MapRequest Datums and Projections

 

MapRequest defines the coordinate system of a map in terms of its datum (model of the earth) and projection (the manner in which the curved surface of the earth is represented on a flat map). The datum and projection of a map are defined in the map’s .properties file (generally using the datum and projection parameters). To use a supported datum or projection, the name must exactly match one of the datums or projections listed here.

 

 

Supported Datums

This list is current as of version 1.0.

MapRequest datum

Name

Comments

AGD66

Australian Geodetic Datum 1966

 

AGD84

Australian Geodetic Datum 1984

 

GD49

New Zealand Geodetic Datum 1949

 

GDA

Geocentric Datum for Australia

Equivalent to WGS84

NAD83

North American Datum 1983

 

OSGB

Ordinance Survey Great Britain

Generally used with the BNG projection

WGS84

World Geocentric System 1984

 

 

Supported Projections

This list is current as of version 1.0.

MapRequest projection

Name

Comments

BNG

British National Grid

MapRequest computes the alphabetic zone identifier from the most significant digit of the northing and easting.

HK80

Hong Kong 1980

Represented as a custom Transverse Mercator grid in Ozi

IL East

IL East (USA) State Plane

Measured in metres

IL East Feet

IL East (USA) State Plane

Measured in US survey feet.

latitude/longitude

Latitude/Longitude

 

NY West

NY West (USA) State Plane

Measured in metres.

NY West Feet

NY West (USA) State Plane

Measured in US survey feet.

NZG

New Zealand Grid

Lat/long measurements (where required) use a UTM approximation of NZG.

South Africa Lo 17

South Africa Transverse Mercator (17 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 19

South Africa Transverse Mercator (19 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 21

South Africa Transverse Mercator (21 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 23

South Africa Transverse Mercator (23 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 25

South Africa Transverse Mercator (25 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 27

South Africa Transverse Mercator (27 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 29

South Africa Transverse Mercator (29 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

South Africa Lo 31

South Africa Transverse Mercator (31 degrees midpoint)

Represented as a custom Transverse Mercator grid in Ozi.

UTM

Universal Transverse Mercator

MapRequest requires the zone and hemisphere parameters specified.

VICGRID66

Victoria (Australia) grid system. (1966 version)

Equivalent to “VICGRID” in OziExplorer.

VICGRID94

Victoria (Australia) grid system. (1994 version)

Represented as Lambert Conformal Conic in OziExplorer.

 

Defining a new Datum or Projection

Each datum or projection is defined in its own .txt file in the cs (for coordinate systems) folder (under the folder where MapRequest is installed). Files defining a datum end in _datum.txt, and files defining a projection end in _projection.txt. MapRequest uses the GeoTools package to do coordinate system transformations, and as such it requires the WKT (Well Known Text format) definition of each datum or projection.

 

A file defining a datum or projection consists of a number of lines, each in the form:

 

parameter=value

 

A datum or projection must define both the “name” and “wkt” parameters. Here is a full list of recognised parameters (current as at 1.0b2):

 

name

The name of the datum or projection in MapRequest (as used by the datum or projection parameters in the map’s .properties file). A datum or projection must have a unique name.

 

wkt

The WKT (Open GIS Well Known Text format) representation of the datum or projection. Note that datum files will normally define a GEOGCS[], and projection files will normally define only a PROJECTION[]. MapRequest will combine these in an appropriate manner to produce a PROJCS[] (unless the “latitude/longitude” projection is used).

 

oziexplorer

The name of the datum or projection in OziExplorer (optional).

 

oziexplorer_code (datums only)

If required, this can be used to specify the base datum in OziExplorer (generally blank or “WGS 84”).

 

oziexplorer_projection_params (projections only)

If the projection requires parameters in OziExplorer (e.g. for Transverse Mercator or Lambert Conformal Conic) then they can be specified using this parameter. E.g. oziexplorer_projection_params=40.000000000,-78.583333333,0.999937500,350000.00,0.00,,,,,

 

oziexplorer_unit_multiplier (projections only)

If the projection is a grid system that uses units other than metric metres (e.g. US Survey Feet) then the multiplier between the units the projection uses and metres is specified here. E.g. oziexplorer_unit_multiplier=0.304800609601219

 

oziexplorer_use_latlong_for_grid (projections only)

If the projection is a custom grid system, but OziExplorer does not allow this type of map to be calibrated using grid coordinates, set “oziexplorer_use_latlong_for_grid=true” to get MapRequest to write the calibration points using latitude/longitude.

 

Using an unsupported Datum or Projection

If a datum or projection name is specified in a map’s .properties file that MapRequest does not recognise, MapRequest can still work with the map, but some functions will be disabled. MapRequest will not calculate the latitude/longitude for the map corners (for moving map mode in OziExplorer), and it will not calculate the metres/pixel (map scale) value.

 

When writing a calibration file, MapRequest will use the name specified in the map’s .properties file (either datum or projection as appropriate) as the name for OziExplorer.

 

MapRequest Documentation