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Cartography
The techniques concerned with
constructing maps from geographic information. Maps are spatial representations
of the environment. Typically, maps take graphic form, appearing on computer
screens or printed on paper, but they may also take tactile or auditory forms
for the visually impaired. Other representations such as digital files of
locational coordinates or even mental images of the environment are also
sometimes considered to be maps, or virtual maps.
Maps and
uses
Because the environment is complex
and everchanging, the variety of maps and map uses is unlimited. For instance,
maps are indispensable tools for navigating over land, sea, or air. Maps are
effective both in exploiting natural resources and in protecting them. They are
used to investigate geographic phenomena, including environmental pollution,
climate change, even the spread of diseases and the distribution of social
phenomena such as poverty and illiteracy. In addition, maps can be used to
communicate insights derived from geographic research through publication in
periodicals and books and through distribution over computer networks and
broadcast media. Every private business, government agency, and academic
discipline whose products, services, or objects of study are geographically
dispersed benefits from detailed, up-to-date maps. Unfortunately, appropriate
maps often are unavailable.
Map scale and geographic
detail
Maps often include insufficient or
excessive detail for the task at hand. The amount of usable detail on a map
varies with its scale, because human visual acuity and the resolution of
printing and imaging devices are limited. Maps that depict extensive areas in
relatively small spaces are called small-scale maps. For example, on a 1-ft-wide
(30-cm) map of the world, on which the ratio of map distance to ground distance
is approximately 1:125,000,000, very little perceptible detail can be preserved.
As the scale of a map increases, so may the level of geographic detail it
represents. Geographic features selected to appear on small-scale maps must be
exaggerated in size and simplified in shape so as to be recognizable by the map
user. These map generalization operations constitute an intriguing field of
research by cartographers attempting to formalize, and ultimately to automate,
the map creation process.
Reference
maps
Topographic maps record the
positions and elevations of physical characteristics of the landscape. They
serve as locational dictionaries for many endeavors, including environmental
planning, resource management, and recreation. The 1:24,000 scale United States
Geological Survey (USGS) series covers the continental
Fig. 1 Large-scale topographic mapping, percent
area coverage area at 1:1000 to 1:36,680 showing the uneven spatial distribution
of map coverage. (After United Nations, World Cartography, vol. 20,
1990)
Thematic
maps
Another problem with available maps
is that they often fail to include a feature of particular interest. Maps that
emphasize one or a few related geographic phenomena in the service of a specific
purpose are called thematic maps. An example is a thematic map that reveals the
uneven distribution of topographic map coverage around the world (Fig. 1).
Thematic maps are powerful alternatives to text, tables, and graphs for
visualizing potentially meaningful patterns in geographic information. Although
the production of large-scale topographic map series requires the resources of
large private or government agencies, individuals and small organizations with
access to relatively inexpensive personal computers, mapping software, and
databases can afford to produce thematic maps in support of business,
scientific, political, and creative endeavors.
Constructing geographic
information
Maps are composed of two kinds of
geographic information: attribute data and locational data. Attribute data are
quantitative or qualitative measures of characteristics of the landscape, such
as terrain elevation, land use, or population density. Locations of features on
the Earth's surface are specified by use of coordinate systems; among these, the
most common is the geographical coordinate system of latitudes and longitudes.
Geographical coordinates describe
positions on the spherical Earth. These must be transformed to positions on a
two-dimensional plane before they can be depicted on a printed sheet or a
computer screen. Hundreds of map projections—mathematical transformations
between spherical and planar coordinates—have been devised, but no map
projection can represent the spherical Earth in two dimensions without
distorting spatial relationships among features on Earth's surface in some way.
One specialized body of knowledge that cartographers bring to science is the
ability to specify map projections that preserve the subset of geometric
characteristics that are most important for particular mapping applications.
Prior to World War II, locational
data were compiled mainly by field surveys. Aerial surveillance techniques
developed for the war effort were then adapted for use in civilian mapmaking.
The scale distortions inherent in aerial photographs can be corrected by
photogrammetric methods, yielding planimetrically correct projections on which
all locations appear to be viewed simultaneously from directly above. Rectified
aerial photographs (orthophotos) can be used either as bases for topographic
mapping or directly as base maps.
See also: Aerial photography; Latitude and longitude; Photogrammetry
Influence of computing
technology
Periodically, cartographic practice
has been transformed by new technologies. Few have had such a profound effect as
the development of computer-based mapping techniques. While printed paper maps
still constitute the richest store of geographic information, cartography has
become as much a digital as a paper-based enterprise. With more and more
geographic data available in digital form, the computer has changed the very
idea of a map from a static caricature of the environment to a dynamic interface
for generating and testing hypotheses about complex environmental and social
processes.
