### GLAS/ICESat Antarctic and Greenland DEMs and Derived Grids

GLAS/ICESat (**Zwally et al., 2002**) was the first laser altimeter to fly on a spacecraft in Earth orbit. The grids available here were generated from the first two years of data (2003 Feb to 2005 May).

The ICESat elevations grids for Greenland and Antarctica are available from NSIDC with elevations relative to WGS84 ellipsoid and the EGM96 geoid. The NSIDC ICESat page has a full description of these grids. All grids on this website have elevations relative to the EGM96 geoid. The elevations grids are identical to the ones at NSIDC.

#### Citation

The following should be used to cite these data: Zwally, H. J., J. P. DiMarzio, and A. C. Brenner, 2012. *GLAS/ICESat Antarctic and Greenland Grids, Digital media.*

### Grid Formats and Projection

Ellipsoid | Topex/Poseidon. For the grids with elevations relative to the WGS-84 ellipsoid and EGM96 geoid, the node locations are still based on the Topex/Poseidon polar-stereographic grid. |

Ellipsoid SemiMajor axis | 6378.1363 km |

Ellipsoid Eccentricity | 0.08181922146 |

File format | Files are gzipped. |

Storage order | Column by column, starting at the upper left corner and ending in the lower right corner: (1,1),(1,2),...,(1,N_{y}),(2,1),(2,2),...,(N_{x},N_{y}) |

Data format | Unformatted: 4-byte (long) signed integer, big endian byte order. ASCII: grids include latitude, longitude, and parameter value. Points without values are not included. |

Undefined value in unformatted files | 2^{31}-1 = 2147483647 |

### Grid Specifications

- Note: UL=upper Left, LL=Lower Left, UR=Upper Right, LR=Lower Right
- x and y are the horizontal and vertical axes of the polar-stereographic coordinate systems.

Parameter | Antarctica | Greenland |
---|---|---|

Standard Latitude | -70° | 70° |

Orientation | Line from S pole along 0° longitude points vertically up on the map | Line from N pole along 315°E points vertically down on the map |

Grid dimensions | 11352 columns x 9368 rows | 1484 columns x 2760 rows |

x/y of center of UL cell | x=3398, y=4423 | x=3858, y=5162 |

Geodetic lat/lon of center of UL cell | Lat=-57.345281, Lon=309.27442 | Lat=81.503104, Lon=269.912123 |

Geodetic lat/lon of UL corner of UL cell | Lat=-57.342282, Lon=309.27498 | Lat=81.503091, Lon=269.868185 |

x/y of center of LL cell | x=14749, y=13790 | x=5341, y=7921 |

Geodetic lat/lon of center of LL cell | Lat=-56.884712, Lon=229.70888 | Lat=58.718847, Lon=304.159350 |

Geodetic lat/lon of LL corner of LL cell | Lat=-56.881714, Lon=229.70843 | Lat=58.713825, Lon=304.152799 |

x/y of center of UR cell | x=14749, y=4423 | x=5341, y=5162 |

Geodetic lat/lon of center of UR cell | Lat=-57.004368, Lon=51.234204 | Lat=80.284817, Lon=6.891585 |

Geodetic lat/lon of UR corner of UR cell | Lat=-57.001376, Lon=51.233605 | Lat=80.284037, Lon=6.929712 |

x/y of center of LR cell | x=14749, y=13790 | x=5341, y=7921 |

Geodetic lat/lon of center of LR cell | Lat=-56.549515, Lon=129.77899 | Lat=58.396343, Lon=328.679882 |

Geodetic lat/lon of LR corner of LR cell | Lat=-56.546525, Lon=129.77949 | Lat=58.391166, Lon=328.685882 |

x/y of pole | x=9022, y=9022 | x=4511, y=4511 |

Nominal cell size | 500 m | 1 km |

Grid dimensions | 11352 columns x 9368 rows | 1484 columns x 2760 rows |

### Images

Links are provided to download the images in either JPEG or POSTSCRIPT format. POSTSCRIPT images are gzipped. Image ranges are limited to display the range of the majority of the data well. Actual limits of values in the grids may be outside the limits shown on the plots. A spike at either end of the histogram colorbar is an indication of this but in some cases the histogram bin size is small enough that the spike may not be visible. Also, the height of these endpoint peaks was limited to the maximum value in the histogram so they would not dominate the figure.

Image axes are labelled with the polar-stereographic x and y coordinates.

