From: Wolfgang Bangerth Date: Fri, 29 Aug 2014 12:32:32 +0000 (-0500) Subject: Clarify where the data comes from. X-Git-Tag: v8.2.0-rc1~169^2~1 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=35cac50d2d31b64ba336bd1f0ee8c54d57195468;p=dealii.git Clarify where the data comes from. --- diff --git a/examples/step-53/doc/intro.dox b/examples/step-53/doc/intro.dox index a3da00ad63..bfcc8c1b16 100644 --- a/examples/step-53/doc/intro.dox +++ b/examples/step-53/doc/intro.dox @@ -174,8 +174,8 @@ of the sphere. However, the Earth is not a sphere: radians (though we will have to change this assumption in the code).
  • It has topography in the form of mountains and valleys. We will account for - this using topography data originally obtained from the US Geologic Survey - (USGS). Using this data set, we can look up elevations on a + this using real topography data (see below for a description of where + this data comes from). Using this data set, we can look up elevations on a latitude-longitude mesh laid over the surface of the Earth. Starting with the box $\hat U=[26,35]\times[-10,5]\times[-500000,0]$, we will therefore first stretch it in vertical direction before handing it off to the WGS 84 @@ -268,9 +268,12 @@ $h(\hat\phi,\hat\theta)$. There is of course no formula for this: Earth is what the best one can do is look up the altitude from some table. This is, in fact what we will do. -The data we use comes from the United States Geologic Survey (USGS) and was provided by -D. Sarah Stamps who also wrote the initial version of the WGS 84 transformation functions. -The topography data is stored in a file topography.txt.gz that, when unpackaed +The data we use was originally created by the Shuttle +Radar Topography Mission, was downloaded from the US Geologic Survey +(USGS) and processed by D. Sarah Stamps who also wrote the initial version of +the WGS 84 transformation functions. The topography data so processed is +stored in a file topography.txt.gz that, when unpacked looks like this: @code 6.983333 25.000000 700 diff --git a/examples/step-53/topography.license b/examples/step-53/topography.license new file mode 100644 index 0000000000..d54582aace --- /dev/null +++ b/examples/step-53/topography.license @@ -0,0 +1,227 @@ +topography.txt.gz contains data from the Shuttle Radar Topography Mission and +was originally downloaded from http://dds.cr.usgs.gov/srtm/version1/, +following by some data processing by D. Sarah Stamps (MIT). The documentation +accompanying SRTM data can be found at +http://dds.cr.usgs.gov/srtm/version1/Documentation/SRTM_Topo.txt and contains +further information about measurement method, data layout (of the original, +not the processed data), copyright holders and conditions and is reproduced in +full below. In particular, note that the SRTM level 1 data corresponds to the +statement "In advance of that, the unedited data are being released for public +use subject to the caveats discussed below." This implies that the data is in +the public domain and can be used for the purpose of this tutorial program and +all modifications you may want to make to it. Similar statements about the +data being in the public domain can be found at + http://dds.cr.usgs.gov/srtm/version1/Documentation/SRTM_Topo.txt + http://wiki.openstreetmap.org/wiki/SRTM + http://www.earthmodels.org/data-and-tools/topography/srtm + https://groups.google.com/forum/#!topic/osmand/9nj3GY1KBGI + +................................................ + +SRTM_Topo (last update 11/05/03) + +SRTM Documentation (best viewed with mono-spaced font, such as courier) + +1.0 Introduction + +The SRTM data sets result from a collaborative effort by the National +Aeronautics and Space Administration (NASA) and the National Imagery and +Mapping Agency (NIMA), as well as the participation of the German and +Italian space agencies, to generate a near-global digital elevation model +(DEM) of the Earth using radar interferometry. The SRTM instrument +consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified +with a Space Station-derived mast and additional antennae to form an +interferometer with a 60 meter long baseline. A description of the SRTM +mission, can be found in Farr and Kobrick (2000). + +Synthetic aperture radars are side-looking instruments and acquire data +along continuous swaths. The SRTM swaths extended from about 30 degrees +off-nadir to about 58 degrees off-nadir from an altitude of 233 km, and +thus were about 225 km wide. During the data flight the instrument was +operated at all times the orbiter was over land and about 1000 individual +swaths were acquired