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Shuttle Radar Topography Mission

Shuttle Radar Topography Mission Pgina 1

GEOG 482: Nature of Geographic Information(Spring 2007)

Project 3: Topic C: Shuttle Radar Topography Mission

Kent StantonJune 19, 2007

The Mission:

In February of 2000 the Space Shuttle Endeavour flew an earth sciences mission called

the Shuttle Radar Topography Mission (SRTM) designed to produce a high-resolution

near-global elevation data set. The eleven-day mission used a single-pass

interferometric synthetic aperture radar sensor to image the continental areas of the

earth between 60 degrees north latitude and 54 degrees south latitude. This provided

coverage of roughly 80% of the earth's land surface. (NASA, Shuttle Radar Topography

Mission Overview)

The Shuttle Radar Topography Mission (SRTM) was a cooperative effort of NASA, the

National Geospatial-Intelligence Agency (NGA - formerly NIMA, the National Imagery

and Mapping Agency), The German Aerospace Center (DIR), and the Italian Space

Agency (ASI).

The SRTM goals called for the creation of digital elevation model (DEM) with a horizontal

resolution of 30 meters. Prior to the SRTM the best global DEM was the GTOPO30 DEM

compiled by the USGS from a variety of existing topographic data sources. Completed in

1996, GTOPO30 has a horizontal spacing of approximately one kilometer with

topographic accuracy dependant on the quality of the original data. (GTOPO30)

Portions of the GTOPO30 data set had been gathered using space borne interferometric

radar, but The SRTM improved on previous efforts to gather topographic data from

space in several important ways. Previous efforts including the use of satellites capable

of collecting elevation data, and two previous shuttle missions in the mid-1990s, relied

on the use of multiple-pass interferometry while the SRTM used a single pass approach.

Radar Interferometry refers to processing two complementary radar signals to derive

additional information by analyzing phase difference between the two signals. Combining

two images to derive elevation data makes interferometry sound very much like

photogrammetry, and there are conceptual similarities. But the derivation of topographic

data from overlapping photographs is based on angle measurements (parallax). Radar

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interferometry derives topography from an analysis of the phase difference between the

two radar signals. (Hannsen 2001, p. 17)

Multi-pass interferometry (the type used prior to the SRTM) relies on the space vehicle

imaging an area on multiple orbits. The data from these separate passes is correlated

and processed to create the 3D representation. But this technique suffers from thedifficulty of precisely correlating data from different passes. Also, changing atmospheric

conditions can introduce variation into the phase comparison that is not easily removed

during processing. The SRTM overcame these problems by capturing the two data sets

at the same time using two physically separated antennae; one in the shuttle cargo bay

and the other at the end of a 60-meter mast.

The use of radar (as opposed to imaging systems that measure other types of

electro-magnetic radiation) provided an additional significant benefit. Because

radar emits the radiation used by the sensor (active sensing), it can be used at

night and without regard to cloud cover (radar penetrates clouds). This was critical

to the mission goal of capturing a topographic snapshot of the earth because in

just 159 orbits. Even with the instrument capturing a a 225 kilometer wide swath,

the mission approached the limit of how long the shuttle can operate in this

manner. For this reason radar is the only way to get this level of coverage in a

short period of time.

JSC2000E-01557 (January 2000) --- This partially computer-generated scene depicts

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anticipated coverage by the Shuttle Radar Topography Mission (SRTM) of topographicfeatures on Earth. Heavy cloud cover, hurricanes and cyclonic storms can prevent opticalcameras on satellites or aircraft from imaging some areas. SRTM radar, with its long

wavelength, will penetrate clouds as well as providing its own illumination, making itindependent of daylight. NASA, Human Space Flight Images

But radar also has limitations as an imaging sensor. The wavelength of the radiation

that we call radar is relatively long, falling in the microwave area of the electromagnetic

spectrum. One outcome of this characteristic of radar is that the size of the antenna

determines the imaging resolution (the larger antenna the higher the resolution). Since

the physical size of the antennas used on the SRTM were limited by the practicalities of

launching and deploying the instruments, an approach called synthetic aperture radar

was used to improve the resolution of the system. Synthetic aperture radar uses a

pulsed radar signal to simulate a larger antenna. Instead of basing a measurement on a

single instantaneous signal, synthetic aperture radar combines multiple signals --

captured in rapid succession while the platform is moving to simulate a larger

antenna. This synthetic antenna produces higher resolution.

