Mapping Toolbox

gtopo30

Read elevation data from GTOPO30 into a regular matrix map

Syntax

• [map,maplegend] = gtopo30(filename,scalefactor)
  [map,maplegend] = gtopo30(filename,scalefactor,latlim,lonlim)
  

Background

A global model of elevations on a 30 arc-second grid (approximately 1 km) has been developed by the Earth Resources Observation System Data Center and is available over the Internet. The model uses a variety of international data sources, but is primarily based on raster data from the Digital Terrain Elevation Model (DTED) and vector data from the Digital Chart of the World (DCW). Gridded elevations are computed from DCW elevation contours, points and drainage lines using ANUDEM, a Digital Elevation Model generator developed by the Australian National University. The dataset consists of a total of 21,600 rows and 43,200 columns on 33 tiles, generally covering 40 by 50 degrees. Each tile requires about 15 megabytes compressed, 80 megabytes uncompressed and twice that to extract.

Description

[map,maplegend] = gtopo30(filename,scalefactor) reads the GTOPO30 files and returns a regular matrix map. The filename is given as a string, which does not include the extension. If the files are not found on the MATLAB path, they can be selected though a dialog box. scalefactor is an integer, which when equal to 1 gives the data at its full resolution. When scalefactor is an integer number larger than one, every n^th point is returned. This scale factor must divide evenly into the number of rows and columns in the data file. The map data is returned as a matrix of elevations and an associated regular matrix map legend.

[map,maplegend] = gtopo30(filename,scalefactor,latlim,lonlim) allows a subset of the map data to be read. The limits of the desired data are specified as vectors of latitudes and longitudes in degrees. The elements of latlim and lonlim must be in ascending order.

Remarks

The data is for elevations only. Elevations are given in meters above mean sea level using WGS 84 as a horizontal datum. Areas with no data, such as the oceans, are coded with NaNs.

The data is available over the Internet via anonymous ftp from:

ftp://edcftp.cr.usgs.gov/pub/data/gtopo30/global.

The data and some documentation is also available over the World-Wide-Web from:

http://edcwww.cr.usgs.gov/landdaac/gtopo30/gtopo30.html and

http://edcwww.cr.usgs.gov/landdaac/gtopo30/README.html.

Examples

    Extract every 20th point from the tile covering the northeastern United States and eastern Canada. Provide an empty file name, and select the file interactively.

• [map,maplegend] = gtopo30([],20);
  size(map)
  ans =
     300   240
  

Extract a subset of the data for Massachusetts at the full resolution.

• latlim = [41 42.5]; lonlim = [-73 -69.9];
  [map,maplegend] = gtopo30('W100N90',1,latlim,lonlim);
  size(map)
  ans =
     180   391
  

Replace the NaNs in the ocean with -1 to color them blue.

• map(isnan(map)) = -1;
  

Extract the data for Thailand. This area straddles two tiles. The data is on CD number 3 distributed by the USGS.

• latlim = [5.22 20.90]; lonlim = [96.72 106.38];
  gtopo30s(latlim,lonlim)
  
  ans = 
  
      'e060n40'
      'e100n40'
  
  [map,maplegend] = gtopo30('f:\',3,latlim,lonlim);
  size(map)
  
  ans =
  
          628 388
  
  

See Also

gtopo30s	Filenames for GTOPO30 data from geographic area.
demdataui	Digital elevation map data user interface
dted	Read Digital Terrain Elevation Data (DTED) data
globedem	Read 1 km GLOBE DEM data
satbath	Global 2-minute topography from satellite bathymetry
tbase	Read data from the TerrainBase dataset
usgsdem	Read USGS digital elevation maps

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