Coordinate Reference Systems

ANUGA works in a projected coordinate system — all domain coordinates are in metres, not degrees. A Geo_reference object records which coordinate system is in use and the local origin offset (xllcorner, yllcorner) that keeps the mesh coordinates numerically small.

Every SWW output file stores the complete geo-reference so downstream tools (QGIS, Python post-processing, etc.) can interpret the coordinates correctly.

—

Why a local origin?

Raw UTM or national-grid coordinates for a real site are typically large numbers (e.g. easting 308 500 m, northing 6 189 000 m for Cairns, Australia). Storing every mesh vertex relative to a local origin keeps the values small and avoids floating-point precision loss during computation.

Coordinates stored in a SWW file are relative to the geo-reference origin. To recover absolute coordinates, add xllcorner / yllcorner back to the mesh x / y arrays.

—

WGS84 UTM (the common case)

Most real-world ANUGA simulations use a WGS84 UTM zone. Pass zone and hemisphere (and optionally the local origin) to Geo_reference:

import anuga

# Cairns, Australia — UTM zone 55, southern hemisphere
geo_ref = anuga.Geo_reference(
    zone       = 55,
    xllcorner  = 363000.0,   # easting of domain origin (m)
    yllcorner  = 8132000.0,  # northing of domain origin (m)
    hemisphere = 'southern',
)

domain = anuga.create_domain_from_regions(
    bounding_polygon,
    boundary_tags,
    geo_reference = geo_ref,
)

The EPSG code is computed automatically from the zone and hemisphere and written to the SWW file:

• Northern hemisphere: EPSG = 32600 + zone (e.g. zone 31N → EPSG 32631)

• Southern hemisphere: EPSG = 32700 + zone (e.g. zone 55S → EPSG 32755)

>>> geo_ref.epsg
32755

Alternatively, supply the EPSG code directly and let ANUGA infer the zone and hemisphere:

geo_ref = anuga.Geo_reference(
    epsg      = 32755,
    xllcorner = 363000.0,
    yllcorner = 8132000.0,
)
>>> geo_ref.zone
55
>>> geo_ref.hemisphere
'southern'

—

National and non-UTM coordinate systems

Some countries use national projected CRS that do not follow the UTM zone structure. Common examples:

Country / region	EPSG code	CRS name
Netherlands	28992	Amersfoort / RD New
Great Britain	27700	OSGB36 / British National Grid
Germany (ETRS89 / UTM zone 32)	25832	ETRS89 / UTM zone 32N
GDA2020 MGA zone 56 (Australia)	7856	GDA2020 / MGA zone 56
WGS84 geographic (lat/lon)	4326	WGS84

These work fine with ANUGA — pass the EPSG code directly. If pyproj is installed, the datum, projection name, and false easting/northing are populated automatically from the EPSG definition:

# Dutch simulation in Rijksdriehoekstelsel (RD New)
geo_ref = anuga.Geo_reference(
    epsg      = 28992,
    xllcorner = 120000.0,   # easting relative to RD New origin (m)
    yllcorner = 480000.0,   # northing relative to RD New origin (m)
)

>>> geo_ref.epsg
28992
>>> geo_ref.projection
'Amersfoort / RD New'
>>> geo_ref.datum
'Amersfoort'
>>> geo_ref.false_easting, geo_ref.false_northing
(155000, 463000)
>>> repr(geo_ref)
'(crs=Amersfoort / RD New, easting=120000.000000, northing=480000.000000, epsg=28992)'
>>> geo_ref.is_located()
True

Note

ANUGA does not perform any reprojection — it treats all coordinates as a flat Cartesian grid in metres. Convert your input data to the target projected CRS before building the domain. Tools such as pyproj, GDAL, or QGIS can reproject shapefiles, rasters, and point clouds as needed.

—

Wavetank and hypothetical simulations

When no real geographic location is involved (e.g. a laboratory wave-tank or an idealised test case) simply omit zone and EPSG:

geo_ref = anuga.Geo_reference()   # zone=-1, no EPSG
geo_ref.is_located()              # False

The domain coordinates are then treated as an arbitrary Cartesian system with no connection to any geographic CRS.

