A palavra uni\u00e3o abrange v\u00e1rias opera\u00e7\u00f5es diferentes, dependendo da ferramenta utilizada:<\/p>\n\n\n\n

Uni\u00e3o (overlay) como no QGIS\/ArcGIS: recorte + concatena\u00e7\u00e3o de atributos.<\/li>\n\n\n\n
ST_Union (PostGIS): fus\u00e3o geom\u00e9trica (dissolu\u00e7\u00e3o global ou por grupo).<\/li>\n\n\n\n
ST_UnaryUnion (PostGIS): fus\u00e3o topol\u00f3gica r\u00e1pida\/robusta (preserva\u00e7\u00e3o da topologia).<\/li>\n\n\n\n

Dissolve por atributo: agregar entidades que compartilham o mesmo valor.<\/li>\n<\/ul>\n\n\n\n

Escolher bem a opera\u00e7\u00e3o evita surpresas… e SQL XXL.<\/p>\n\n\n\n

\n\n\n\n

<\/span>1) Uni\u00e3o (overlay) \u2013 a \u201cverdadeira\u201d uni\u00e3o no estilo QGIS\/ArcGIS<\/span><\/h2>\n\n\n\n

<\/span>O que ela faz<\/span><\/h3>\n\n\n\n

Mant\u00e9m todas as partes geom\u00e9tricas resultantes das sobreposi\u00e7\u00f5es e n\u00e3o sobreposi\u00e7\u00f5es.<\/li>\n\n\n\n
Concatena os atributos das duas camadas (com NULL no lado ausente).<\/li>\n\n\n\n

Ideal para an\u00e1lises multicrit\u00e9rio (zonas que se sobrep\u00f5em parcialmente).<\/li>\n<\/ul>\n\n\n\n

<\/span>A ter em conta<\/span><\/h3>\n\n\n\n

Trata-se de uma opera\u00e7\u00e3o de sobreposi\u00e7\u00e3o e n\u00e3o de uma fus\u00e3o de geometrias.<\/li>\n\n\n\n

O resultado pode conter v\u00e1rios objetos (cada intersec\u00e7\u00e3o cria fragmentos).<\/li>\n<\/ul>\n\n\n\n

