{"id":8034,"date":"2019-02-06T00:04:27","date_gmt":"2019-02-05T23:04:27","guid":{"rendered":"http:\/\/www.sigterritoires.fr\/?p=8034"},"modified":"2019-02-06T08:34:36","modified_gmt":"2019-02-06T07:34:36","slug":"gis-and-local-authorities-4-some-gis-technical-fundamentals","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/gis-and-local-authorities-4-some-gis-technical-fundamentals\/","title":{"rendered":"GIS and local authorities (4) &#8211; Some GIS technical fundamentals"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">A geographical information \u00ab\u00a0system\u00a0\u00bb\nis composed of a combination of software, data, operators and users, processing\nand treatments which underlie a certain organization. <br>\nWe will discuss here the technical fundamentals of\nthe system: the management (capture, storage, extraction), the analysis\n(interrogation and processing), the communication (map production, plans and\nreports), and finally the simulation (modeling and production of scenarios ). <br>\nAs technical fundamentals we include the definition\nof a series of words that take on a particular meaning when it comes to GIS. <\/p>\n\n\n\n<!--more-->\n\n\n\n<p class=\"wp-block-paragraph\">The geographical entities <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A system manages, by definition, customized\nentities: a banking system will have the bank accounts as entities, the Health\nInsurance manages the entities \u00ab\u00a0social insured\u00a0\u00bb, etc. In the case of a geographical information system the entities will be the\nobjects of the same type (buildings, roads, hydrography, monuments, &#8230;).\nThese entities will be grouped into\nsets called information layers. <br>\nCompared to other types of management\nof geographical objects (CAD-CAD, &#8230;) the geographical objects consist of\nthree different conceptual levels: <\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>the geometric level:<\/strong> the object is represented as a point, a line, a\nsurface, a volume. Each of these representations is created\nby one or more points that constitute the object, each of these points,\nnecessarily, having a location in the space (geographical coordinates). <\/li><li><strong>the attribute level:<\/strong> each object has alphanumeric information describing\nits characteristics (name of the commune, address, type of building, &#8230;) <\/li><li><strong>the relational level:<\/strong> the geographical objects maintain spatial and\nsemantic relations between them. The spatial relations are of the type building in plot, plot in commune,\nplot on the right side of the road, river crosses the commune, etc. This is called topological relations. Semantic relationships may include land belonging\nto an owner, areas with special protection, and so on. At finer levels, there are other types of\nGIS-specific relationships such as aggregation (of communes forming a department)\nor inheritance (each section of a watercourse inherits the name of the\nwatercourse). . <\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Real world representations<\/strong> <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The set of geographical objects can be\nrepresented as two main forms: the image form (raster) or the vector form\n(point, line, polygon). <\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>the raster mode:<\/strong> the real world is represented by a series of\nregular pixels each containing one or more values \u200b\u200bthat characterize the\nobject located in this pixel. The most\ncommon raster data are aerial photographs, scanned maps or plans, digital terrain\nmodels or satellite images. Thus, the\nreal world is represented by a regular matrix of cells where each object\ncorresponds to a cell or a group of adjacent cells. <\/li><li><strong>the vector mode:<\/strong> each object is represented in an identified way and\nindependent of other objects. It is\nrepresented by a point, a line, a polygon or a volume having a unique\nidentifier. This unique identifier is also\npresent in the attributes table and makes it possible to link the geometry of\nthe object with its attribute properties. <\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The GIS data <\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There are two major types of data in a GIS: <\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>the reference data <\/li><li>the proper or business data <\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The reference data<\/strong> will help you, when creating new\ndata, to position them accurately in the real world. This \u00ab\u00a0accuracy\u00a0\u00bb\nwill be relative to the chosen reference. The production of the most common\ngeographical references is the responsibility of public institutions such as\nIGN (National Geographic Institute), INSEE (National Institute of Statistics\nand Economic Studies), DGFIP (General Directorate of Public Finance) , MEDDE\n(Ministry of Ecology, Sustainable Development and Energy), the BRGM (Bureau of\nGeological and Mining Research). At municipal level, the two most frequently\nused digital map funds are the cadastral plan and the orthophotoplan\n(orthorectified aerial photography). <br>\nRelying on a given reference source makes it possible to ensure the consistency\nof the data produced and the possibility of exchanging data with other\npartners. But you should know that the different reference sources are not\nalways completely consistent among them: the positioning accuracy of each one can\nbe different and the objects present depend on the rate of update of each source.\nIn any case, it must be acknowledged that efforts have been made in recent\nyears to bring together all the available references. <br>\n<strong>Business data<\/strong> may have an external origin (service providers, thematic\ndata providers, etc.) or internal, in the latter case, special attention should\nbe given to metadata information. These famous metadata will be the subject of\nthe next article in this series. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>A geographical information \u00ab\u00a0system\u00a0\u00bb is composed of a combination of software, data, operators and users, processing and treatments which underlie a certain organization. We will discuss here the technical fundamentals of the system: the management (capture,&hellip;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_bbp_topic_count":0,"_bbp_reply_count":0,"_bbp_total_topic_count":0,"_bbp_total_reply_count":0,"_bbp_voice_count":0,"_bbp_anonymous_reply_count":0,"_bbp_topic_count_hidden":0,"_bbp_reply_count_hidden":0,"_bbp_forum_subforum_count":0,"sfsi_plus_gutenberg_text_before_share":"","sfsi_plus_gutenberg_show_text_before_share":"","sfsi_plus_gutenberg_icon_type":"","sfsi_plus_gutenberg_icon_alignemt":"","sfsi_plus_gutenburg_max_per_row":"","_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[1260],"tags":[],"class_list":["post-8034","post","type-post","status-publish","format-standard","hentry","category-non-classe-en"],"aioseo_notices":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p6XU0A-25A","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/8034","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/comments?post=8034"}],"version-history":[{"count":0,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/8034\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/media?parent=8034"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/categories?post=8034"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/tags?post=8034"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}