—

Checking what is stored in a SWW file

The geo-reference metadata can be read directly from a SWW file:

from anuga.file.netcdf import NetCDFFile
from anuga.coordinate_transforms.geo_reference import Geo_reference
from anuga.config import netcdf_mode_r

fid = NetCDFFile('my_simulation.sww', netcdf_mode_r)
geo_ref = Geo_reference(NetCDFObject=fid)
fid.close()

print(geo_ref)          # e.g. (zone=55, ..., epsg=32755)
print(geo_ref.epsg)     # EPSG code stored in file
print(geo_ref.is_located())

—

Reference

class anuga.Geo_reference(zone=None, xllcorner=0.0, yllcorner=0.0, datum='wgs84', projection='UTM', units='m', false_easting=None, false_northing=None, hemisphere='undefined', epsg=None, NetCDFObject=None, ASCIIFile=None, read_title=None)

Coordinate reference system for an ANUGA domain.

Attributes

zoneint

UTM zone (1–60), or -1 (DEFAULT_ZONE) for non-UTM / arbitrary origin (e.g. a wave-tank simulation).

hemispherestr

'southern', 'northern', or 'undefined'.

xllcornerfloat

X coordinate (easting) of the local origin relative to the UTM grid.

yllcornerfloat

Y coordinate (northing) of the local origin relative to the UTM grid.

datumstr

Geodetic datum (default 'wgs84').

projectionstr

Map projection (default 'UTM').

unitsstr

Units of measurement (default 'm').

false_eastingint

False easting offset (500 000 m for WGS84 UTM).

false_northingint

False northing offset (10 000 000 m southern, 0 m northern).

epsgint or None

EPSG code for the coordinate reference system. For WGS84 UTM this is auto-computed from zone and hemisphere (32600 + zone for northern, 32700 + zone for southern). Returns None when the zone is DEFAULT_ZONE or the CRS cannot be determined.

property epsg

EPSG code for this coordinate reference system.

For WGS84 UTM projections the code is computed automatically from zone and hemisphere when it has not been set explicitly:

• Northern hemisphere: 32600 + zone (e.g. zone 55N → EPSG 32655)

• Southern hemisphere: 32700 + zone (e.g. zone 55S → EPSG 32755)

Returns None when the zone is DEFAULT_ZONE (-1) or the CRS cannot be determined.

Returns

int or None

get_absolute(points)

Given a set of points geo referenced to this instance, return the points as absolute values.

get_epsg()

Return the EPSG code for this coordinate reference system.

Returns

int or None

EPSG code, or None if unknown.

get_hemisphere()

Check if this georef has a defined hemisphere.

get_origin()

Get origin of this geo_reference.

get_relative(points)

Convert points to relative measurement.

points Points to convert to relative measurements

Returns a set of points relative to the geo_reference instance.

This is the inverse of get_absolute().

get_xllcorner()

Get the X coordinate of the origin of this georef.

get_yllcorner()

Get the Y coordinate of the origin of this georef.

get_zone()

Get the zone of this georef.

is_located()

Return True if this geo-reference describes a real, located CRS.

A geo-reference is located when either:

• zone is a valid UTM zone (1–60), or

• an EPSG code has been set (covers national grids, geographic CRS, and any other projected CRS that does not use UTM zones, e.g. EPSG:28992 Netherlands RD New, EPSG:27700 British National Grid).

A geo-reference with zone == DEFAULT_ZONE (-1) and no EPSG code is unlocated — this is the case for wavetank or other hypothetical simulations with an arbitrary local origin.

Returns

bool

read_NetCDF(infile)

Set georef attributes from open NetCDF file.

Parameters

infilefile handle

Handle to an open NetCDF file.

write_NetCDF(outfile)

Write georef attributes to an open NetCDF file.

Parameters

outfilefile handle

Handle to an open NetCDF file.