Abstract
AbstractGlaciers depicted on old maps reveal their historical extents, before the advent of aerial and satellite remote sensing. Digital glacier inventories produced from these maps can be employed in assessments of centennial-scale glacier change. This study reconstructs the ~1899 (covering the period 1882–1916) glacier extent in Nordland, northern Norway, from historical gradteigskart maps, with an emphasis on examining the accuracy of the mapped glaciers. Glacier outlines were digitised from georectified scans of the analogue maps in a raster graphics editor and were subsequently inventoried in a GIS. The accuracy of the historical glacier extent was established from written descriptions and landscape photographs created during the original field surveys, and further validated against independent glacier outlines of (1) the maximum Little Ice Age extent derived from geomorphological evidence, and (2) the 1945 extent derived from vertical aerial photographs. An overall uncertainty of ±17% is associated with our inventory. Nordland's glaciers covered an area of 1712 ± 291 km2 in 1899. By 2000, total ice cover had decreased by 47% (807 ± 137 km2) at a rate of 6% 10 a−1 (80 ± 14 km2 10 a−1). The approach presented here may serve as a blueprint for future studies intending to derive glacier inventories from historical maps.
Highlights
Measurements of the area and extent of glaciers in the form of glacier inventories provide crucial input data for quantifying glacier volume (e.g. Bahr and others, 1997, 2009; Farinotti and others, 2009; Radić and Hock, 2010; Linsbauer and others, 2012; Vaughan and others, 2013) and modelling glacier mass change (e.g. Marzeion and others, 2012; Radić and others, 2014), both of which are needed for estimates of the glacier contribution to sea-level rise
The aims of our research are (1) to examine the history of the Nordland gradteigskartene maps and the accuracy of the mapped glaciers for their suitability as a glacier inventory; (2) to build a digital GIS inventory of the glacier outlines displayed on the old maps; (3) to independently evaluate the accuracy of the historical glacier extents from geomorphological data and early aerial photographs; (4) to compare the new dataset with existing glacier inventories in order to quantify 20th-century glacier change in Nordland and (5) to offer general recommendations for creating glacier inventories from historical map sources
Since the meridians converge towards the poles, the maps of northern Norway cover a whole degree of longitude (Harsson, 2009; Harsson and Aanrud, 2016) (Fig. 1, Table 2)
Summary
Measurements of the area and extent of glaciers in the form of glacier inventories provide crucial input data for quantifying glacier volume (e.g. Bahr and others, 1997, 2009; Farinotti and others, 2009; Radić and Hock, 2010; Linsbauer and others, 2012; Vaughan and others, 2013) and modelling glacier mass change (e.g. Marzeion and others, 2012; Radić and others, 2014), both of which are needed for estimates of the glacier contribution to sea-level rise. Glacier inventories from historical maps are, in a strict sense, a representation of what the cartographers at the time chose to draw in as glacier ice (this is a noteworthy difference to most satellite-derived inventories, where semi-automatic image classification selects all existing snow and ice bodies, and it is up to the inventory creators to decide which to include/exclude) Their reliability depends on, first, the quality of the historical map source and the precision of the original glacier mapping, and second, how thoroughly map uncertainties were assessed and taken into account by the inventory creators. In Norway, only a few attempts to produce early- or pre-20thcentury glacier outlines from historical maps have been carried out, focusing on individual plateau icefields (Winsvold and others, 2014; Weber and others, 2019) These studies used the 1 : 100 000 scale gradteigskartene (‘quadrangle maps’), which were Norway’s main map series from the mid-1890s (Harsson and Aanrud, 2016). The aims of our research are (1) to examine the history of the Nordland gradteigskartene maps and the accuracy of the mapped glaciers for their suitability as a glacier inventory; (2) to build a digital GIS inventory of the glacier outlines displayed on the old maps; (3) to independently evaluate the accuracy of the historical glacier extents from geomorphological data and early aerial photographs; (4) to compare the new dataset with existing glacier inventories in order to quantify 20th-century glacier change in Nordland and (5) to offer general recommendations for creating glacier inventories from historical map sources
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