The mission also made use of radars operating at two different frequencies; a C-band

system provided by NASA and an X-band system provided by the DIR. The German X-

band system yielded higher resolution but at the cost of imaging a much narrower

ground swath on each pass (providing approximately half the coverage of the C-band

instrument). However, the data produced by the X-band system was needed to fill voids

in areas not captured effectively by the C-band sensor. (Hanssen, 2001)

The Shuttle Radar Topography Mission presented NASA with significant engineering

challenges and resulted in several important space firsts. In particular, using the

distance from the tip of the shuttle's opposite wing to the edge of the outboard antenna

(the mast), the structure spanned 83 meters (272 feet). This made it the largest rigid

structure ever flown in space. (NASA Shuttle Radar Topography Mission FAQs) And the

characteristics of this mast in a large part determined the horizontal resolution of the

data. Because the mast was not perfectly rigid, and flexed when the shuttle thrusters

were fired, special instrumentation was used to constantly measure and track the

location of the mast radar sensor. The resolution of the system depended on knowing

the exact position of the mast borne radar antenna relative to the main sensor in the

Shuttle's cargo bay, so an additional system called the AODA was used to track and

continuously record the positional data as well. This AODA data was used during post

mission processing of the data to remove noise caused by variation in the relative

positions of the two radar sensors. (Farr 2007)

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JSC2000E01559 (January 2000) -- An artist's concept depicts the Space Shuttle Endeavourwith its 200-ft. (60 meter) mast deployed in Earth orbit. NASA, Human Space Flight Images

The Data:

With the mast deployed, and the sensor activated, the radar system captured data

continuously while over land resulting in the accumulation of over ten terabytes of data

over the course of the mission. The data capture rate of the sensors exceeded the

maximum data transfer capability of the shuttle by a factor of six so it was not possible

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to transmit the bulk of this data to earth in real-time (NASA Shuttle Radar Topography

Mission FAQs). Instead the data was stored on the shuttle --on 330 high-density

magnetic tapes-- and became available for processing only after the shuttle had landed.

During the mission limited amounts of the data were transferred to earth for verification

and alignment purposes. (Farr 2007)

After the shuttle returned to earth the C-band data tapes were transferred to the Jet

Propulsion Laboratory in Pasadena California for preliminary processing and the X-band

data was sent to the DIR for processing. It took nearly two years for the JPL to fully

process the raw radar echoes to produce what is called the unfinisheddata set. This

step consisted of the interferometric processing --comparing the phase relationship of

the two signals to produce a 3D data set-- while at the same time correlating the data

with the AODA data. Considerable error checking and correction was also applied and

the raw data was cross-checked with a variety of existing topographic data sources to

detect and correct errors. (Farr 2007)

The data was processed on a continent-by-continent basis to create one-degree by one-

degree tiles that are named using the coordinates of the southwest corner of the tile.

The tiles, formatted according to the Digital Terrain Elevation Data (DTED) specification,

were then forwarded to the NGA for further refinement. The raw data set is at a

resolution of 1 arc second (30 meters), but tiles using this resolution have been released

only for United States and its territories. All other areas of the world are limited to three

arc second resolution (90 meter). ( Shuttle Radar Topography Mission)

One limitation of the system was that the broad swath imaged by the C-band radar

resulted in radar shadows in areas of high relief. This was especially true near the edges

of a swath where the beam struck the earth at an angle. In these areas mountains and

canyons sometimes created radar shadows that resulted in data voids. In some cases

these voids could be filled using data from other passes (the swaths captured on each

pass overlapped), or using data captured by the X-band system. However, for some

areas no SRTM data was available and the voids were filled using alternative data

sources and/or interpolation. These areas amount to less than .2% of the total area

covered but tend to be concentrated. Considerable post-processing effort has been

applied to fill these voids with accurate data. (Farr 2007)