Digital geographic
data
There are two major approaches to
encoding geographic data for computer processing. One, commonly called raster
encoding, involves sampling attribute values at some regular interval across the
landscape. Imagery scanned from Earth-observing satellites works this way,
recording surface reflectance values for grid cells (pixels) from 80 to 30 m
(250 to 100 ft) or less in resolution. Digital elevation models are matrices of
terrain elevations derived from satellite imagery or sampled from topographic
maps (Fig. 2).
Fig. 2 Computer rendering of the topography of
the 48 contiguous
A second method, known as vector
encoding, involves digitizing outlines of landscape features that are
homogeneous with regard to some attribute, such as a river, a watershed, a road,
or a state boundary. Vector encoding is more expensive than raster encoding, but
it is more flexible for many applications. One
Although the raster and vector
approaches for digital encoding predominate, these have been implemented in
dozens of idiosyncratic data formats designed to suit individual mapping
agencies and computer vendors. Incompatible formats have impeded data sharing
and have resulted in expensive redundancies in database construction. A
committee of
Constructing geographic
understanding
Geographic illiteracy is thought to
put afflicted societies at a disadvantage in an increasingly integrated
international economy. Access to geographic information is a necessary but
insufficient condition for constructing geographic understanding. Access to
analytical expertise is required to learn from the available information.
Geographic information
systems
Guiding much of the research and
development efforts of academic cartographers is the concept of automated
geography—an amalgam of computer databases and procedures by which analysts
might model, simulate, and ideally predict the behavior of physical and social
systems on the landscape. Concurrently, increasing social concern for
environmental protection stimulates a market for computerized geographic
information systems (GIS) that combine mapping capabilities with techniques in
quantitative spatial analysis. Many of the analytical procedures in these
systems—such as calculations of distances, areas, and volumes; of terrain
surface slope and aspect; defining buffer regions surrounding landscape
features; and generating maps of new features formed by the intersection of
several related map layers—have been codified by cartographers. Some
cartographers have investigated the potential of computerized expert systems
that may be used to assist nonspecialists in performing quantitative geographic
analyses. See also: Expert systems;
Geographic information systems
Cartographic
communication
With few exceptions, the outcomes of
analyses performed with geographic information systems are maps. Just as
careless or biased quantitative analyses result in erroneous conclusions, so can
unskilled map designs mislead users, and even the analysts themselves. Concern
for the integrity of thematic maps motivated cartography's largest research
project—the search for objective guidelines, based on psychological research, to
optimize map communication. Map design issues that have garnered the most
attention include classification techniques for grouping attribute data into
discernible map categories and logical systems for choosing appropriate graphic
symbols for representing different types of attribute data.
Interactive
cartography
Although many broadly applicable map
design principles have been established, the goal of specifying an optimal map
for a particular task is less compelling than it once was. Instead, there is
interest in the potential of providing map users with multiple, modifiable
representations via dynamic media such as CD-ROM, computer networks, and
interactive television. Even as computer graphics technologies and numerical
models provide ever more realistic environmental simulations, innovative, highly
abstract display methods are being developed and tested to help analysts
discover meaningful patterns in multivariate geographic data sets (Fig. 3).
Maps, graphs, diagrams, movies, text, and sound can be incorporated in
multimedia software applications that enable users to navigate through vast
electronic archives of geographic information. Interactive computer graphics are
eliminating the distinction between the mapmaker and the map user. Modern
cartography's challenge is to provide access to geographic information and to
cartographic expertise through well-designed user interfaces. See also: Land-use planning; Map design;
Map projections
David DiBiase
Fig. 3 Three geographic data variables related
in a scatterplot matrix and linked to a map. Locations of observations selected
in a scatterplot are automatically highlighted on the map. (From M. Monmonier,
Geographic brushing: Enhancing exploratory analysis of the scatterplot matrix,
Geog. Anal., 21(1):81–84, 1989)
Bibliography
G. L. Gaile and C. J. Willmott,
Geography in
J. Makower (ed.), The Map Catalog: Every
Kind of Map and Chart on Earth and Even Some Above It, 3d ed., 1992
P. C. Muehrcke, Map Use:
D. Wood, The Power of Maps, 1992
Ali
fazeli=egeology.blogfa.com
Brachiopoda
A phylum of solitary,
exclusively marine, coelomate, bivalved animals, with both valves symmetrical
about a median longitudinal plane. Brachiopods are typically attached to the
substrate by a posteriorly located cuticle-covered stalk called a pedicle.
Anteriorly, a relatively large mantle cavity is always developed between the
valves, and the ciliated tentacular (filamentous) feeding organ, or lophophore,
is suspended within, projecting anteriorly from the anterior body wall (Fig. 1).
The name Brachiopoda means “arm foot” and refers to the morphology and location
of the lophophore.