over the ten days of mapping operations. Length of the +acquired swaths range from a few hundred to several thousand km. Each +individual data acquisition is referred to as a "data take." + +SRTM was the primary (and pretty much only) payload on the STS-99 mission +of the Space Shuttle Endeavour, which launched February 11, 2000 and flew +for 11 days. Following several hours for instrument deployment, activation +and checkout, systematic interferometric data were collected for 222.4 +consecutive hours. The instrument operated virtually flawlessly and imaged +99.96% of the targeted landmass at least one time, 94.59% at least twice +and about 50% at least three or more times. The goal was to image each +terrain segment at least twice from different angles (on ascending, or +north-going, and descending orbit passes) to fill in areas shadowed from +the radar beam by terrain. + +This 'targeted landmass' consisted of all land between 56 degrees south and +60 degrees north latitude, which comprises almost exactly 80% of the total +landmass. + +2.0 Data Set Characteristics + +2.1 General + +SRTM data were processed in a systematic fashion using the SRTM Ground +Data Processing System (GDPS) supercomputer system at the Jet Propulsion +Laboratory. Data were mosaicked into approximately 15,000 one degree by +one degree cells and formatted according to the Digital Terrain Elevation +Data (DTED) specification for delivery to NIMA, who will use it to update +and extend their DTED products. Data were processed on a +continent-by-continent basis beginning with North America. NIMA is applying +several post-processing steps to these data including editing, spike and well +removal, water body leveling and coastline definition. Following these +"finishing" steps data will be returned to NASA for distribution to the +scientific and civil user communities, as well as the public. In advance of +that, the unedited data are being released for public use subject to the +caveats discussed below. + +2.2 Organization + +SRTM data are organized into individual rasterized cells, or tiles, each +covering one degree by one degree in latitude and longitude. Sample spacing +for individual data points is either 1 arc-second or 3 arc-seconds, +referred to as SRTM-1 and SRTM-3, respectively. Since one arc-second at the +equator corresponds to roughly 30 meters in horizontal extent, the sets are +sometimes referred to as "30 meter" or "90 meter" data. + +Unedited SRTM-3 data are being released continent-by-continent, with the +definitions of the continents displayed in the file Continent_def.gif. +By agreement with NIMA unedited SRTM-1 data for the United States and its +territories and possessions are also being released and can be found in +the directory /United_States_1arcsec./ Cells that straddle the border with +neighboring countries have been masked with quarter degree quantiation +such that data outside the U.S. have the void value. + +2.3 Elevation mosaics + +Each SRTM data tile contains a mosaic of elevations generated by averaging +all data takes that fall within that tile. Since the primary error source +in synthetic aperture radar data is speckle, which has the characteristics +of random noise, combining data through averaging reduces the error by the +square root of the number of data takes used. In the case of SRTM the +number of data takes could range from a minimum of one (in a very few +cases) up to as many as ten or more. + +3.0 Data Formats + +The names of individual data tiles refer to the longitude and latitude of +the lower-left (southwest) corner of the tile (this follows the DTED +convention as opposed to the GTOPO30 standard). For example, the +coordinates of the lower-left corner of tile N40W118 are 40 degrees north +latitude and 118 degrees west longitude. To be more exact, these +coordinates refer to the geometric center of the lower left pixel, which in +the case of SRTM-1 data will be about 30 meters in extent. + +SRTM-1 data are sampled at one arc-second of latitude and longitude and +each file contains 3601 lines and 3601 samples. The rows at the north +and south ecges as well as the columns at the east and west edges of each +cell overlap and are identical to the edge rows and columns in the adjacent +cell. + +SRTM-3 data are sampled at three arc-seconds and contain 1201 lines and +1201 samples with similar overlapping rows and columns. This organization +also follows the DTED convention. Unlike DTED, however, 3 arc-second data +are generated in each case by 3x3 averaging of the 1 arc-second data - thus +9 samples are combined in each 3 arc-second