A geographical information system (SIG) is computer tools making it possible to organize and present Donnée S Alphanumérique S spatially referred, like producing Plan S and charts. Its Usage S covers the activities Géomatique S of treatment and diffusion of the geographical Information. The representation is generally in two dimensions, but one returned 3D or an animation presenting of the temporal variations on a territory is possible.

The role of the Information system is to propose a more or less realistic representation of the space environment by basing on primitive graphic such as points, Vecteur S (arcs), Polygone S or Maillage S (raster). With these primitives is associated qualitative information such as nature (Route, Railway, Forêt, etc) or any other contextual information (many inhabitants, type or surface of a commune e.g.).

Geographical information can be defined like the whole of the description of an object and its geographical position on the surface of the Ground.

In France, in its current meaning, the term refers to the tools Logiciel S. Cependant, the concept includes the unit consisted the Logiciel S, the Donnée S, the material and know-how related to the use of the latter. One can also speak about Information system to space reference (SIRS) for the Donnée S and their structuring. Acronym SIT (information system on the territory) is also used in some countries Francophone S. Enfin, initials BDU (urban data bank), even BDT (data bank on the territory), older, can meet here and there.

History

The development of the SIG in science and the Town and country planning was allowed by the projection of technology Informatique, and encouraged by catch of environmental Conscience and scientific new approaches transdisciplinaires, integrating. Since the Years 1970, in particular since the top of the Earth in Rio de Janeiro in 1992 which generated an increasing demand of charts presenting the state of the environment and useful to measure the impacts of the development.

Maguire and Al (1991) distinguish three principal periods in the evolution from the SIG:

• fine of the Years 1950 - medium of the Years 1970: beginning of data processing, first automatic cartographies;
• medium of the Years 1970 - beginning of the Years 1980: diffusion of the tools for cartography automatique/SIG in the organizations of State (armed, topographic Land register, services,…) ;
• since the Years 1980: growth of the Software market of the S, developments of the applications on PC, put in network (Databases distributed, with since the Years 1990, of the applications on Internet) and a vulgarizing of the Use of the geographical Information (cartography on Internet, calculation of road routes, use of embarked solutions related to GPS…).

Some concepts

Components of the SIG

A SIG consists of five major components:

Software

They provide the 5 following functions (sometimes gathered under the term of the ` 5A '):

• seized by geographical information under digital form ( Acquisition )
• data base management ( Filing )
• handling and interrogation of the geographical data ( Analysis )
• formatted and visualization ( Posting )
• representation of the real-world ( Abstraction).

Given S

They are the base of the SIG. The Données geographical are imported starting from files or seizures by an operator. See the following paragraph.

The Computer materials

The Traitement of the Donnée S is done using the software on a Ordinateur. Customer-server systems in Intranet, Extranet even via facilitating Internet then and more and more diffusion of the results.

Know-how

A SIG calls upon with a technical Connaissance and with various know-how and thus various trades which can be carried out by one or more people. The “ sigist ” must mobilize competences in Géodésie (knowledge of the concepts of and system frame of reference of projection), in analysis of the Donnée S, processes and of modeling (analyzes Merise, language UML for example), in statistical processing, graphic and cartographic semiology, graphic treatment. It must know to translate into data-processing requests the questions that one poses to him.

Users

As all the user S of SIG are not inevitably specialists, a SIG proposes a series of toolboxes that the user assembles to carry out his project. No matter who can, one day or the other, being brought to use a SIG. However, one does not improvise geomatician: a good knowledge of the Given S handled and nature of the treatments carried out by the Logiciel S only makes it possible to suitably interpret the quality of the results obtained.

Questions which the SIG can answer, and limits

A SIG must answer five questions, whatever the scope of application:

• Where: where is the field of study and which is its extent Géographique?

• What: which objects can one find on studied space?
• How: how the objects are distributed in studied space, and which are their relations? It is the space Analyze.
• When: which is the age of an object or a phenomenon? It is the temporal analysis.
• And if: what would it occur if it occurred such event?

The SIG have like limits the relevance, the richness, and the occurrence of update their databases, but also sometimes the restrictions of accessibility as well as the royalties on certain data and information which can prevent the diffusion of charts, or to prevent their realization for work sharing the data of several SIG. Accessibility can also suffer from measurements taken to protect from the particular entities when the sample size is too small (statistical confidentiality), or by the presence on a layer from data of information strategic and/or protected. Finally certain requests require a time or a computing power nonavailable.

Given S of the SIG

The Données geographical have four components:

• the geometrical Donnée S return to the form and the localization of the objects or phenomena;
• the descriptive Donnée S (which belong to the data assignees) return to the whole of the descriptive attributes of the objects and phenomena except for the form and of the localization;
• the Donnée S graphs return to the parameters of posting of the objects (standard of feature, color…) ;
• the Metadatum S associated, i.e. data on the data (date of acquisition, name of the owner, methods of acquisition…).

Given S assignees

It is about Donnée S associated with an object or a geographical location, either to describe a geographical object, or to locate information: name of a road, type of a building located by its address, many inhabitants of a building located by its coordinates Lambert, flow of a river, tension of a line of transport of energy, type of trees in an orchard located by its piece, etc The data assignees are connected to the geometry of the object.

Geographical objects

Three types of geographical entities can be represented:

• the specific point (X, there) or ;
• the line ((x1, y1),…, (xn, yn)) or linear ;
• the surface polygon or .

At present, no SIG completely manages the voluminal polyhedrons, or . In the best of the cases, that of the software known as 2D ½ , at a point (X, there) can be associated a dimension (Z) and only one.

