**Land Topography Picture**

[**SRTM**](http://srtm.usgs.gov/) (Shuttle Radio Topography Mission):
has topography data using radar onboard a space shuttle. It surveyed 80%
of land cover. It has only topographic data. However, at the [mission
website](http://www.jpl.nasa.gov/srtm/dataprod.htm), it has done some
processing. On various pictures in the gallery, it has overlaid
photographic images of the land on top of topography images. Other
interpretations of the topographic data use false color to give a sense
of relief. The topography data is distributed through [EROS data
center](http://edc.usgs.gov/). Resolution is at 30 meters, and the
largest error is 16 meters at the 90 % confidence level. It has data in
the X-band and C-band. The X-band is being processed by the German
Aerospace Center, DLR and the C-band is being processed by NASA. C-band
and X-band are the different wavelengths that were used to collect the
data.

Getting SRTM would be free and is available from the USGS's [EROS data
center](http://edc.usgs.gov/).

"The NIMA (National Imagery and Mapping Agency) -NASA [memorandum of
understanding (MOU)](http://srtm.usgs.gov/USGSRole/MOU.html) states that
NASA has the right to distribute data obtained from SRTM and that 'the
data will be archived at the appropriate NASA data center and made
available to all users without restriction and at no more than the cost
of fulfilling the user request."

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  ![](media/image1.jpeg){width="5.125in" height="3.202777777777778in"}

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SRTM Coverage Map

**GTOPO30** also has topographic data set, available via
<ftp://edcftp.cr.usgs.gov/>. Log into the ftp server with user name:
**anonymous** and password: **your email address**. After successfully
entering the system go to the directory:
[/pub/data/gtopo30/global](ftp://edcftp.cr.usgs.gov/pub/data/gtopo30/global).
Change to binary mode for transfer type and download the data. The data
for Antarctica is a separate data set at
[/pub/data/gtopo30/Antarctica](ftp://edcftp.cr.usgs.gov/pub/data/gtopo30/Antarctica).
GTOPO30 is a global digital elevation model. It comes in *filename*.DEM
file format. There is a ReadMe file on how to view the DEM files at the
FTP location. But it can be viewed with ArcInfo/View, which is available
at ESRI website ([http://www.esri.com](http://www.esri.com/)), by
changing the file extension name from .dem to .bil. GTOPO30 is a
collection of vector data and raster data sets. So it's a mix of data
sets. The accuracy of the GTOPO30 highly depends on where the data set
comes from. Area where the data is derived from raster data sets has a
higher degree of accuracy compared to the vector data sets, because
there is less preprocessing done to raster source data than vector
source data. More preprocessing introduce noise and error as the vector
source data are processed more times before being integrated into the
DEM. The complete GTOPO30 data set has a varying range of data error
that can be compounded by further processing. The resolution at 30-arc
seconds is approximately 1 km.

  ----------------------------------- -----------------------------------
  **Raster Source Data**              **Vector Source Data**

  Digital Terrain Elevation Data      Digital Chart of the World (DCW)
  (DTED)                              

  USGS 1-degree DEM                   Army Map Services Maps

  New Zealand DEM                     International Map of the World

                                      Peru Map

                                      Antarctic Digital Database
  ----------------------------------- -----------------------------------

![](media/image2.png){width="5.25in" height="2.625in"}

GTOPO30 Sources

**EROS data center** has data covering aerial photography, survey maps
(distance valid), elevation (topography), satellite imaging (remote
sensing, e.g., phot, infra-red, radar, etc.), and land cover (e.g., land
use, vegetations, etc.). The EROS (USGS) collection is broad. It can
look through the NASA DAACs since it manages the [EOS
Gateway](http://eosims.cr.usgs.gov/imswelcome) (NASA), it also employs 3
other data management system: [Earth
Explorer](http://earthexplorer.usgs.gov/), [Photo
Finder](http://edc.usgs.gov/Webglis/glisbin/finder_main.pl?dataset_name=NAPP),
and [Map
Finder](http://edc.usgs.gov/Webglis/glisbin/finder_main.pl?dataset_name=MAPS_LARGE).
The [EROS data center](http://edc.usgs.gov/) is good place to start to
look for geographic data.

[**NOAA National Geophysical Data
Center**](http://www.ngdc.noaa.gov/mgg/global/global.html): Offers
2-minute and 5-minute global gridded data. However, the latitude minute
changes as latitude changes. Therefore, the data is generalized. The
2-min latitude/longitude grid resolution is approximately 3.7 km. And
5-min resolution is at 9.26 km. The difference is enormous. The data
provided here at shaded with false color for differentiate elevations.
The coverage is the same at GTOPO30. The resolution is not as good as
the GTOPO30, but it uses smaller sources of data set, so there's less
difference in error between going from one data set's boundary to
another. The data can be obtained from the [NOAA National Geophysical
Data Center](http://www.ngdc.noaa.gov/mgg/global/global.html).

![](media/image3.jpeg){width="5.0in" height="2.5in"}

ETOPO2 Sources

SRTM should be the primary choice of topographic data over GTOPO30 and
ETOPO2. It offers the best resolution as shown in the picture below.
Also, the error of the topography is much less than either of the DEM
(Digital Elevation Model) datasets, GTOPO30 and ETOPO2.

![](media/image4.jpeg){width="5.322916666666667in"
height="2.6666666666666665in"}

Comparison of GTOPO30 dataset with SRTM C-band data\
(GTOPO30 on the left & SRTM on the right)