From a paper in the March 2006 edition of Photogrammetric Engineering & Remote

Sensing, "Some of these voids can be attributed to the complex nature of IFSAR

technology, while topographic shadowing can cause others." This paper, Filling SRTM
ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/MIL-PDF-89020B.pdfftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/MIL-PDF-89020B.pdf

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Voids, the Delta Surface Fill Method, describes both the issues that cause data voids and

the ways in which the problems were be fixed. This paper is detailed, but readable, and

provides an excellent introduction to the issues encountered in processing the SRTM

data set. (Grohman, 2006)

An example of an unedited tile and the finished version of the same tile. Notice the data void the topcenter portion of the left image. The void has been filled in the edited image.

(EROS SRTM Topo Documentation)

A second enhancement of the data done by the NGA was the use of existing water

boundary files to create distinct and accurate water boundaries in the DEM. The radar

used on the SRTM does not generally show bodies of water as smooth even-elevation

surfaces. So, existing boundaries were used to improve the accuracy of water body

boundaries. The SRTM data was then averaged to associate a single elevation with the

body of water boundary. A by-product of this process was the creation of an SRTM water

body data file -- essentially a 30-meter resolution water mask. This data has been made

available to the user community along with the other SRTM data sets. (Gesch 2006)

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(A comparison of water boundaries before and after the water boundary enhancementprocessing. Slater, The Generation and Dissemination of "Finished" SRTM Data Products byNGA National Geospatial-Intelligence Agency, Slide Presentation at the SRTM Data Validation

and Applications Workshop,June 14-16,2005 Slide 9)

An additional distinction that separates the SRTM based DEM from other digital elevation

models is that the SRTM radar produced elevations based on the height of man-made

structures and the vegetative canopy. This is known as first return and it is opposed to

bare earth elevations that some other types of systems produce. This sometimes

results in discrepancies when the SRTM data is matched up with other topographic data

sets and it greatly complicated the use of external data to fill voids in the SRTM data.

The volume of data in the SRTM data set has also created challenges for researchers

attempting to use the data for spatial analysis. Toma noted that software that has been

used to successfully process lower resolution DEMs --like the GTOPO30 data-- can be

overwhelmed by the number of points in SRTM based data products. For example, whenused with the SRTM data, commonly used algorithms for applying flow routing analysis

to large areas were too slow to be of practical use and Toma describes new algorithms

developed to apply this kind of analysis to the SRTM data set. (Toma, 2001)

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Data Quality:

The SRTM mission data quality requirements were very specific:

1. The linear vertical absolute height error shall be less than 16 m for 90% of

the data.

2. The linear vertical relative height error shall be less than 10 m for 90% of

the data.

3. The circular absolute geolocation error shall be less than 20 m for 90% of

the data.

4. The circular relative geolocation error shall be less than 15 m for 90% of the

data.

The 90% figure cited in the requirement is a confidence level in keeping the National

Map Accuracy Standards. It means that at least 90% of the data meets the statedaccuracy requirement.

To verify that the requirements had been met NASA and the NGA compared data from

the SRTM with data from a variety of sources and from ground measurements called

ground truth data. This review showed that in general the data collected during the

SRTM exceeded the performance requirements of the mission by a factor of 2. As part of

this verification process, and to aid in the alignment and processing of the data, GPS

equipped vehicles were driven over long distances to create transects of location and

elevation data that could be compared with the SRTM data set. (Rodrguez)

The Data Products:

The SRTM produced a highly-accurate global-scale digital elevation model (DEM). A DEM

is a regular grid of location points with associated elevations that can be used for a

variety of mapping, map generation, and map analysis purposes. Because a DEM

consists of a regular array of points, the topography encoded in the DEM can be directly

represented as a raster by associating a pixel (or a regularly shaped group of pixels)

with each point. However, it is also possible to create a vector representation of a DEM

by creating a triangulated irregular network (TIN).