Fig. 1 Principal organs of a brachiopod as
typified by Terebratulina. (After R. C. Moore, ed., Treatise on Invertebrate
Paleontology, pt. H, Geological Society of America, Inc., and University of
Kansas Press, 1965)
This brief description
serves to differentiate a brachiopod from any other animal. Within the
Brachiopoda, three groups currently regarded as subphyla are recognized, each
named after their most ancient living representative: Linguliformea,
Craniiformea, Rhynchonelliformea. Phoroniformea are recognized by some as a
fourth subphylum. The two classes (Inarticulata and Articulata), formerly
distinguished by the presence or absence of articulation between the two valves
of the shell, are no longer recognized as distinct taxa; rather, they appear to
represent the ends of a spectrum of articulatory types, some well delineated,
others less so. Rhynchonelliforms have impunctate, endopunctate, or
pseudopunctate calcitic shells with a fibrous (or laminar) shell structure; most
are articulated, with the valves typically hinged together by a pair of ventral
teeth with complementary sockets in the dorsal valve; most have pedicles with a
core of connective tissue. Craniiforms have punctate calcitic shells with a
laminar (or tabular) shell structure; all lack articulation, with valves that
are held together only by the soft tissue of the living animal, and all lack
pedicles. Linguliforms have canaliculate organophosphatic shells with a
stratiform shell structure; all lack articulation; and all have pedicles with a
coelomate core. See also:
Inarticulata; Lingulida; Rhynchonelliformea
Phylogeny and
classification
Molecular sequence
data from nuclear and mitochondrial genes in extant brachiopods provide strong
support for brachiopods as protostomous organisms, and not deuterostomes, as
some earlier morphological and developmental data seemed to suggest. These data
also indicate that the Phoronida is nested within the Brachiopoda, and is not
the brachiopod sister group, as previously thought. Lophophorates alone do not
appear to form a clade; the Bryozoa are more distantly related within the
Lophotrochozoa, a rather poorly resolved clade that also includes mollusks and
annelids, among other phyla (Fig. 2). Only approximately 5% of named brachiopod
genera (and most likely, species) are extant; most of our current understanding
of brachiopod phylogeny results from a composite of molecular data from a small
taxonomic sample, and morphological (and stratigraphic) data from a much larger
sample of extinct fossil taxa. See
also: Bryozoa; Phoronida; Rhynchonelliformea
Fig. 2 Phylogenetic relationships among
selected metazoans. (Simplified from K. M. Halanych and Y. Passamaneck, Amer.
Zool., 41:629–639, 2001; K. M. Halanych, Annu. Rev. Ecol. Evol. Sys.,
35:229–256, 2004; and B. L. Cohen and A. Weydmann, Organisms, Diversity and
Evolution, 5(4):253–273, 2005)
The phylum is
currently classified as follows:
Subphylum
Linguliformea
Class
Lingulata
Order:
Lingulida
Siphonotretida
Acrotretida
Class
Paterinata
Order
Paterinida
Subphylum Craniiformea
Class
Craniata
Order:
Craniida
Craniopsida
Trimerellida
Subphylum
Phoroniformea (?)
Subphylum
Rhynchonelliformea
Class
Chileata
Order:
Chileida
Dictyonellida
Class
Obolellata
Order:
Obolellida
Naukatida
Class
Kutorginata
Order:
Kutorginida
Class
Strophomenata
Order:
Strophomenida
Billingsellida
Orthotetida
Productida
Class
Rhynchonellata
Order:
Protorthida
Orthida
Pentamerida
Rhynchonellida
Atrypida
Spiriferida
Spiriferinida
Thecideida
Athyridida
Terebratulida
Orientation
The two brachiopod
valves are currently referred to as the dorsal and ventral valves, not brachial
and pedicle, because not all brachiopods possess pedicles. This body orientation
distinguishes them from bivalved mollusks, which have valves on the left and
right sides of the body. While topologically dorsal and ventral in adults, their
developmental orientation is not entirely clear; both valves of some extant
craniiforms may derive from the dorsal surface of the developing, modified
trochophore larva. The pedicle either protrudes between the valves (through the
delthyrium and notothyrium, which are small triangular apertures serving as
pedicle openings), or (Fig. 3) more commonly emerges from a variably modified
opening (pedicle foramen) in the ventral valve, which, in articulated forms, is
the valve bearing the hinge teeth. Whatever the form of the pedicle opening, it
is always at the posterior end of the animal and its enclosing shells, the
opposite end being regarded as anterior. Both valves have a characteristic
distribution of muscles, and any skeletal support for the lophophore is
invariably developed from the dorsal valve (Fig. 4).
Fig. 3 Diagrammatic representation of the
external features of a generalized brachiopod seen in (a) posterior, (b) left
lateral, (c) dorsal, and (d) dorsolateral views. (After R. C. Moore, ed.,
Treatise on Invertebrate Paleontology, pt. H, Geological Society of America,
Inc., and University of Kansas Press, 1965)
Fig. 4 Generalized representation of
distribution of epithelium in relation to other tissues and organs in (a)
lingulides and (b) terebratulides. (After R. C. Moore, ed., Treatise on
Invertebrate Paleontology, pt. H, Geological Society of America, Inc., and
University of Kansas Press, 1965)
Anatomy
The pedicle is the
only organ protruding outside the valves, for the remainder of the animal is
enclosed in the space between them. This space is divided into two unequal
parts, a smaller posteriorly located body cavity and an anterior mantle cavity.