data point. Since the primary +error source in the elevation data has the characteristics of random noise +this reduces that error by roughly a factor of three. + +This sampling scheme is sometimes called a "geographic projection", but of +course it is not actually a projection in the mapping sense. It does not +possess any of the characteristics usually present in true map projections, +for example it is not conformal, so that if it is displayed as an image +geographic features will be distorted. However it is quite easy to handle +mathematically, can be easily imported into most image processing and GIS +software packages, and multiple cells can be assembled easily into a larger +mosaic (unlike the pesky UTM projection, for example.) + +3.1 DEM File (.HGT) + +The DEM is provided as 16-bit signed integer data in a simple binary +raster. There are no header or trailer bytes embedded in the file. The data +are stored in row major order (all the data for row 1, followed by all the +data for row 2, etc.). + +All elevations are in meters referenced to the WGS84 EGM96 geoid as +documented at http://www.nima.mil/GandG/wgsegm/. + +Byte order is Motorola ("big-endian") standard with the most significant +byte first. Since they are signed integers elevations can range from -32767 +to 32767 meters, encompassing the range of elevation to be found on the +Earth. + +In these preliminary data there commonly will be data voids from a number of +causes such as shadowing, phase unwrapping anomalies, or other +radar-specific causes. Voids are flagged with the value -32768. + + +4.0 Notes and Hints for SRTM Data Users + +4.1 Data Encoding + +Because the DEM data are stored in a 16-bit binary format, users must be +aware of how the bytes are addressed on their computers. The DEM data are +provided in Motorola or IEEE byte order, which stores the most significant +byte first ("big endian"). Systems such as Sun SPARC and Silicon Graphics +workstations use the Motorola byte order. The Intel byte order, which +stores the least significant byte first ("little endian"), is used on DEC +Alpha systems and most PCs. Users with systems that address bytes in the +Intel byte order may have to "swap bytes" of the DEM data unless their +application software performs the conversion during ingest. + +4.3 SRTM Caveats + +As with all digital geospatial data sets, users of SRTM must be aware of +certain characteristics of the data set (resolution, accuracy, method of +production and any resulting artifacts, etc.) in order to better judge its +suitability for a specific application. A characteristic of SRTM that +renders it unsuitable for one application may have no relevance as a +limiting factor for its use in a different application. + +In particular, data produced by the PI processor should be considered as +"research grade" data suitable for scientific investigations and +development and testing of various civil applications. + +No editing has been performed on the data, and the elevation data in +particular contain numerous voids and other spurious points such as +anomalously high (spike) or low (well) values. Water bodies will generally +not be well-defined - in fact since water surfaces generally produce very +low radar backscatter they will appear quite "noisy" or rough, in the +elevations data. Similarly, coastlines will not be well-defined. + +5.0 References + +Farr, T.G., M. Kobrick, 2000, Shuttle Radar Topography Mission produces a +wealth of data, Amer. Geophys. Union Eos, v. 81, p. 583-585. + +Rosen, P.A., S. Hensley, I.R. Joughin, F.K. Li, S.N. Madsen, E. Rodriguez, +R.M. Goldstein, 2000, Synthetic aperture radar interferometry, Proc. IEEE, +v. 88, p. 333-382. + +DMATR 8350.2, Dept. of Defense World Geodetic System 1984, Its Definition +and Relationship with Local Geodetic Systems, Third Edition, 4 July 1997. +http://164.214.2.59/GandG/tr8350_2.html + +Lemoine, F.G. et al, NASA/TP-1998-206861, The Development of the Joint NASA +GSFC and NIMA Geopotential Model EGM96, NASA Goddard Space Flight Center, +Greenbelt, MD 20771, U.S.A., July 1998. + +Other Web sites of interest: + +NASA/JPL SRTM: http://www.jpl.nasa.gov/srtm/ + +NIMA: http://164.214.2.59/nimahome.html + +STS-99 Press Kit: http://www.shuttlepresskit.com/STS-99/index.htm + +Johnson Space Center STS-99: +http://spaceflight.nasa.gov/shuttle/archives/sts-99/index.html + +German Space Agency: http://www.dlr.de/srtm + +Italian Space Agency: http://srtm.det.unifi.it/index.htm + +U.S. Geological Survey, EROS Data Center: http://edc.usgs.gov/ + +Note: DTED is a trademark of the National Imagery and Mapping Agency