As discussed previously, processing the raw data to create usable data products was in

itself a sizeable project and in the seven years since the shuttle flight the raw data

gathered on the mission has been processed for use in a variety of ways. To understand

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what SRTM based data is available its helpful to first the put the different kinds of data

into classes:

SRTM Data Product Classes:

Class: Comments:

Unfinished (raw) The data as processed by the JPL without extra processing. The raw form data i

generally used by researchers.

Finished The finished class data sets have been enhanced by filling voids and fixing errors know

as spikes and wells. In addition, water boundaries have been enhanced. The finishe

data sets are most appropriate for map making and analysis using GIS.

Enhanced Enhanced data is based on the finished SRTM data but with additions and enhancemen

added by commercial data vendors. Enhanced DEMs based on the SRTM data ar

available from a variety of vendors.

Combined The SRTM DEM combined with other data sets

SRTM Based Data Sets:

Source: Comments:

National Elevation

Data (NED)

Class: combined

The SRTM has been

used to fill-in and

enhance the other

data sets used to

create the NED.

USGS

Online Access:

http://seamless.usgs.gov/

For Information:

http://eros.usgs.gov/products/elevation/ned.html

Also available on DVD at minimal cost

As of November of 2006 theNED replaced a variety ofdigital elevation data setsprevious offered by the USGSand simply called DEMs. TheDEMs were available atvarious levels of spatiaresolution and with varyinglevels of accuracy.

SRTM DETD Level 1

Class: finished

EROS

Online Access:


For Information:



3 arc second (90 meter)

global coverage

Where ever the level 1 and

level 2 data sets coincide they

have been aligned so that the

elevations on the two grids

match
http://eros.usgs.gov/products/elevation/ned.htmlhttp://eros.usgs.gov/products/elevation/ned.htmlhttp://eros.usgs.gov/products/elevation/ned.html

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SRTM DETD Level 2

Class: finished

EROS

Online Access:


For Information:



1 arc second (30) coverage ofthe United States andterritories

(Source: USGS SRTM Products Page - http://edc.usgs.gov/products/elevation.html, retrieved on June 182007)

One potential frustration of working with the SRTM data is that numerous references on

the Internet to the data and its availability are no longer valid. For example, the initia

release of the finished data set (in 2003 and 2004) was via an FTP server at the USGSs

EROS data center. Many journal articles and web resources refer to obtaining the data

from this source, but that server is no longer available.

The primary source for acquiring SRTM based data sets is now USGS seamless

distribution system found on the web athttp://seamless.usgs.gov/.

A good place to find an overview of the digital elevation data available from the USGS is:

http://eros.usgs.gov/products/elevation.html

Additional Sources for SRTM based data sets:

USGS U.S. Geological Survey (USGS)viewing, download, andmedia copies (http://seamless.usgs.gov/ or http://eros.usgs.gov/products/elevation.html or ftp://e0srp01u.ecs.nasa.gov/srtm/

University of Maryland Global LandCover Facility

http://glcf.umiacs.umd.edu/data/srtm/

Consultative Group for InternationalAgriculture Research

http://srtm.csi.cgiar.org/

German Aerospace Center http://www.dlr.de/srtm/index_en.html

Integrated Committee on EarthObservation Satellites(CEOS) European Data Server

http://iceds.ge.ucl.ac.uk/

(Gesch 2006)
http://eros.usgs.gov/products/elevation/ned.htmlhttp://eros.usgs.gov/products/elevation/ned.htmlhttp://seamless.usgs.gov/http://seamless.usgs.gov/http://seamless.usgs.gov/http://eros.usgs.gov/products/elevation.htmlhttp://eros.usgs.gov/products/elevation.htmlhttp://glcf.umiacs.umd.edu/data/srtm/http://glcf.umiacs.umd.edu/data/srtm/http://glcf.umiacs.umd.edu/data/srtm/http://www.dlr.de/srtm/index_en.htmlhttp://www.dlr.de/srtm/index_en.htmlhttp://www.dlr.de/srtm/index_en.htmlhttp://www.dlr.de/srtm/index_en.htmlhttp://glcf.umiacs.umd.edu/data/srtm/http://eros.usgs.gov/products/elevation.htmlhttp://seamless.usgs.gov/http://eros.usgs.gov/products/elevation/ned.html

