Alaska North Slope 100 m Digital Elevation Model (DEM)

Summary:

This data set is a 100-m resolution digital elevation model (DEM) for the 
Brooks Range and the North Slope of Alaska.  The DEM was created by merging 
100 National Elevation Dataset DEM tiles for areas of Alaska north of 
67 degrees north latitude.  A small area of Canada, derived from the global 
GTOPO30 DEM, is also included.  The resulting DEM is in the Alaska Albers 
conical equal area projection, using the North American Datum of 1927 (NAD27).

Data  are available in GeoTIFF, Arc/INFO grid, and Band Interleaved by Line 
(BIL) formats, and are accessible via ftp.

National Science Foundation grant #OPP0100120 supported the development of this 
digital elevation model.

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Spatial and temporal coverage (range and resolution) of the data set:

Bounding coordinates in Alaska Albers projection are xmin -545300, ymin
1943200, xmax 534800, ymax 2398000. 

Latitude and Longitude Bounding Coordinates:
      West Bounding Coordinate: -168.8067
      East Bounding Coordinate: -139.4672
      North Bounding Coordinate: 71.5834
      South Bounding Coordinate: 66.9846

Spatial Data Organization Information:
  Direct Spatial Reference Method: Raster
  Raster Object Information:
    Raster Object Type: Grid Cell
    Row Count: 4548
    Column Count: 10801
    
Spatial Reference Information:
  Horizontal Coordinate System Definition:
    Planar:
      Map Projection:
	Map Projection Name: Albers Conical Equal Area
	Albers Conical Equal Area:
	  Standard Parallel: 55
	  Standard Parallel: 65
	  Longitude of Central Meridian: -154
	  Latitude of Projection Origin: 50
	  False Easting: 0
	  False Northing: 0
      Planar Coordinate Information:
	Planar Coordinate Encoding Method: coordinate pair
	Coordinate Representation:
	  Abscissa Resolution: 100
	  Ordinate Resolution: 100
	Planar Distance Units: meters
    Geodetic Model:
      Horizontal Datum Name: North American Datum of 1927
      Ellipsoid Name: Clarke 1866
      Semi-major Axis: 6378206.4
      Denominator of Flattening Ratio: 294.98
      
Time Period of Content:
	Calendar Date of compilation: 20010615

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Description of data collection techniques, including instrumentation used, 
sensors, calibrations, acquisition algorithms, etc:

Description:
    Abstract: Digital elevation model (DEM) 100m grid for the Brooks
    Range and North Slope of Alaska, created from USGS NED DEM tiles.

Process Step:
      Process Description: The first step in producing the grid was
      merging the 100 1 x 1 degree NED DEM tiles for areas of Alaska
      north of 67 deg (provided by the USGS EROS Alaska Field Office;
      see http://agdc.usgs.gov). Map projection, datum, data type, and
      grid cell spacing for the original NED tiles are geographic,
      NAD27, floating point, 2 arcsec. After using the ArcInfo MERGE
      command with all 100 NED tiles, the resulting very large grid
      (still geographic, NAD27, floating point, 2 arcsec) had 46,800
      columns and 9000 rows. This grid was then reprojected to Alaska
      Albers, NAD27, floating point, 100 m using bilinear sampling, and
      subsequently converted to an integer grid. This preliminary
      integer grid was brought into the MFWorks raster GIS program, and
      clipped to eliminate areas beyond the Brooks Range and north
      slope. A small triangular sliver for nearby Canada was separately
      processed starting with a Canada 30 Sec DEM (which was derived
      from the global GTOPO30 dem).  This sliver was projected from
      Alaska Albers, 720 m, to Alaska Albers, 100 m (oversampling resulting 
      in less horizontal and vertical accuracy than for the US portion of 
      ans_dem).  The Canadian portion was combined with US portion, including 
      interpolation for smoothing of a 500 m buffer extending into Canada.  
      Areas of NODATA in the Chukchi and Beaufort Seas were recoded to zero. 
      This grid was finally brought back into ArcInfo as a raster GRID file. 
      For additional usability, the grid was exported to binary format using 
      the GRIDFLOAT command.

Process Date: 2001-06-15

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Specify data collection problems, bad data flags, data dropouts, and
other quality control factors:

Data Quality Information:
  Attribute Accuracy:
    Attribute Accuracy Report: See Horizontal Accuracy.
  Logical Consistency Report: Not applicable for raster data.
  Completeness Report: None.
  Positional Accuracy:
    Horizontal Positional Accuracy:
      Horizontal Positional Accuracy Report: See the USGS National
      Elevation Dataset website, http://edcnts12.cr.usgs.gov/ned/
    Vertical Positional Accuracy:
      Vertical Positional Accuracy Report: See Horizontal Accuracy.

