Commonly used geographic vector objects include three levels: featureCollection, feature, and geometry. dissolve and union will be compared in the context of geometry.

dissolve

Aggregates some geometris into one geometry.

• It works on one object, eg feature or geometryCollection. The latter is a list of geometris and it may be one of multipolygon, multilines, and points.
• If two geometris touch, they will be unioned as one geometry.
• If two geometris untouch, they will be gathered to one multi-object geometry. Considering the sole input, nothing will change for a geometryCollection.
• Due to only one geometry for each feature, it is used to dissolve a multigemotry feature or geometryCollection coverted from multi features.
• A similar operation in ArcGIS, Edit -> Select the features to merge -> merge. It may produce a unioned feature or a multigeometry feature.
• unary-union in shapely have the same function.

union

Computes a geometric union of the input geometris.

• It works on at leat two objects.
• If two input geometris touch, they will be unioned as one geometry.
• If two input geometris untouch, they will be gathered to one multi-object geometry. Two untoched geomtry objects has merged to one multigeometry object.
• A similar operation in ArcGIS, Edit -> Select the features to merge -> merge. It may produce a unioned feature or a multigeometry feature.
• Note that union of analysis tool box in ArcGIS won’t create a unioned geometry for thoese touched each other. It will put all in one and split them by bounds.
• union in shapely have the same function.

At end, we can say, dissolve or unary-union combines the sub-objects for a given geometryCollection object. union works for two independent objects. The outputs of them are very similar.

buffer

Google Earth Engine

• If two geometris touched, they will be unioned as one geometry after ee.Geometry.buffer operation.

GeoPandas

• The number of geometris returen by GeoSeries.buffer() remains the same as input object.

Reference

• Union (analysis) in ArcGIS
• Dissolve (Data Management) in ArcGIS
• Aggregation with dissolve - GeoPandas
• Merging features in the same layer - ArcGIS
• union - shapely
• unary-union - shapely
• GeoSeries.buffer()

Related

Ports of python geospatial analysis tools gis

A simple way to map landuse ArcGIS

神奇的Copy Raster ArcGIS

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