decrease in glacier area from 1988 to 2003 at Hardangerjøkulen (15 years).

Wavelength

Spa

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a) b)

Literature Paul, F., L.M. Andreassen and S.H. Winsvold. 2011. A new glacier inventory for the Jostedalsbreen region, Norway, from Landsat TM scenes of 2006 and changes since 1966. Annals of Glaciology. 52(59), 153-162. Andreassen, L.M., F. Paul, A. Kääb, and J.E. Hausberg. 2008. Landsat-derived glacier inventory for Jotunheimen, Norway, and deduced glacier changes since the 1930s. The Cryosphere, 2, 131-145.

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