def _create_square(x, y, zoom) -> geojson.MultiPolygon:
"""
Function for creating a geojson.MultiPolygon square representing a single OSM tile grid square
:param x: osm tile grid x
:param y: osm tile grid y
:param zoom: osm tile grid zoom level
:return: geojson.MultiPolygon in EPSG:4326
"""
# Maximum resolution
MAXRESOLUTION = 156543.0339
# X/Y axis limit
max = MAXRESOLUTION * 256 / 2
# calculate extents
step = max / (2**(zoom - 1))
xmin = x * step - max
ymin = y * step - max
xmax = (x + 1) * step - max
ymax = (y + 1) * step - max
# make a shapely multipolygon
multipolygon = MultiPolygon([
Polygon([(xmin, ymin), (xmax, ymin), (xmax, ymax), (xmin, ymax)])
])
# use the database to transform the geometry from 3857 to 4326
transformed_geometry = ST_Transform(
shape.from_shape(multipolygon, 3857), 4326)
# use DB to get the geometry as geojson
return geojson.loads(
db.engine.execute(transformed_geometry.ST_AsGeoJSON()).scalar())
def _make_4326_polygon_from_bbox(bbox: list, srid: int) -> Polygon:
""" make a shapely Polygon in SRID 4326 from bbox and srid"""
try:
polygon = box(bbox[0], bbox[1], bbox[2], bbox[3])
if not srid == 4326:
geometry = shape.from_shape(polygon, srid)
geom_4326 = db.engine.execute(ST_Transform(geometry, 4326)).scalar()
polygon = shape.to_shape(geom_4326)
except Exception as e:
raise ProjectSearchServiceError(f"error making polygon: {e}")
return polygon
def _get_area_sqm(polygon: Polygon) -> float:
""" get the area of the polygon in square metres """
return db.engine.execute(
ST_Area(ST_Transform(shape.from_shape(polygon, 4326),
3857))).scalar()