<\/span>Exemplo em Python (QGIS Processing)<\/span><\/h3>\n\n\n

<\/div>

<\/div><\/div>

<\/p>\n\n\n\n

import processing\nfrom qgis.core import QgsProject\n\n# Deux couches charg\u00e9es dans QGIS (QgsVectorLayer) : layer_a, layer_b\nresult = processing.run(\"qgis:union\", {\n    \"INPUT\": layer_a,\n    \"OVERLAY\": layer_b,\n    \"OUTPUT\": \"memory:\"  # ou un GPKG\/PG\n})\n\nunion_layer = result[\"OUTPUT\"]\nQgsProject.instance().addMapLayer(union_layer)\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>Exemplo em PostGIS (modelo SQL \u00aboverlay union\u00bb)<\/span><\/h3>\n\n\n\nVers\u00e3o compacta sem atributos (apenas o ID dos pares + geometria).<\/p>\n\n\n
<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nWITH pairs AS (\n  SELECT a.gid AS ida, b.gid AS idb, ST_Intersection(a.geom, b.geom) AS geom\n  FROM plugin.communes a\n  JOIN plugin.logements b ON ST_Intersects(a.geom, b.geom)\n  WHERE NOT ST_IsEmpty(ST_Intersection(a.geom, b.geom))\n),\ninter_union AS (\n  SELECT ST_UnaryUnion(geom) AS geom FROM pairs\n),\na_only AS (\n  SELECT a.gid AS ida, NULL::integer AS idb,\n         ST_Difference(a.geom, (SELECT geom FROM inter_union)) AS geom\n  FROM plugin.communes a\n  WHERE NOT ST_IsEmpty(ST_Difference(a.geom, (SELECT geom FROM inter_union)))\n),\nb_only AS (\n  SELECT NULL::integer AS ida, b.gid AS idb,\n         ST_Difference(b.geom, (SELECT geom FROM inter_union)) AS geom\n  FROM plugin.logements b\n  WHERE NOT ST_IsEmpty(ST_Difference(b.geom, (SELECT geom FROM inter_union)))\n)\nSELECT * FROM pairs\nUNION ALL\nSELECT * FROM a_only\nUNION ALL\nSELECT * FROM b_only;\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>2) ST_Union: fus\u00e3o geom\u00e9trica (dissolu\u00e7\u00e3o global)<\/span><\/h2>\n\n\n\n<\/span>Funcionamento<\/span><\/h3>\n\n\n\n\nFunde um conjunto de geometrias em uma \u00fanica (Multi*) sem manter os atributos.<\/li>\n\n\n\n
Por padr\u00e3o, gerencia a topologia, mas ST_UnaryUnion \u00e9 geralmente mais robusto\/r\u00e1pido.<\/li>\n<\/ul>\n\n\n\n<\/span>Quando usar<\/span><\/h3>\n\n\n\n\nPara dissolver pol\u00edgonos cont\u00edguos (por exemplo, criar a \u00e1rea total).<\/li>\n\n\n\n
Para pr\u00e9-agregar antes de um buffer ou c\u00e1lculo de \u00e1rea.<\/li>\n<\/ul>\n\n\n\n<\/span>Exemplo em SQL: fundir duas tabelas em uma \u00fanica geometria.<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nCREATE TABLE plugin.emprise_globale AS\nSELECT ST_UnaryUnion(geom) AS geom\nFROM (\n  SELECT geom FROM plugin.communes\n  UNION ALL\n  SELECT geom FROM plugin.logements\n) s;\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>Exemplo em Python (psycopg2)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport psycopg2\nconn = psycopg2.connect(\"dbname=... user=... password=... host=... port=...\")\ncur = conn.cursor()\ncur.execute(\"\"\"\n  DROP TABLE IF EXISTS plugin.emprise_globale;\n  CREATE TABLE plugin.emprise_globale AS\n  SELECT ST_UnaryUnion(geom) AS geom\n  FROM (\n    SELECT geom FROM plugin.communes\n    UNION ALL\n    SELECT geom FROM plugin.logements\n  ) s;\n\"\"\")\nconn.commit()\ncur.close()\nconn.close()\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>3) ST_UnaryUnion \u2013 fus\u00e3o topol\u00f3gica (mais robusta)<\/span><\/h2>\n\n\n\n<\/span>
O que faz<\/span><\/h3>\n\n\n\n\nComo ST_Union, mas preservando a topologia e geralmente mais r\u00e1pido.<\/li>\n\n\n\n
Corrija melhor as sobreposi\u00e7\u00f5es m\u00faltiplas, an\u00e9is compartilhados e pequenos erros de topologia.<\/li>\n<\/ul>\n\n\n\n<\/span>
Quando usar<\/span><\/h3>\n\n\n\n\nSempre para camadas grandes ou com geometrias imperfeitas.<\/li>\n\n\n\n
Como bloco dentro de outras consultas (por exemplo, overlay union).<\/li>\n<\/ul>\n\n\n\n<\/span>