The ectodermal outer epithelium underlying the shell bounding the body cavity is
in a single layer; but anteriorly, laterally, and even posteriorly in most
inarticulated brachiopods, it is prolonged as a pair of folds forming the
ventral and dorsal mantles, from which the shells are mineralized (at the
generative zones). In extant species, the two mantles approach each other and
ultimately fuse along the posterior margin of rhynchonelliform brachiopods; in
contrast, the mantles are invariably discrete in craniiforms and linguliforms,
and are separated by a strip of body-wall inner epithelium (Fig. 4).
The body cavity
contains the musculature; the alimentary canal; the mixonephridia, which are
paired excretory organs also functioning as gonoducts; the reproductive organs;
and the rather poorly understood circulatory and nervous systems. Except for the
openings through the mixonephridia, the body cavity is enclosed, but the mantle
cavity communicates freely with the sea when the valves are opened. The
lophophore is a feeding and respiratory organ, typically suspended from the
anterior body wall within the mantle cavity, and is always symmetrically
disposed about the median plane. The lophophore consists of a ciliated,
filament-bearing tube with two arms in varying configurations. The ciliary beat
produces a laminar flow of water into and then out of the mantle cavity, flowing
across the filaments while inside. The latter trap food particles which are
carried along a groove in the lophophore to the mouth situated medially between
the two arms. See also: Lophophore
The alimentary canal
of all brachiopods is broadly similar in structure, but differs in orientation
in the body cavity. The mouth opens into a muscular tube, the esophagus, which
continues to a stomach and intestine. In addition, there are a variable number
of digestive diverticula that communicate with the stomach through narrow ducts
(Fig. 1). In rhynchonelliform brachiopods, the intestine curves into a C-shape
and ends blindly (with no anus) in a posteroventral location; in linguliforms,
it curves into a U-shape, leaving the anus right-lateral or ventrolateral; in
phoronids, it is also U-shaped, but the anus is anterodorsal; in craniiforms,
the intestine is straight and does not curve or fold, leaving the anus
medioposterior.
The diductor and
adductor muscles that control the opening and closing of the valves are
contained within the body cavity (Fig. 1), but their distribution varies among
the three brachiopod subphyla. In most rhynchonelliforms, they are disposed to
effect a rotation of the valves about a hinge axis located on the valves; in
craniiforms and linguliforms, the hinge axis is located in the viscera between
the valves; in selected linguliforms, the two valves can “scissor” past one
another, allowing burrowing in a soft substrate. Adjustor muscles effect
movement of the shell relative to the pedicle, and other muscles may be present
that can move the lophophore slightly relative to the valves. Because of a
differential rate of secretion of shell material by the epithelium at the bases
of the muscles, the site of muscle attachment is commonly impressed in the
valves, producing muscle scars. Rarely, the muscle scars may be elevated above
the adjacent shell.
Although some
small-bodied species are herma-phroditic and brood their larvae, the sexes are
separate in the majority of brachiopods. The gonads, or reproductive organs, are
located either within the body cavity (as in the lingulids and discinids) or
more typically in slender tubelike extensions of the body cavity which project
into the mantle, called the mantle canals (in craniids and rhynchonelliforms).
The mantle canal pattern may be retained on the inner surface of the valves,
even in fossils, by processes of differential secretion comparable with those
producing the muscle scars.
Given the phylogenetic
nesting of phoronids within brachiopods, the anatomy of Phoronida can be
interpreted as having evolved from a brachiopod body plan; phoronid anatomy is
modified largely as a reflection of the loss of the two mineralized valves. See also: Phoronida
Embryology and
ontogeny
Linguliforms,
craniiforms, phoronids, and rhynchonelliforms share numerous aspects of
embryogenesis and larval development, yet each has distinct characteristics not
shared with any others (Fig. 5). Extant linguliforms and phoronids are
planktotrophic and have likely remained so from their origin in the Cambrian (or
earlier); extant craniiforms and rhynchonelliforms are lecithotrophic, and have
each evolved independently from a planktotrophic ancestral state. Planktotrophic
larvae remain in the water column for weeks to months, while lecithotrophic
larvae remain in the water column only briefly, usually less than a week; these
differences have important implications for the timing of differentiation of
adult structures, and for the ability of the larvae to disperse
biogeographically. Planktotrophic brachiopods develop more structures as larvae,
prior to metamorphosis, and lecithotrophs develop more structures following
metamorphosis, as young juveniles. Shell deposition begins at metamorphosis in
all three subphyla, although mantle formation can begin during embryogenesis or,
more commonly, during the larval growth period, just prior to metamorphosis.
