In a Ruby-on-Rails application, I need the area in square meters (or any other surface measure, be it ares, hectares, square kilometers, but not in pseudo surface measure like square degrees) of a geographic polygon.
The application already uses rgeo and PostGIS (via the activerecord-postgis-adapter) and already requires rgeo's optional libGEOS support. So I'd like to make use of these tools for the area calculation, too.
I can use PostGIS (via the activerecord-postgis-adapter) to compute the area of any rgeo geometry:
def calculate_area(geometry) ActiveRecord::Base.connection.execute(" SELECT ST_Area( geography( ST_Transform( ST_GeomFromText('#{geometry.as_json}', #{geometry.srid}), 4326 ) ) ) AS area; ")[0]['area'].to_fend(Nesting ST_Area()'s argument in geography() is what causes the result to be in square meters.)
However, having to extract the result from execute()'s return value and having the lengthy SQL code embedded in Ruby is rather ugly.
Also, rgeo already provides an area method on the RGeo::Feature::Surface module, which RGeo::Feature:
olygon includes. Its default implementation just isn't all that useful:
srs_db = RGeo::CoordSys::SRSDatabase::ActiveRecordTable.newwgs84_factory = RGeo::Geographic.spherical_factory(srid: 4326, srs_database: srs_db)# 1/8 of the Earth, should be around 6.4e13 sqmpolygon = wgs84_factory.parse_wkt('POLYGON ((0 0, 0 90, 90 0, 0 0))')polygon.area # RGeo::Error::UnsupportedOperation: Method Surface#area not defined.I have installed rgeo such that it can use libGEOS, which I can verify like so:
RGeo::GEOS.supported? # trueThus, I thought that I might be luckier with a GEOS-specific factory:
geos_wgs84_factory = RGeo::Geos.factory(srid: 4326, srs_database: srs_db)geos_polygon = RGeo::Feature.cast(polygon, geos_wgs84_factory)geos_polygon.area # 4050.0
Well, that's the area in square degrees (90°
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The application already uses rgeo and PostGIS (via the activerecord-postgis-adapter) and already requires rgeo's optional libGEOS support. So I'd like to make use of these tools for the area calculation, too.
I can use PostGIS (via the activerecord-postgis-adapter) to compute the area of any rgeo geometry:
def calculate_area(geometry) ActiveRecord::Base.connection.execute(" SELECT ST_Area( geography( ST_Transform( ST_GeomFromText('#{geometry.as_json}', #{geometry.srid}), 4326 ) ) ) AS area; ")[0]['area'].to_fend(Nesting ST_Area()'s argument in geography() is what causes the result to be in square meters.)
However, having to extract the result from execute()'s return value and having the lengthy SQL code embedded in Ruby is rather ugly.
Also, rgeo already provides an area method on the RGeo::Feature::Surface module, which RGeo::Feature:
olygon includes. Its default implementation just isn't all that useful:srs_db = RGeo::CoordSys::SRSDatabase::ActiveRecordTable.newwgs84_factory = RGeo::Geographic.spherical_factory(srid: 4326, srs_database: srs_db)# 1/8 of the Earth, should be around 6.4e13 sqmpolygon = wgs84_factory.parse_wkt('POLYGON ((0 0, 0 90, 90 0, 0 0))')polygon.area # RGeo::Error::UnsupportedOperation: Method Surface#area not defined.I have installed rgeo such that it can use libGEOS, which I can verify like so:
RGeo::GEOS.supported? # trueThus, I thought that I might be luckier with a GEOS-specific factory:
geos_wgs84_factory = RGeo::Geos.factory(srid: 4326, srs_database: srs_db)geos_polygon = RGeo::Feature.cast(polygon, geos_wgs84_factory)geos_polygon.area # 4050.0
Well, that's the area in square degrees (90°
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