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Using the Data:

The USGS Seamless Data Distribution system can be used to view SRTM data online

using a web browser and data sets can also be selected and downloaded. This site allows

data sets to be defined by selecting map boundary features or by dragging out a

selection rectangle to define the extent of the desired area. Using this site you can select

and download data from the finished Version Two 1 Arc Second (30 meter) or 3 Arc

Second (90 meter) data sets among many others.

A screen capture of the USGS Seamless Data Distribution system user interface. This map shows NewYork State's Catskill Mountains. The Hudson River is visible in the lower right corner. (retrieved on June21, 2007)

ftp://e0srp01u.ecs.nasa.gov/srtm/. In addition to the data itself, files found on this

server provide a good overview of how to use the data with popular GIS software suchas ArcView. Both the version one (unfinished) and version two (finished) data sets are

available from this server.

The data on this site is provided in the .HGT file format and can be opened directly the

the Global Mapper Application.

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A one degree square data SRTM DEM tiledisplayed in Global Mapper.

However, using the SRTM data in this format with GIS software like ArcView requires the

use of add-in software and/or special techniques. One approach, which involves the

creation of header file to allow a DEM tile to be opened and used in ArcView directly, is

described in detail on the FTP server referenced above. And a variety of tools to facilitate

using this data with both AcrView and other GIS software are readily available on theInternet.

But the easiest way to put the SRTM to use is download it from the Seamless Data

Distribution site. Files from this site are provided in the Arc shapefile format and can be

opened directly in ArcView.

The seamless DEM daThis file was downloDistribution site andshriking the image fosignificant loss of reso

Conclusion:

The Shuttle Radar Topography Mission is regarded by many as the most important

science mission ever flown by the Space Shuttle. The consistent coverage and accuracy

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of the SRTM data set has made it useful in research endeavors ranging from the

mapping and discovery of ancient irrigation channels in Mesopotamia (Hritz 2006) to the

creation of new algorithms for dealing with high resolution DEMs as the basis for spatia

analysis (Toma 2001).

The total value of the SRTM lies not only in the initial results, in this case the SRTM dataset, but also in the engineering and research done to support the project. There is every

reason to expect that new uses for the SRTM data will continue to be found for years to

come. And the snapshot of global topography that the mission produced might well

prove to be invaluable at some future time, possibly for reasons not yet even imagined.

Sources:

EROS SRTM Topo Documentation,

ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/SRTM_Topo.pdf, retrieved June 19, 2007

Farr, T. G., et al. (2007)The Shuttle Radar Topography Mission, Rev. Geophys., 45, RG2004,doi:10.1029/2005RG000183.http://www2.jpl.nasa.gov/srtm/SRTM_paper.pdf - retrieved June 19, 2007

Farr, Tom G.; Gesch, Dean B., Muller, Jan-Peter, The Shuttle Radar Topography Mission Data Validationand ApplicationsPHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, VOLUME 72, NUMBER 3, March 2006

Grohman, Greg; George Kroenung, and John Strebeck, Filling STRM Voids, the Delta Surface Fill Method,VOLUME 72, NUMBER 3PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING March 2006http://www.asprs.org/publications/pers/2006journal/march/ retrieved June 19, 2007

GTOPO30 Project Home Page, http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html, retrievedon June 18, 2007

Hanssen, Ramon F.; Radar Interferometry. Data Interpretation and Error Analysis, Kluwer AcademicPublishers 2001

Hritz, Carrie; Wilkinson, T. J.. Using Shuttle Radar Topography to map ancient water channels inMesopotamia. Antiquity, Jun2006, Vol. 80 Issue 308, p415-424, 10p; (AN 21735723)