Fig. 5 Comparison of fate maps and selected
developmental stages of the three brachiopod subphyla and Phoroniformea,
considered by some to be a fourth brachiopod subphylum. (a) Fate maps of
uncleaved eggs. (b) 16-cell embryos. (c) Late gastrula stage. (d) Lateral views
of the early larva. (Reprinted from G. Freeman, Developmental Biology,
261:263–287, 2003, with permission from
Elsevier)
Larvae in all three
subphyla share a three-part body organization: an apical lobe, from which the
lophophore differentiates in the larval stage of linguliforms and immediately
following metamorphosis in craniiforms and rhynchonelliforms; a mantle lobe,
from which the mantle (on which the periostracum and shells later mineralize),
alimentary canal, and most of the viscera develop; and a pedicle lobe, from
which the pedicle develops in linguliforms during the larval stage, and in
rhynchonelliforms following settlement and metamorphosis. In rhynchonelliforms
alone, the mantles are reversed after metamorphosis so that they are oriented
anteriorly, partially covering what was the apical lobe of the larva, and with
the original inner surface of the mantle forming the outer surface of the
organism. The mantles subsequently secrete the earliest, first-formed shell, the
protegulum, which enlarges by terminal accretion during later shell growth. Only
after metamorphosis does the rudimentary lophophore, alimentary canal, and adult
musculature develop. Rhynchonelliforms and phoronids share many aspects of
embryogenesis (Fig. 5a–c), providing further support for genetic data suggesting
they evolved among brachiopods. Craniiforms do not develop pedicles or a larval
pedicle lobe, but the larvae settle at their posteriormost ends, comparable to
where a pedicle lobe would be located. Craniiforms do not undergo mantle
reversal.
Linguliform larvae are
much like miniature adults that undergo minimal morphological change at
metamorphosis, having developed most of their adult features while larvae in the
plankton. The two mantle rudiments are separated from each other early in larval
life, and are not fused along the posterior margin in the manner characteristic
of rhynchonelliform brachiopods.
Shell morphology
The two valves are
commonly of unequal size, with the ventral valve typically larger. Because the
shell increases in size by increments laid down at the mantle margin,
ontogenetic changes in shape are faithfully recorded by growth lines. The valves
may be further ornamented by concentric folds (rugae), growth lamellae, or
radially disposed ribs of various amplitude and wavelength. Numerous extinct
genera are characterized by extravagant development of spines. In
rhynchonelliforms, the posterior region of one or both valves is commonly
differentiated from the remainder of the valve as a somewhat flattened cardinal
area. The ventral area is typically further modified by the pedicle opening;
among articulated brachiopods this consists of a triangular delthyrium which may
be partially closed by a pseudodeltidium or a pair of deltidial plates. A
corresponding triangular opening, the notothyrium, may be developed on the
dorsal cardinal area. Internally, muscle scars and mantle canal impressions may
be apparent, together with structures of varying complexity associated with
articulation (for example, dental and socket plates) and support of the
lophophore (crura, spiralia, loops). Calcite spicules are present in the mantle
tissues of some extant terebratulides. Linguliform valves are less heavily
mineralized and more organic-rich than rhynchonelliforms. Extant craniiforms,
cemented to a hard substrate, have ventral valves that are flat and extremely
thin, with gently cap-shaped dorsal valves.
Shell morphology
varies considerably externally in overall shape and size, relative biconvexity,
and degree of ornamentation. Shell morphology also varies internally in
structures involved in articulation, lophophore support, and muscle position,
from order to order, among the 26 orders recognized. This extensive variation
makes it difficult to succinctly characterize overall trends. See also: Rhynchonellida; Terebratulida
Ecology and
biogeography
All modern brachiopods
are marine, and there is little doubt from the fossil record that brachiopods
have always been confined to the sea. A few genera, however, notably the closely
related linguliform brachiopods Lingula and Glottidia, can tolerate reduced
salinities and may survive in environments that would be lethal to the majority
of forms. Recent brachiopods occur commonly beneath the relatively shallow
waters of the continental shelves, which seem to have been the most favored
environment in terms of diversity and abundance, but the bathymetric range of
the phylum is very large. Some modern species live intertidally and, at the
other extreme, some have been dredged from depths of over 16,500 ft (5000 m).
The majority of
brachiopods form part of the sessile benthos and are attached by their pedicle
during postlarval life. Paleozoic brachiopods not uncommonly exhibit boreholes
penetrating their shells, suggesting death by boring predation; interestingly,
modern brachiopods have very few known predators, boring or otherwise, even
though most are epifaunal, typically living on hard substrates. Glottidia and
Lingula are exceptional in being infaunal and making burrows with the help of
complex musculature connecting the two valves that lack articulation. The loss
of the pedicle has occurred multiple times over the course of brachiopod
evolution, by either complete suppression or atrophy early in the life of the
individual. Such forms lie free on the sea floor, are attached by cementation of
part or all of the ventral valve, or are anchored by spines. The geographic
distribution and geological setting of some fossil species suggest that they may
have been epiplanktonic, attached to floating weed, but such a mode of life is
unknown in modern faunas.
