area change calculations
Our surveys use NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) bands 01 and 02 imagery gridded using Terry Haran’s MODIS Swath to Grid Toolbox (MS2GT).
The glaciers surveyed are the widest in Greenland. All are marine-terminating, as opposed to the relatively sluggish land-terminating glaciers. Considering the widest glaciers, the observed area changes are actually resolvable with the moderate resolution (250 m is considered coarse resolution by many) MODIS pixels.
For each glacier, researcher David Decker repeats a manual area outlining with ENVI region of interest tool 3 to 5 times. Comparing the multiple attempts to quantify area change, we find that the area changes are usually sufficiently larger than the uncertainty (1.0 sq km), that is, the signal is larger than the noise, that we can measure statistically “significant” areas.
Time of Year Chosen
Terminus position is chosen for the clear image date just before the return of permanent (for that season) winter snow. Choosing the annual minimum extent is another approach and may more accurately reflect the stage changes of the glacier.
Effective Length Change
We divide the observed area change by the average width of the glacier to assess the effective length change (L’). The average glacier width is measured at the front using Google Earth.
Completeness of Survey
The fraction of the total Greenland glacier width is represented by our survey so far is but 13% of the estimated 261 glacier total that are 1 km or wider. 424.8 km is the total width of these 35 glaciers out of 1415.7 km total Greenland marine terminating glacier width. 30.0% of Greenland’s glacier width is thus surveyed.
Sources of error include: geolocation uncertainty, ice edge manual classification error.
The names and coordinates of the surveyed glaciers is given here.