Two modes of representations are possible:

• vectorial (format vector): the objects are represented by points, lines, polygons or polygons with holes;
• matric (format raster ): it is of an image, a plan or a photograph digitized and posted in the SIG as an image.

A terrestrial frame of reference (spherical or projective) makes it possible to refer the objects in space and to position the whole of the objects the ones compared to the others. The objects are generally organized in layers, each layer gathering the whole of the homogeneous objects (built, rivers, roadway system, pieces, etc).

==> To show an example of organization in layers.

Examples of data “raster”:

• a Orthophotographie is an image obtained by digitalization of an air stereotype silver (or, now, taken of numerical sight) which one corrected of the deformations due:

• with the relief of the ground photographed,
• with the distortion of the camera,
• with the slope of the catch of sight.
• a scan was an image scannée starting from a chart paper. Most known are the series of the Scan 25,100 and 250 resulting from the charts 1:25000, 1:100000 and 1:250000 of IGN.

Topology

One of the advantages of the SIG is that the relations between the objects can be calculated and give rise to points of intersection. It is the Topologie. This makes it possible to avoid the repetition of superimposed objects. A piece bordering a road will have the same definite tops as those for the road.

Relation Objects Given S assignees

Géo-referencing is the technique of comparison organized of the geographical objects and the Donnée S assignees. It supposes the installation in the SIG of a system of locating normalized, whose role is the equivalent of the functional dependences in the relational databases. Thus alphanumeric items, resulting from external files to the SIG itself, could be cross with geographical information of the SIG, giving place to uses of Géoanalyse.

The Metadatum S

See also: Metadatum

The Donnée S handled by a SIG come from various sources and databases. An organization which obtains such a system must have in heart to control these sources, in order to ensure itself:

• that it is well with the fact of the whole of the layers of Donnée S available in the organization,

• which it can trust with the results obtained during their use,
• that it controls the Gestion intern of it,
• that it controls the Mise and costs of acquisition of them up to date,
• that it is in measurement, if necessary, to provide whole or part of its data to thirds, by giving a sufficient visibility on quality of the supply.

This is why any source of Données geographical is not limited to its contents assignee and geographical, but is accompanied by information characterizing the source itself, i.e. data on the data (they are called Métadonnée S).

Some examples of Metadatum S (among much of others):

• general Description

• description and nature of the data
• system of projection and geographical extent
• producing organization

• Quality of the data:

• date of seizure or validity - if a Donnée is old compared to the evolutions of the entities which it represents, one can always utilize it in calculations, but the results are to interpret with prudence;
• precision of the seizure - to cross Donnée S of quality centimetric with data of hectometric quality never gives but results of a hectometric precision!

• Gestion interns

• Responsable and localization
• date for acquisition
• Fréquence of update
• date of last update

The whole of this information must be able to be easily accessible and shareable by all the actors intervening on some level that it is in the Life cycle Donnée S within the organization. The definition of a wallet of metadata (Registre of metadata) remains a stake for any organization which makes of its SIG an important part of its activity, and what it is supplier of Donnée S or simple user.

In order to facilitate the exchanges of Metadatum S, they can be structured according to the standard ISO 19115. This work of standardization should allow the constitution of large directories of Données geographical, which will allow an optimal use of these last.

Some software SIG

See also: List of software SIG

Example of databases/referential

• Corine Land Cover (occupation of the ground)

• Cadrastal map Computerized “NCV-vector” (property of the DGI)
• data base CARTO: First database vectorial produced by IGN starting from its charts to the 1/50 000. The data base Carto proposes ten layers of information going from the occupation of the ground to the highway network and the toponynie.
• DATA BASE TOPO: Topographic component of the Reference frame to large scales (RGE). Database of precision metric and 3D, it specifies the available data on data base Carto. It will be the index reference of charts IGN to the 1/25 000.
• data base ALTI: Database of the level lines. One also speaks about MNT or digital model of ground.
• DATA BASE ORTHO: Orthophotographic component of the RGE. The national cover is now finished with a pixel of 50 cm.
• DATA BASE NYME. The data base Nyme is the database of the toponyms (name of the localities) managed by IGN (integrated in the data base Topo)
• data base Addresses: Third component of the RGE, this database géoréférence postal addresses and tax.
Compartmental
• data base: Last component of the RGE, the Compartmental data base ensures the space continuum between different the scannées or vectorized cadastral boards.
• GTopo 30
• Geological maps to the 1/50 000 produced by BRGM
• Given and referential on water (Sites measuring, Works related to water, River BDCarthage, hydrogeologic Entities BDRHF,…) products by the Information system on Water (SIE) and published by the National Service of Administration of the Data and Reference frames on Water (PIKE PERCH)
• etc

Use

• space Analyzes;

• Decision-making aid, in particular for the Town and country planning;
• Definition of Market radius, establishment of sales outlets, assistance with the Mediaplanning in particular in Posting, optimization of the distribution of I.S.A. (printed papers form without addresses).
• Cartography.

Outlines

Applications SIG/SIRS develop quickly especially these last years with the evolution of the performances of data processing. Today, the evolution of the SIG tends towards an accessibility for the Web with:

• cartographic Waiters,
• SIRS divided on the Web.

Moreover, wandering tools SIG/SIRS appear thanks to the PDA in the world of the Agriculture of precision.

However, it always misses a component with the SIG which is the “Temps”, still difficult to manage.

In the European Union, the Directive INSPIRES aims at harmonizing the geographical Informations between the Member States, in particular by the use of Métadonnée S.

Notes and references of the article

Random links:

Rensis Likert | Quincampoix | School of Engineers de Genève | James Peter Allen | Varadin