Biogeographically,
brachiopods today exhibit an antitropical distribution, with their highest
diversity closer to the Poles than to the tropics, and higher in the Southern
Hemisphere than the Northern Hemisphere. Paleobiogeographical data from the
distribution of fossil species indicate that Paleozoic brachiopods exhibited a
more typical latitudinal diversity gradient, with high tropical diversity,
decreasing toward the polar regions, such as is observed today in the majority
of marine invertebrates.
Taxonomic diversity and stratigraphic
distribution
Brachiopods formed a
major part of skeletonized marine faunas throughout the Paleozoic; linguliforms
follow a pattern of diversity through time more or less typical for the Cambrian
Evolutionary Fauna described by J. J. Sepkoski, while rhynchonelliforms typify
the Paleozoic Evolutionary Fauna. The oldest undoubted brachiopods, the
paterinates, occur in the Lower Cambrian Tommotian stage. Representatives of
seven of the eight classes are known from the Lower Cambrian, indicating that
morphological diversity was high even very early in the history of the phylum.
Throughout the Cambrian, linguliforms were more abundant than rhynchonelliforms;
but during the Early Ordovician radiation, rhynchonelliform diversity and
abundance both increased dramatically and remained high until the end-Permian.
In the late Paleozoic, a number of major brachiopod groups became extinct, and
the end-Permian extinction event caused diversity to plummet. Four of the five
extant orders survived this extinction event, and rediversified, although much
more modestly through the Mesozoic and Cenozoic than in the Paleozoic. In the
present-day oceans, approximately 120 genera are recognized, dominated by
rhynchonelliforms (terebratulides and rhynchonellides in particular). See also: Extinction (biology);
Paterinida; Rhombifera
S. J. Carlson
A. J. Rowell
Bibliography
S. J. Carlson and M. R.
Sandy (eds.), Brachiopods Ancient and Modern: A Tribute to G. Arthur Cooper,
Paleontol. Soc. Pap. 7, 2001
R. L. Kaesler (ed.),
Treatise on Invertebrate Paleontology, pt. H: Brachiopoda (revised), 6 vols.,
Geological Society of America and
M. J. S. Rudwick, Living
and Fossil Brachiopods,
Additional
C. H. C., Brunton, L. R.
M. Cocks, and S. L. Long (eds.), Brachiopods Past and Present: Proceedings of
the Millennium Brachiopod Congress, 2000, Systematics Ass. Spec. Vol. Ser. 63,
S. J. Carlson,
Phylogenetic relationships among extant brachiopods, Cladistics, 11:131–197,
1995
B. L. Cohen and A.
Weydmann, Molecular evidence that phoronids are a subtaxon of brachiopods
(Brachiopoda: Phoronata) and that genetic divergence of metazoan phyla began
long before the Early Cambrian, Organisms, Diversity & Evolution,
5(4):253–273, 2005
P. Copper and J. Jin
(eds.), Brachiopods: Proceedings of the Third International Brachiopod Congress,
Sudbury, Ontario, Canada, 2-5 September 1995, A. A. Balkema, Rotterdam, 1996
G. Freeman, Regional
specification during embryogenesis in Rhynchonelliform brachiopods, Dev. Biol.,
261:268–287, 2003
Paleopolis
Tree of Life Web Project
Global Positioning System (GPS
The Global Positioning System is a
satellite-based system providing worldwide continuous position, velocity, time,
and related data to civil and military users. It has a growing number of
applications in the fields of marine, land, and aerospace navigation and precise
time and time transfer, as in surveying, geodesy, and mapping; precision
farming; air-traffic control; asset location and tracking; and timing of
communication systems and power grids. Since the 1960s, GPS has grown from a
navigation concept to an operational system of about 24 spacecraft (Fig. 1)
serving millions of users. Over a million GPS receivers a year were produced
during 1997–1999.
Fig. 1 Constellation of operational GPS
spacecraft.
GPS has performed extremely well and has
generally exceeded expectations. However, a number of deficiencies have been
identified, and some significant improvements are needed that could be
implemented with the new GPS replenishment spacecraft.
Satellites
The satellites' limited lifetime in orbit,
about 7.5 years for the current operational Block II and IIA spacecraft (Fig.
2), establishes the need and deployment schedule for their replacements.
Twenty-one third-generation (Block IIR) replenishment spacecraft have been
ordered by the U.S. Department of Defense to continue the GPS constellation to
2010 and possibly beyond. In July 1997, the first of these was launched to
replace the Block II and IIA spacecraft that will phase out by about 2005.
Fig. 2 Generations of GPS spacecraft. (a)
The Department of Defense has also
contracted for 6 of a planned 30 fourth-generation, follow-on (IIF) spacecraft.
