def geometry(obj, tp_arcs, transform=None):
"""
Converts a topology object to a geometry object.
The topology object is a dict with 'type' and 'arcs' items, such as
-- {'type': "LineString", 'arcs': [0, 1, 2]}
"""
if obj["type"] == "GeometryCollection":
geometries = [geometry(feat, tp_arcs) for feat in obj["geometries"]]
return {"type": obj["type"], "geometries": geometries}
if obj["type"] == "MultiPoint":
scale = transform["scale"]
translate = transform["translate"]
coords = obj["coordinates"]
point_coords = dequantize(np.array(coords), scale, translate).tolist()
return {"type": obj["type"], "coordinates": point_coords}
if obj["type"] == "Point":
scale = transform["scale"]
translate = transform["translate"]
coords = [obj["coordinates"]]
point_coord = dequantize(np.array(coords), scale, translate).tolist()
return {"type": obj["type"], "coordinates": point_coord[0]}
else:
return {"type": obj["type"], "coordinates": coordinates(obj["arcs"], tp_arcs)}
def get_gridcell_geometry():
"""
Calculates the grid cell area and dimensions.
The length expressions are derived from the haversine formula:
hav(d/r)= hav(lat2-lat1) + cos(lat1) * cos(lat2) * hav(lon2-lon1)
See: http://math.stackexchange.com/a/479459
The expression for the area comes from: http://gis.stackexchange.com/a/29743
See also: https://badc.nerc.ac.uk/help/coordinates/cell-surf-area.html
"""
# FIXME: these 3 lines should be a get_lat_lon() function
latnrs, lonnrs = get_latnrs_lonnrs()
with netCDF4.MFDataset("%s/*.sp.nc" %
wam2layers_config.data_dir) as dataset:
latitude = dataset.variables['latitude'][latnrs]
geometry = collections.namedtuple(
'geometry', ['area', 'top_length', 'bottom_length', 'side_length'])
grid_size = np.abs(latitude[0] - latitude[1])
tops = np.radians(np.minimum(latitude + 0.5 * grid_size, +90))
bottoms = np.radians(np.maximum(latitude - 0.5 * grid_size, -90))
grid_size = np.radians(grid_size)
# define the haversine and inverse haversine functions:
hav = lambda x: np.sin(x / 2)**2
inv_hav = lambda x: 2 * np.arcsin(np.sqrt(x))
# For the "side" length, lon2 = lon1, then: hav(lon2-lon1) = 0, and the
# haversine formula becomes: hav(d/r) = hav(lat2-lat1)
# i.e.: d = r * (lat2-lat1)
side_length = AUTHALIC_EARTH_RADIUS * grid_size
# For the top and bottom lengths, lat2 = lat1, then:
# hav(lat2-lat1) = 0
# cos(lat1)*cos(lat2) = cos(lat1)**2
# and the haversine formula becomes: hav(d/r) = cos(lat1)**2 * hav(lon2-lon1)
# i.e.: d = r * inv_hav(cos(lat1)**2 * hav(lon2-lon1))
top_length = AUTHALIC_EARTH_RADIUS * inv_hav(
np.cos(tops)**2 * hav(grid_size))
bottom_length = AUTHALIC_EARTH_RADIUS * inv_hav(
np.cos(bottoms)**2 * hav(grid_size))
area = (AUTHALIC_EARTH_RADIUS**2 * grid_size *
np.abs(np.sin(tops) - np.sin(bottoms)))
return geometry(area[np.newaxis, :, np.newaxis], top_length[np.newaxis, :,
np.newaxis],
bottom_length[np.newaxis, :, np.newaxis], side_length)
def serialize_as_geojson(
topo_object,
fp=None,
pretty=False,
indent=4,
maxlinelength=88,
validate=False,
objectname="data",
):
from shapely.geometry import asShape
# prepare arcs from topology object
arcs = topo_object["arcs"]
transform = None
if "transform" in topo_object.keys():
transform = topo_object["transform"]
scale = transform["scale"]
translate = transform["translate"]
if arcs:
np_arcs = np_array_from_arcs(arcs)
# dequantize if quantization is applied
np_arcs = dequantize(np_arcs, scale, translate)
else:
np_arcs = None
# select object member from topology object
features = topo_object["objects"][objectname]["geometries"]
# prepare geojson featurecollection
fc = {"type": "FeatureCollection", "features": []}
# fill the featurecollection with geometry object members
for index, feature in enumerate(features):
f = {"id": index, "type": "Feature"}
if "properties" in feature.keys():
f["properties"] = feature["properties"].copy()
# the transform is only used in cases of points or multipoints
geommap = geometry(feature, np_arcs, transform)
if validate:
geom = asShape(geommap).buffer(0)
assert geom.is_valid
f["geometry"] = geom.__geo_interface__
else:
f["geometry"] = geommap
fc["features"].append(f)
return fc