5.3 Coordinate Reference Frame

5.3.1 Command Status

Required - If using the recommended TUFLOW QGIS Plugin model initialisation process (Section 5.2) the coordinate reference frame commands are automatically populated and this section is provided for reference.

5.3.2 Description

The coordinate reference frame defines how spatial locations and units are interpreted within the TUFLOW FV model domain. It determines the projection system, horizontal geometry (cartesian or spherical) and the unit system used for all spatial inputs and calculations. All input and output files must be defined in the same coordinate reference frame.

TUFLOW FV offers three coordinate reference frame model implementations as presented in Table 5.1. These implementations represent predefined combinations of horizontal geometry and unit system. Links in the Model Implementation column provide direct access to further information and example syntax for each implementation.

The coordinate reference frame is configured using the following five steps.

  1. Set the model projection. For QGIS or ArcGIS use the SHP Projection command. For MapInfo use the MI Projection command
  2. Set the GIS output format. For QGIS or ArcGIS use GIS Format == SHP. For MapInfo use GIS Format == MIF
  3. Generate empty template GIS input layers in the model coordinate system using the Write Empty GIS Files command
  4. Set the computational coordinate system to either Cartesian or Spherical using the Spherical command
  5. Select the model units as either Metric or US Customary via the Units command

The coordinate reference frame commands are summarised in Table 5.2.

Table 5.1: Coordinate Reference Frame Model Implementations
Model Implementation Description
Cartesian (Metric Units) Projected coordinate system with eastings and northings in meters.
Spherical (Metric Units) Geographical coordinate system with longitudes and latitudes in decimal degrees.
Cartesian (US Customary Units) Projected coordinate system with eastings and northings in feet.
Table 5.2: Coordinate Reference Frame Commands
Command Description
GIS Format Required - Sets the format of output GIS files.
Write Empty GIS Layers Required - Writes empty template GIS files used for TUFLOW FV model development in the choosen model projection.
SHP Projection Conditional - Required if reading ESRI shape file layers or if GIS Format == SHP. Defines a projection string or .prj file that sets the geographical coordinate system for all input and output shape file GIS layers.
MI Projection Conditional - Required if reading Mapinfo .mif file layers or if using GIS Format == MIF. Defines a projection string or .mif file that sets the geographical coordinate system for all input and output GIS .mif layers.
GIS Projection Check Optional - Sets the error level when comparing input GIS layer projections with the model projection.
Spherical Optional - Defines the computational coordinate system used by TUFLOW FV’s solver to Spherical (Longitude and Latitude) or Cartesian (Eastings and Northings) coordinates.
Latitude Conditional - Required if Spherical == 0. Sets the latitude for Coriolis calculations when a Cartesian coordinate system is used.
Units Optional - Sets the model to Metric or US Customary Units.

5.3.3 Cartesian (Metric Units)

Cartesian models use projected coordinates in horizontal units of meters. This coordinate reference frame implementation is typically used for models with domains less than 100km in size. This is the default coordinate reference frame.

! Cartesian (Eastings and Northings) Model in Metric Units
SHP Projection == ..\model\gis\projection.prj ! Projection string - No default
GIS Format == SHP ! {MIF} | SHP
! Write Empty GIS Layers == ..\model\shp\empty ! Directory to write empty GIS vector layers. Only need to run this once during model setup
Units == Metric ! {Metric} | US Customary | Imperial | English
Spherical == 0 ! {0} Cartesian | 1 Spherical
Latitude == -32.0 ! {0.0} Representative latitude for study area

5.3.4 Spherical (Metric Units)

Spherical models use longitude and latitude coordinates in horizontal units of decimal degrees. This coordinate reference frame implementation is typically used for coastal and ocean model domains.

! Spherical (Longitude and Latitude) Model in Metric Units
SHP Projection == ..\model\gis\projection.prj ! Projection string - No default
GIS Format == SHP ! {MIF} | SHP
! Write Empty GIS Layers == ..\model\shp\empty ! Directory to write empty GIS vector layers. Only need to run this once during model setup
Units == Metric ! {Metric} | US Customary | Imperial | English
Spherical == 1 ! {0} Cartesian | 1 Spherical

5.3.5 Cartesian (US Customary Units)

The Cartesian (US Customary Units) coordinate reference frame implementation allows model inputs and outputs to be specified in US Customary units, as listed in Table 5.3. All governing equations are evaluated internally using metric (SI) units. Input values provided in US Customary units are converted to metric units prior to solution, and model results are converted back to US Customary units during output. This unit conversion does not alter the numerical formulation of the governing equations. This implementation is supported for the 2D HD simulation class only.

Table 5.3: Metric and Equivalent US Customary Units
Parameter Metric Units US Customary Units
Length m ft
Velocity m/s ft/s
Flow m3/s ft3/s
Eddy Viscosity m2/s ft2/s
Bed Shear Stress N/m2 lbf/ft2

This coordinate reference frame implementation is typically used for models with domains less than 50 miles in size.

! Cartesian (Eastings and Northings) Model in US Customary Units
SHP Projection == ..\model\gis\projection.prj ! Projection string - No default
GIS Format == SHP ! {MIF} | SHP
! Write Empty GIS Layers == ..\model\shp\empty ! Directory to write empty GIS vector layers. Only need to run this once during model setup
Units == US Customary ! {Metric} | US Customary | Imperial | English
Spherical == 0 ! {0} Cartesian | 1 Spherical
Latitude == 27.5 ! {0.0} Representative latitude for study area