These are to replace the IIR spacecraft and will carry the GPS constellation
well beyond 2010. The Delta 2 launch vehicle (Fig. 3) carries these spacecraft
to their medium-altitude orbits (20,180 km or 10,898 nautical miles above the
Earth).
Fig. 3 Delta 2 ready to launch a Block II
spacecraft.
Modernization
activities
A
number of committees have investigated the needs and deficiencies of the GPS in
order to determine what capabilities and features should be incorporated into a
future system to satisfy both military and civil users. The modernization will
include a new frequency, new signals, higher signal power levels, more extensive
ground tracking, and more frequent spacecraft position updates, all of which
will dramatically improve accuracy, integrity, and other aspects of performance.
The management of GPS has also changed and now involves coordinated civil and
military funding and oversight. These factors, combined with the increasing
worldwide importance of navigation systems and services, provide a basis for
integrating GPS into an international Global Navigation Satellite System (GNSS)
consisting of a number of independent but coordinated elements. The modernized
GPS will continue to play a central role in providing position, velocity,
attitude, and time services in an economical manner.
Selective
availability
Removal of selective availability (SA) is
scheduled between 2000 and 2006, in accordance with the Presidential Decision
Directive on GPS of March 29, 1996. This removal will provide undegraded
accuracy of the signals for civil users. This modification, together with the
additional civil signal frequencies (which include means for correcting
ionospheric delay errors), will improve civil GPS performance by an order of
magnitude or more, to a position determination accuracy of 5 m (15 ft) or
better, by 2010 (Fig. 4).
Civil
signals
New civil signals are planned, including
one centered at frequency L2 (1227.6 MHz), which has heretofore been used
exclusively by the military (Fig. 5). The long-standing civil signal centered at
L1 (1575.42 MHz) will be retained. On January 25, 1999, Vice President Gore
announced that a third civil frequency, L5, had been selected at 1176.45 MHz in
the Aeronautical Radionavigation Services band. This selection was intended
principally to satisfy aviation safety concerns but also to benefit applications
requiring real-time kinematic measurements. (Basically, these are precision
measurements of carrier phases for a number of GPS signals that are measured
between two differential receivers. They are done in real time, or near real
time, and provide almost survey-quality position information, currently about
5–20 cm or 2–8 in.) The new arrangement provides to civil users capabilities for
correction of errors caused by ionospheric delays, increased signal robustness,
and improved techniques for resolving the cycle ambiguities associated with
precision carrier-phase measurements.
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Fig. 5 GPS current and modernized signals. (a)
Civil signal spectrum. (b) Military signal
spectrum.
Signal
structures
The new civil signal at L5 is planned at a
code rate ten times that of the Coarse/ Acquisition (C/A) code, which is now
available to civilian users, and with a 1-millisecond period. These
specifications will improve measurement accuracy, reduce noise, and provide
improved mitigation of multipath errors. New military signals named M codes will
provide improved measurement accuracy, a desirable power distribution in the
spectrum, and a capability for direct access. (In order to acquire the P/Y code,
currently employed in military applications, authorized users must normally
first access the civil C/A code, which contains information about the timing of
the military code. No such procedure is needed to access the M code.) The civil
signals have most of their power in the center of their bands, while the
military signals have most of their power in the outer regions of their bands
(Fig. 5). The existing civil and military signals will remain available
throughout the decade 2000–2010, while the new military (M-code) signals and the
new civil signals at L2 and L5 are to be introduced during the latter part of
the decade.
Receiver
system
Improved receivers such as narrow
correlator types, which provide considerably lower noise and other desirable
performance characteristics, will become generally available and commonly
employed. Also, the use of carrier-phase (real-time kinematic) measurements to
obtain precision position, velocity, attitude, and time determinations will
become commonplace. For example, position precision at the 2–10-cm (1–4 in.)
level will become available in moderate-cost receivers using phase and wide-lane
measurements of the three civil frequencies. (Wide-lane measurements are based
on the “lanes” formed by the wavelength corresponding to the difference in
frequency between signals. For example, the wide lane formed by the L1 and L2
frequencies, separated by 347.72 MHz, is about 86 cm.)
Spacecraft
The GPS spacecraft will offer increased
signal availability and power, and also have greater reliability and longer
lifetimes. Power in the new civil L2 (C/A-code) signal is to be consistent with
the L1 civil signal, which may be increased in power level by about 6 dB for
greater system robustness. Power in the military M-code signals is to be
substantially greater (by about 6–10 dB or more at times) than the current P/Y
military code signal power levels. While an increased number of spacecraft
(30–36) in the GPS constellation cannot be assured, there is strong interest in
this expansion.
Error
reduction
Systematic errors will be reduced not only
by the removal of selective availability and by ionospheric error correction but
also by substantial improvements in GPS receivers and in the control segment.
