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Chapter 13 Digital Landscaping
and topographic features from orbit, resulting in a highly accurate digital
elevation model. At the mission’s close, roughly 80% of Earth was examined
and modeled as 90-meter DEMs (and 30-meter DEM data is available for the
United States). See Hands-on Application 13.3: SRTM Imagery Online for more
about STRM data.
An additional remote sensing method of terrain mapping is called LIDAR
(Light Detection and Ranging). Rather than firing a microwave pulse at a
ground target like a RADAR system would, LIDAR uses a laser beam to measure the terrain. In LIDAR, a plane flies over the ground equipped with a
special system that fires a series of laser beams (between 2000 and 5000 pulses per second) at the ground. The laser beams are reflected from the ground
back to the plane, and based on the distance from the plane to targets on the
ground, the elevation of the landscape (as well as objects on the surface of
the terrain) can be determined. GPS (see Chapter 4) is used in part to determine where the beams were striking the ground. After the data is collected
and processed, the end result of the LIDAR mission is a highly accurate set
of x/y locations with a z-value. The elevations of the DEM products derived
from LIDAR have a vertical accuracy of 15 centimeters (see Figure 13.7 for an
example of a LIDAR-derived DEM).
The USGS has numerous types of DEMs available, depending on the size
of the area being covered and the DEM resolution. The USGS distributes this
elevation data for free download via the NED (National Elevation Dataset).
The purpose of the NED was to have a digital elevation product that covers
the entire United States at the best resolution possible and to eliminate any
gaps in the data. The NED is designed to be “seamless” in that its data use the
same datum, projection, and elevation unit, without having separate tiles of
data that the user needs to match up. With the NED, users can select which
sections of the national elevation dataset they need and download those
areas.
NED data is made freely available online via the USGS and the National Map Seamless Server, a very versatile online utility for downloading
LIDAR Light Detection
and Ranging. A process
in which a series of
laser beams fired at the
ground from an aircraft
is used for creation of
highly accurate DEMs.
NASA maintains an online gallery of SRTM imagery
from the 2000 mission, as well as other products
related to SRTM. Open your Web browser and go
to http://www2.jpl.nasa.gov/srtm. Select the option for Gallery of Images to view image results
of SRTM results from around the globe. Check out
several of the examples of SRTM imagery that are
set up in perspective view—among others, check
out the image galleries of North America—United
States (Alaska, Utah, and Washington State), Central
America ( Costa Rica), and Africa (Congo). Note that
they’re only example graphics of SRTM results, not a
way to download the actual SRTM data.
There’s also a Multimedia option on the main
page, which will allow you to view a series of video
files (in Real Player or Quicktime) of SRTM in action.
These should help give you a feel for how terrain
can be mapped from space.
Hands-on Application 13.3
SRTM Imagery Online
NED the National
Elevation Dataset,
which provides digital
elevation coverage
of the entire United
States.
Seamless Server an
online utility used
by the USGS for the
download of different
datasets, including
elevation data.
Chapter 13 Digital Landscaping
and topographic features from orbit, resulting in a highly accurate digital
elevation model. At the mission’s close, roughly 80% of Earth was examined
and modeled as 90-meter DEMs (and 30-meter DEM data is available for the
United States). See Hands-on Application 13.3: SRTM Imagery Online for more
about STRM data.
An additional remote sensing method of terrain mapping is called LIDAR
(Light Detection and Ranging). Rather than firing a microwave pulse at a
ground target like a RADAR system would, LIDAR uses a laser beam to measure the terrain. In LIDAR, a plane flies over the ground equipped with a
special system that fires a series of laser beams (between 2000 and 5000 pulses per second) at the ground. The laser beams are reflected from the ground
back to the plane, and based on the distance from the plane to targets on the
ground, the elevation of the landscape (as well as objects on the surface of
the terrain) can be determined. GPS (see Chapter 4) is used in part to determine where the beams were striking the ground. After the data is collected
and processed, the end result of the LIDAR mission is a highly accurate set
of x/y locations with a z-value. The elevations of the DEM products derived
from LIDAR have a vertical accuracy of 15 centimeters (see Figure 13.7 for an
example of a LIDAR-derived DEM).
The USGS has numerous types of DEMs available, depending on the size
of the area being covered and the DEM resolution. The USGS distributes this
elevation data for free download via the NED (National Elevation Dataset).
The purpose of the NED was to have a digital elevation product that covers
the entire United States at the best resolution possible and to eliminate any
gaps in the data. The NED is designed to be “seamless” in that its data use the
same datum, projection, and elevation unit, without having separate tiles of
data that the user needs to match up. With the NED, users can select which
sections of the national elevation dataset they need and download those
areas.
NED data is made freely available online via the USGS and the National Map Seamless Server, a very versatile online utility for downloading
LIDAR Light Detection
and Ranging. A process
in which a series of
laser beams fired at the
ground from an aircraft
is used for creation of
highly accurate DEMs.
NASA maintains an online gallery of SRTM imagery
from the 2000 mission, as well as other products
related to SRTM. Open your Web browser and go
to http://www2.jpl.nasa.gov/srtm. Select the option for Gallery of Images to view image results
of SRTM results from around the globe. Check out
several of the examples of SRTM imagery that are
set up in perspective view—among others, check
out the image galleries of North America—United
States (Alaska, Utah, and Washington State), Central
America ( Costa Rica), and Africa (Congo). Note that
they’re only example graphics of SRTM results, not a
way to download the actual SRTM data.
There’s also a Multimedia option on the main
page, which will allow you to view a series of video
files (in Real Player or Quicktime) of SRTM in action.
These should help give you a feel for how terrain
can be mapped from space.
Hands-on Application 13.3
SRTM Imagery Online
NED the National
Elevation Dataset,
which provides digital
elevation coverage
of the entire United
States.
Seamless Server an
online utility used
by the USGS for the
download of different
datasets, including
elevation data.