### Antarctic grids

Elevation |
Slope |
Azimuth |

dz/dx |
dz/dy |

### Greenland Grids

Elevation |
Slope |
Azimuth |

dz/dx |
dz/dy |

### Grids

Antarctica | Antarctica | Greenland | Greenland | |||
---|---|---|---|---|---|---|

Parameter | Description | Data units in unformatted files | Links to data | File size (MB)* | Links to data | File size (MB)* |

Elevation relative to EGM96 ellipsoid | Elevations computed using biquadratic fits. | mm | Unformatted | ASCII | 425.3 | 1,916 | Unformatted | ASCII | 16.3 | 79.8 |

Latitude | Latitude of the center of each cell | microdegrees | Unformatted | 425.3 | Unformatted | 16.3 |

Longitude | Longitude of the center of each cell | microdegrees | Unformatted | 425.3 | Unformatted | 16.3 |

Slope | Absolute value of slope | millidegrees | Unformatted | ASCII | 425.3 | 916 | Unformatted | ASCII | 16.3 | 62 |

Azimuth | Upslope direction measured clockwise with 0°=-y direction (upward in the map). Can also be described as the downslope direction measured clockwise with 0°=+y direction (upward in the map) | millidegrees | Unformatted | ASCII | 425.3 | 1,916 | Unformatted | ASCII | 16.3 | 62 |

dz/dx | Tangent of slope in the x direction on the map | mm/km | Unformatted | ASCII | 425.3 | 1,917 | Unformatted | ASCII | 16.3 | 64 |

dz/dy | Tangent of slope in the y direction on the map. | mm/km | Unformatted | ASCII | 425.3 | 1,917 | Unformatted | ASCII | 16.3 | 64 |

All data | ASCII | 4,044 | ASCII | 171 |

##### * Sizes are for the uncompressed files. All files are distributed in gzipped format.

#### Derivation

For the derivation of the DEMs see section 4 (Data Acquisition and Processing) of the NSIDC ICESat page.

The slope, azimuth, and directional slope (dz/dx and dz/dy) grids were generated from grids of elevation relative to EGM96, but given the scale of the grids (500m or 1km), these data can be considered independent of EGM96. Elevations relative to WGS84 or Topex/Poseidon would have given the same results with data to within the uncertainties in the data.

Latitudes and longitudes were computed using standard equations for the polar-stereographic projection (Snyder, 1982).

The directional slopes dz/dx and dz/dy were computed as central differences where possible:

dz(x_{i},y_{j})/dx = (z(x_{i+1},y_{j})-z(x_{i-1},y_{j}))/(x_{i+1}-x_{i-1}) dz(x_{i},y_{j})/dy = (z(x_{i},y_{j+1})-z(x_{i},y_{j-1}))/(y_{j+1}-y_{j-1})

and by forward differences

dz(x_{i},y_{j})/dx = (z(x_{i+1},y_{j})-z(x_{i},y_{j}))/(x_{i+1}-x_{i}) dz(x_{i},y_{j})/dy = (z(x_{i},y_{j+1})-z(x_{i},y_{j}))/(y_{j+1}-y_{j})

or backward differences

dz(x_{i},y_{j})/dx = (z(x_{i},y_{j})-z(x_{i-1},y_{j}))/(x_{i}-x_{i-1}) dz(x_{i},y_{j})/dy = (z(x_{i},y_{j})-z(x_{i},y_{j-1}))/(y_{j}-y_{j-1})

along the edges or where data were missing.

The magnitude of the slope, θ, was then computed from

tan^{2}θ = (dz/dx)^{2}+ (dz/dy)^{2}

and the aziumth (Φ') relative to the polar-stereographic coordinate system was computed from

tan Φ' = (dz/dy)/(dz/dx)

This was stored as an angle measured clockwise from the -y (upward pointing) axis. To convert the azimuth Φ_{1} = Φ (x_{1},y_{1}) to an angle relative to north, start with the latitude (L) and longitude (b) of the points at (x_{1},y_{1}) and (x_{1},y_{2}), where y_{2}=y_{1}-1. Using the spherical trigonometric sine and cosine laws,

cos c = cos(90-L_{1})cos(90-L_{2}) + sin(90-L_{1})sin(90-L_{2})cos(b_{2}-b_{1}) sin B = sin(90-L_{2})sin(b_{2}-b_{1})/sin c

and the azimuth measured clockwise relative to north is

Φ = Φ_{1}- B