The GPS ground control system will be expanded by the addition of six or more
tracking stations, principally by incorporating those of the National Imagery
and Mapping Agency. More frequent uploads to the GPS spacecraft are also
planned. Control segment determinations of spacecraft position and prediction
errors will improve from about 2 m (80 in.) to 10–50 cm (4–20 in.).
Augmentations
Augmentations supporting improved
performance will become available worldwide before 2010. Augmentations include
the U.S. Coast Guard Differential Network (available now), the U.S. Federal
Aviation Administration's Wide Area Augmentation System (WAAS) and Local Area
Augmentation System (LAAS), the European Geostationary Navigation Overlay System
(EGNOS), and the Japanese Mobile Satellite Augmentation System (MSAS). Also, a
large number of other differential GPS systems are in use or will become
available that can provide highly precise position, velocity, attitude, and time
measurements.
International
implications
The GPS has become the de facto standard
for navigation satellite system operations, but there have been long-standing
concerns internationally because of the United States military origin and
control of the system. However, system and institutional changes have occurred
such that GPS now has a joint civil-military management structure and provides
independent civil and military capabilities, both of which are being
considerably improved. Additionally, GPS has an important role as a resource
worldwide. The management of GPS appears ready to take a significant role in an
international Global Navigation Satellite System. On February 10, 1999, the
European Commission requested the governments of the 15 states in the European
Union to support the development of an advanced system called the Galileo
project. Proposals have been made for the GPS frequencies and civil signal
structure to be integrated into the Galileo system and possibly into the MSAS as
well. The Europeans indicate a strong desire for Galileo to support their launch
vehicle, spacecraft, and ground control system industries. There is also the
potential for a coordinated global navigation satellite system capability
involving GPS as a principal element. The transition to an international system
is likely to occur before 2010.
Keith D. McDonald
Bibliography
K. D. McDonald, The GPS
modernization dilemma and some topics for resolution, Quart. Newsl. Inst.
Navig., vol. 8, no. 2, summer 1998
K. D. McDonald, The
modernization mantra, GPS World, Directions '99, 9(12):46, December 1998
K. D. McDonald, Technology, implementation and policy issues for the modernization of GPS and its role in a GNSS, J. Navig., vol. 51, no. 3, Royal Institute of Navigation, September 1998
Ali fazeli=egeology.blogfa.com
Additional
Galileo to Set Pace in
Satellite Navigation, Improve Safety, Generate Jobs, Commission Says, Public
Information Release, Pub. IP/99/102, European Commission,
K. D. McDonald et al.,
GPS: A shared national asset—A summary of the National Research Council report
on the future of GPS, Proceedings of the 8th International Technical Meeting of
the Satellite Division of the
New Global Positioning
System Modernization Initiative, Public Announcement, Office of the Vice
President, January 25, 1999
Presidential Decision
Directive on the Global Positioning System, announced by Vice President Albert
Gore and Secretary of Transportation Federico Pena, Doc. DOT 62–96, March 29,
1996
J. J. Spilker et al., A
family of split spectrum GPS civil signals, Proceedings of the 11th
International Technical Meeting of the Satellite Division of the Institute of
Navigation, Part 2, ION GPS-98, September 15–18, 1998
A. J. Van Dierendonck, L5
Signal Requirements, Revision 2, presentation to RTCA Working Group 1 on the
second and third civil frequency signals,
Stratigraphy
A
discipline involving the description and interpretation of layered sediments and
rocks, and especially their correlation and dating. Correlation is a procedure
for determining the relative age of one deposit with respect to another. The
term “dating” refers to any technique employed to obtain a numerical age, for
example, by making use of the decay of radioactive isotopes found in some
minerals in sedimentary rocks or, more commonly, in associated igneous rocks. To
a large extent, layered rocks are ones that accumulated through sedimentary
processes beneath the sea, within lakes, or by the action of rivers, the wind,
or glaciers; but in places such deposits contain significant amounts of volcanic
material emplaced as lava flows or as ash ejected from volcanoes during
explosive eruptions. See also:
Dating methods; Igneous rocks; Rock age determination; Sedimentary rocks
Sedimentary successions are locally many thousands of meters thick owing
to subsidence of the Earth's crust over millions of years. Sedimentary basins
therefore provide the best available record of Earth history over nearly 4
billion years. That record includes information about surficial processes and
the varying environment at the Earth's surface, and about climate, changing sea
level, the history of life, variations in ocean chemistry, and reversals of the
Earth's magnetic field. Sediments also provide a record of crustal deformation
(folding and faulting) and of large-scale horizontal motions of the Earth's
lithospheric plates (continental drift). Stratigraphy applies not only to strata
that have remained flat-lying and little altered since their time of deposition,
but also to rocks that may have been strongly deformed or recrystallized
(metamorphosed) at great depths within the Earth's crust, and subsequently
exposed at the Earth's surface as a result of uplift and erosion. As long as
original depositional layers can be identified, some form of stratigraphy can be
undertaken. See also: Basin;
Continental drift; Fault and fault structures; Sedimentology
Basic principles