Exemplo em Python (GeoPandas)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport geopandas as gpd\n\ngdf = gpd.read_file(\"commun.es.gpkg\")\ngeom_unary = gdf.unary_union  # \u00e9quivalent \u00e0 ST_UnaryUnion\n# -> geom_unary est un (Multi)Polygon shapely\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>4) Dissolver por atributo (ST_Union + GROUP BY)<\/span><\/h2>\n\n\n\n<\/span>O que faz<\/span><\/h3>\n\n\n\n\nAgrupa as entidades por uma chave e funde a geometria dentro de cada grupo.<\/li>\n\n\n\n
Os atributos n\u00e3o s\u00e3o agregados automaticamente (exceto a chave) \u2192 adicione seus agregados.<\/li>\n<\/ul>\n\n\n\n<\/span>Exemplo SQL: dissolver os munic\u00edpios por EPCI<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nCREATE TABLE plugin.communes_par_epci AS\nSELECT epci,\n       ST_UnaryUnion(geom) AS geom\nFROM plugin.communes\nGROUP BY epci;\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>Exemplo Python (QGIS Processing \u2013 Dissolve)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport processing\n\nres = processing.run(\"native:dissolve\", {\n    \"INPUT\": layer_communes,\n    \"FIELD\": [\"epci\"],\n    \"SEPARATE_DISJOINT\": False,\n    \"OUTPUT\": \"memory:\"\n})\ndissolved = res[\"OUTPUT\"]\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>5) B\u00f4nus: GeoPandas \u2013 overlay(\u2026, how=\u00a0\u00bbunion\u00a0\u00bb)<\/span><\/h2>\n\n\n\n<\/span>O que faz<\/span><\/h3>\n\n\n\nEquivalente \u00e0 Uni\u00e3o overlay QGIS\/ArcGIS no lado Python cient\u00edfico.<\/p>\n\n\n
<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport geopandas as gpd\n\na = gpd.read_file(\"plugin_communes.gpkg\")\nb = gpd.read_file(\"plugin_logements.gpkg\")\n\nu = gpd.overlay(a, b, how=\"union\")\nu.to_file(\"union_overlay.gpkg\")\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>Como escolher?<\/span><\/h2>\n\n\n\n\nQuer manter os atributos das duas camadas + manter as \u00e1reas n\u00e3o sobrepostas?
\u2192 Uni\u00e3o (overlay).<\/li>\n\n\n\n
Quer uma \u00fanica geometria mesclada (para uma \u00e1rea de influ\u00eancia, uma m\u00e1scara)?
\u2192 ST_UnaryUnion (ou ST_Union, se n\u00e3o houver outra op\u00e7\u00e3o).<\/li>\n\n\n\n
Voc\u00ea deseja dissolver por uma chave (departamentos, EPCI\u2026)?
\u2192 GROUP BY + ST_UnaryUnion.<\/li>\n\n\n\n
Voc\u00ea quer fazer tudo em Python QGIS sem SQL?
\u2192 qgis:union (sobreposi\u00e7\u00e3o) ou native:dissolve (dissolver por atributo).<\/li>\n<\/ul>\n\n\n\n<\/span>Resumo comparativo<\/span><\/h3>\n\n\n\nTipo de uni\u00e3o<\/th> Natureza<\/th> Atributos<\/th> Resultado geom\u00e9trico<\/th> Exemplo de uso<\/th><\/tr><\/thead>
ST_Union<\/code> (PostGIS)<\/td> Fun\u00e7\u00e3o geom\u00e9trica<\/td> N\u00e3o (exceto GROUP BY)<\/td> Fus\u00e3o de pol\u00edgonos<\/td> Dissolver os munic\u00edpios de uma regi\u00e3o<\/td><\/tr>
UNION<\/code> (QGIS\/PostGIS overlay)<\/td> Sobreposi\u00e7\u00e3o<\/td> Sim (todos mantidos)<\/td> Recorte + combina\u00e7\u00e3o<\/td> Cruzar munic\u00edpios e habita\u00e7\u00f5es<\/td><\/tr>
Union<\/code> (ArcGIS)<\/td> Sobreposi\u00e7\u00e3o<\/td> Sim (todos mantidos)<\/td> Parti\u00e7\u00e3o completa<\/td> An\u00e1lise multicrit\u00e9rio<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n<\/span>Armadilhas e boas pr\u00e1ticas<\/span><\/h2>\n\n\n\n\nAtributos vs geometria: ST_Union n\u00e3o cola os atributos \u21d2 sem sobreposi\u00e7\u00e3o.<\/li>\n\n\n\n
Tipos de atributos: em sobreposi\u00e7\u00e3o SQL caseira, alinhe os tipos (ou converta em texto) antes de UNION ALL.<\/li>\n\n\n\n
Topologia: prefira ST_UnaryUnion para conjuntos volumosos ou um pouco \u201csujos\u201d.<\/li>\n\n\n\n
SRID: certifique-se de que as duas camadas compartilham o mesmo SRID; caso contr\u00e1rio, reprojete antes.<\/li>\n\n\n\n