Meade, Charles; Sandwll David T., Synthetic Aperture Radar for Geodesy, Science, New Series, Vol.273, No. 5279. (Aug. 30, 1996), pp. 1181-1182. JSTOR Stable URL: http://links.jstor.org/sici?sici=0036-8075%2819960830%293%3A273%3A5279%3C1181%3ASARFG%3E2.0.CO%3B2-N

NASA, Human Space Flight Images, keyword search: SRTMhttp://spaceflight.nasa.gov/gallery/search.cgi

NASA, Shuttle Radar Topography Mission Overview, NASA SRTM Websitehttp://www2.jpl.nasa.gov/srtm/missionoverview.html - retrieved on June 10, 2007

NASA, Shuttle Radar Topography Mission FAQs, NASA SRTM Website,http://www2.jpl.nasa.gov/srtm/faq.html - retrieved on June 9, 2007

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NASA, What Part of the Earth was Mapped?, NASA SRTM Websitehttp://www2.jpl.nasa.gov/srtm/coverage.html - retrieved on June 9, 2007

Slater, J.; The Generation and Dissemination of "Finished" SRTM Data Products by NGA NationalGeospatial-Intelligence Agency, Slide Presentation at the SRTM Data Validation and ApplicationsWorkshop, June 14-16 2005,The Generation and Dissemination of "Finished" SRTM Data Products by NGA

Toma, Laura; Rajiv Wickremesinghe, Lars Arge, Jeffery S. Chase, Jeffery Scott Vitter, Patrick N. Halpin,Dean Urban., Proceedings of the 9th ACM international symposium on Advances in geographic informationsystems GIS '01, November 2001,ACM Press

Wikipedia, Interferometric_synthetic_aperture_radar,

http://en.wikipedia.org/wiki/Interferometric_synthetic_aperture_radar, retrieved on June 18, 2007

Wikipedia, Shuttle Radar Topography Mission, http://en.wikipedia.org/wiki/SRTM, retrieved on June18, 2007

This document is published in fulfillment of an assignment by a student enrolled in an

educational offering of The Pennsylvania State University. The student, named above, retains

all rights to the document and responsibility for its accuracy and originality.

Page Modified: June 3, 2011

http://www.personal.psu.edu/kas667/geog482/project3/project3.html
http://eros.usgs.gov/conferences/SRTM/presentations/Slides02_Slater.pdfhttp://eros.usgs.gov/conferences/SRTM/presentations/Slides02_Slater.pdfhttp://www.personal.psu.edu/kas667/geog482/project3/project3.htmlhttp://www.personal.psu.edu/kas667/geog482/project3/project3.htmlhttp://www.personal.psu.edu/kas667/geog482/project3/project3.htmlhttp://eros.usgs.gov/conferences/SRTM/presentations/Slides02_Slater.pdf

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Cuestionario:

A. Responda: Valor 4.0

1. Cules son los Principales objetivos de la Misin topogrfica Radar Shuttle (SRTM)2. Qu instituciones trabajan en la SRTM?3. Qu modelo de elevacin previo (A escala mundial) exista antes de la SRTM?, Qu resolucin espacial tiene

este modelo?

4. El tamao de la antena determina alguna caracterstica en la SRTM? Por qu? Como corregirlo?5. Describa los problemas presentados en la transferencia y conservacin de los datos captados.6. Porque hay vacios en algunos modelos de elevacin?, cmo corregir esos vacios?7. Una segunda mejora en la informacin es realizada por la NGA debido a cuerpos de agua. Comente los

incidentes.

8. Que especificaciones en la calidad de los datos, cumple la SRTM9. Cual es la resolucin de los datos en la SRTM? (30 metros, 90 metros, los dos?)Que tipo de productos genera

la SRTM? quienes pueden usarlos?

B. Argumente Valor 1.0

10.Explique el principio usado en la adquisicin de datos, por el cual se genera el modelo de elevaciones. Quclase de sensor fue usado en esta misin? Estos sensores son activos o pasivos?

11.Qu importancia cree usted que tiene la SRTM en los estudios relacionados con la Geologa?

+

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