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Data Structure:

Description of header and data records including size and array:
Spatial Data Organization Information:
  Direct Spatial Reference Method: Raster
  Raster Object Information:
    Raster Object Type: Grid Cell
    Row Count: 4548
    Column Count: 10801
Minimum value: 0
Maximum value: 2734
NODATA value: -9999

Data Type, and Data Set Format:
Raster data.
Data is provided in 3 formats. GeoTIFF, Arc/INFO Grid and Band
Interleaved by Line (BIL)

Header record format: There are 2 header files, ans_dem.blw and
ans_dem.hdr. Both of these header files accompany the ans_dem.bil file.
The "blw" file is a BIL world file. The first record is the cellsize,
100 and the last 2 records are the x (upper left hand corner) and y
(upper left hand corner) locations on the earth in the Albers Conical
Equal Area projection. The ".hdr" file is an Arcview header file for
use with image files. A description of each parameter is below:

nrows - the number of rows in the image. Rows are parallel to the
x-axis of the map coordinate system. There is no default.

ncols - the number of columns in the image. Columns are parallel to the
y-axis of the map coordinate system. There is no default.

nbands - the number of spectral bands in the image. The default is 1.

nbits - the number of bits per pixel per band. Acceptable values are 1,
4, 8, 16 and 32. The default value is 8-bits per pixel per band. For a
true color image with three bands (R, G, B) stored using 8-bits for
each pixel in each band, nbits equals 8 and nbands equals 3, for a
total of 24 bits per pixel. For an image with nbits equal to 1, nbands
must also equal 1.

byteorder - the byte order in which image pixel values are stored. The
byte order is important for 16-bit images, with two bytes per pixel.
Acceptable values are:

I - Intel byte order (Silicon Graphics, DEC Alpha, PC) - also known as
little-endian

M - Motorola byte order (Sun, HP, etc.) - also known as
big-endian

The default byte order is the same as that of the host machine
executing the software.

layout - the organization of the bands in the image file. Acceptable
values are:

bil - band interleaved by linebip - band interleaved by pixelbsq - band
sequential

The default layout is bil.

Mapunits - map measurement units

skipbytes - the number of bytes of data in the image file to skip in
order to reach the start of the image data. This keyword allows you to
bypass any existing image header information in the file. The default
value is 0 bytes.

ulxmap - the x-axis map coordinate of the center of the upper-left
pixel. If you specify this parameter, set ulymap, too; otherwise, a
default value is used.

ulymap - the y-axis map coordinate of the center of the upper-left
pixel. If this parameter is specified, ulxmap must also be set;
otherwise, a default value is used.

xdim - the x dimension of a pixel in map units. If this parameter is
specified, ydim, ulxmap, and ulymap must also be set; otherwise, a
default value is used.

ydim - the y dimension of a pixel in map units. If this parameter is
specified, xdim, ulxmap, and ulymap must also be set; otherwise, a
default value is used.

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Variable Name and Description: value, elevation in meters

Data Type (e.g. alphanumeric, integer, real): integer

Units of Measurement: meters

Level of Precision:
Data Quality Information:
  Attribute Accuracy:
    Attribute Accuracy Report: See Horizontal Accuracy.
  Logical Consistency Report: Not applicable for raster data.
  Completeness Report: None.
  Positional Accuracy:
    Horizontal Positional Accuracy:
      Horizontal Positional Accuracy Report: See the USGS National
      Elevation Dataset website, http://edcnts12.cr.usgs.gov/ned/
    Vertical Positional Accuracy:
      Vertical Positional Accuracy Report: See Horizontal Accuracy.

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Development Machine:

  Native Data Set Environment: Windows NT4, ArcInfo 8.0.1, ArcView 3.2,
  MFWorks 2.6.3

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Supporting Information:

Source Information:
      Source Citation:
	Citation Information:
	  Originator: U.S. Geological Survey (USGS), EROS Data Center
	  Publication Date: 19990711
	  Title: National Elevation Dataset
	  Publication Information:
	    Publication Place: Reston, VA
	    Publisher: U.S. Geological Survey
      Type of Source Media: digital database file
      Source Time Period of Content:
	Time Period Information:
	  Single Date/Time:
	    Calendar Date: 19990711
	Source Currentness Reference: Publication Date
      Source Citation Abbreviation: None
      Source Contribution: The US portion of the dataset was derived
      from this source.
    Source Information:
      Source Citation:
	Citation Information:
	  Originator: National Park Service, Alaska Support Office
	  Publication Date: 20000518
	  Title: Canada 30 Second digital elevation model (DEM)
	  Publication Information:
	    Publication Place: Anchorage, Alaska
	    Publisher: National Park Service, Alaska Support Office
      Type of Source Media: digital database file
      Source Time Period of Content:
	Time Period Information:
	  Single Date/Time:
	    Calendar Date: 20000518
	Source Currentness Reference: Publication Date
      Source Citation Abbreviation: None
      Source Contribution: The Canadian portion of the dataset was
      derived from this source.

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Conditions for use and citation:

Access Constraints: None

Use Constraints: It is not recommended that the data be used at a
  scale larger than 1 to 100,000.  Please cite the data set as shown below.
  
Citation Information:
  Manley, W.F. 2002. Alaska North Slope 100 m Digital Elevation Model (DEM). 
  Boulder, CO: National Snow and Ice Data Center. Digital media.