Pol\u00edgono vs Multi: force ST_Multi(\u2026) se voc\u00ea precisar de um tipo homog\u00eaneo.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n<\/span>Resumo dos trechos de Python (para copiar e colar)<\/span><\/h2>\n\n\n\n<\/span>A) Overlay Union (QGIS)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport processing\nfrom qgis.core import QgsProject\n\nresult = processing.run(\"qgis:union\", {\n    \"INPUT\": layer_a,\n    \"OVERLAY\": layer_b,\n    \"OUTPUT\": \"memory:\"\n})\nQgsProject.instance().addMapLayer(result[\"OUTPUT\"])\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>B) Fus\u00e3o global (PostGIS)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport psycopg2\nconn = psycopg2.connect(\"dbname=... user=... password=... host=... port=...\")\ncur = conn.cursor()\ncur.execute(\"\"\"\n  DROP TABLE IF EXISTS plugin.emprise_globale;\n  CREATE TABLE plugin.emprise_globale AS\n  SELECT ST_UnaryUnion(geom) AS geom\n  FROM (\n    SELECT geom FROM plugin.communes\n    UNION ALL\n    SELECT geom FROM plugin.logements\n  ) s;\n\"\"\")\nconn.commit(); cur.close(); conn.close()\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>C) Dissolve por atributo (QGIS)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport processing\n\nres = processing.run(\"native:dissolve\", {\n    \"INPUT\": layer_communes,\n    \"FIELD\": [\"epci\"],\n    \"OUTPUT\": \"memory:\"\n})\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
<\/span>D) Overlay Union (GeoPandas)<\/span><\/h3>\n\n\n<\/div> <\/div><\/div><\/div><\/p>\n\n\n\nimport geopandas as gpd\nu = gpd.overlay(gpd.read_file(\"a.gpkg\"), gpd.read_file(\"b.gpkg\"), how=\"union\")\nu.to_file(\"union_overlay.gpkg\")\n<\/code><\/pre>\n\n\n\n<\/div><\/div>\n\n\n\n
\n\n\n\n<\/span>Conclus\u00e3o<\/span><\/h2>\n\n\n\nO termo uni\u00e3o abrange, na verdade, duas l\u00f3gicas diferentes:<\/p>\n\n\n\n
\nfus\u00e3o geom\u00e9trica (ST_Union, Dissolve) \u2192 para simplificar os contornos.<\/li>\n\n\n\n
sobreposi\u00e7\u00e3o com combina\u00e7\u00e3o de atributos (UNION, ArcGIS Union) \u2192 para an\u00e1lise multicrit\u00e9rio.<\/li>\n<\/ul>\n\n\n\nA escolha depende, portanto, do objetivo: simplificar ou analisar.<\/p>\n\n\n\n
\n\n\n\nEm um pr\u00f3ximo artigo, poderemos explorar em detalhes o desempenho dessas diferentes abordagens, pois as uni\u00f5es podem rapidamente se tornar dispendiosas em tempo de c\u00e1lculo quando se manipula milh\u00f5es de geometrias.<\/p>\n\n\n\n
\n","protected":false},"excerpt":{"rendered":"Entendendo as diferentes formas de uni\u00e3o em SIG A uni\u00e3o espacial \u00e9 uma das opera\u00e7\u00f5es fundamentais em geom\u00e1tica. Ela permite combinar v\u00e1rias camadas ou entidades para produzir novas geometrias ou enriquecer os atributos. No entanto, por…<\/p>\n","protected":false},"author":1,"featured_media":15379,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"give_campaign_id":0,"_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_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[2134],"tags":[3680,3682,3684,2803,2160,3284,3686,3648],"class_list":["post-15441","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-qgis","tag-disolve-pt","tag-fusion-pt","tag-overlay-pt","tag-python-pt","tag-qgis-pt","tag-sql-pt","tag-unary-union-pt","tag-union-pt"],"aioseo_notices":[],"jetpack_featured_media_url":"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2025\/08\/union.jpg?fit=614%2C409&ssl=1","jetpack_shortlink":"https:\/\/wp.me\/p6XU0A-413","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/15441","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=15441"}],"version-history":[{"count":0,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/posts\/15441\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/media\/15379"}],"wp:attachment":[{"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/media?parent=15441"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/categories?post=15441"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sigterritoires.fr\/index.php\/wp-json\/wp\/v2\/tags?post=15441"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}