## Polar Products  
Many remote sensing products may be difficult for an inexperienced user to process and interpret. Raw data are processed, projected, resampled or combined to produce derived products for users. Indeed, even for experienced users, a derived product is sometimes more helpful than the raw data. Examples include surface temperature, albedo, sea ice extent, glacier outlines or grounding lines and snow water equivalent/snow depth, vegetation and lithology maps. Based on the provided metadata, it is crucial to understand where products come from, the biases, potential errors and what implications these have for eventual application. Many of these datasets are easily available, with metadata, through centralized sources, such as the National Snow and Ice Data Center (NSIDC, [[http://nsidc.org/](http://nsidc.org/)]([http://nsidc.org/](http://nsidc.org/))), Global Cryosphere Watch ([[http://globalcryospherewatch.org](http://globalcryospherewatch.org)]([http://globalcryospherewatch.org](http://globalcryospherewatch.org))), the World Glacier Monitoring Service ([[http://www.wgms.ch](http://www.wgms.ch)]([http://www.wgms.ch](http://www.wgms.ch))), the Ocean and Sea Ice Satellite Applications Facility ([[http://osisaf.met.no/p/ice/](http://osisaf.met.no/p/ice/)]([http://osisaf.met.no/p/ice/](http://osisaf.met.no/p/ice/))) and the Polar Geospatial Center ([[http://www.pgc.umn.edu/](http://www.pgc.umn.edu/)]([http://www.pgc.umn.edu/](http://www.pgc.umn.edu/))), although others are distributed by individual principal investigators (PIs), or through institutions and consortia, as well. For a list of some polar product providers and notable products themselves, see Table 2. Cryolist, a semi-moderated e-mail distribution list ([[http://cryloist.org/](http://cryloist.org/)]([http://cryloist.org/](http://cryloist.org/))), also deserves special mention here; this online community of polar and cryospheric researchers can be a helpful resources to find data, to ask questions about tools or methods and to disseminate news about new or updated datasets. An important set of products is that which uses altimetry data, interferometry or stereo imagery to produce digital elevation models (DEMs). DEMs can be produced utilizing a wide variety of data types and with varying methodologies, but there are three broad categories: stereo imagery, InSAR (interferometric synthetic aperture radar) and altimeters (both laser and radar). Stereo pairs of optical imagery were used to produce the 30-m ASTER GDEM and GDEM-2 [99], SPIRIT DEMs from the French SPOT satellites instruments (e.g., [100, 101]), some historical DEMs (e.g., [32, 33]) and increasingly for stereo WorldView images. With WorldView, the Ames Stereo Pipeline (ASP) is a freely available and popular tool to create DEMs [102, 103], although private tools like SETSM [104] and proprietary tools like ERDAS Imagine are also common. A Greenland-specific 30-m DEM created with a combination of stereo imaging and photoclinometry is available at [[http://bprc.osu.edu/GDG/gimpdem.php](http://bprc.osu.edu/GDG/gimpdem.php)]([http://bprc.osu.edu/GDG/gimpdem.php](http://bprc.osu.edu/GDG/gimpdem.php)) [105]. Interferometric synthetic aperture radar was used to produce the 90-m Shuttle Radar Topography Mission (SRTM) DEM ([[http://www2.jpl.nasa.gov/srtm/](http://www2.jpl.nasa.gov/srtm/)]([http://www2.jpl.nasa.gov/srtm/](http://www2.jpl.nasa.gov/srtm/)); [[http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-database-v4-1](http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-database-v4-1)]([http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-database-v4-1](http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-database-v4-1))) [106, 107, 108], although this DEM does suffer from penetration in low density surfaces (e.g., [109, 110]).