DMA Release 0.8  Copyright ©1995 by Genesis Project, Ltd.  All Rights Reserved.





MiscIrelandCoordConverter




Inherits From: MiscPlanetCoordConverter : MiscCoordConverter : Object
Declared In: misckit/miscgiskit/MiscIrelandCoordConverter.h



Class Description

A MiscIrelandCoordConverter object is able to convert an Irish UTM Grid Coordinate to World Coordinates and vice versa.  An instance of it is created and used by MiscIrelandUTMCoord and MiscIrelandOldUTMCoord objects.  There is little reason for it to be used outside of those classes.  It exists as a subclass of MiscPlanetaryCoord because the Irish grid uses slightly different equations than the UK National Grid or the International UTM Grid.

This converter will respond to the convert:to: method for the following coordinate conversions:

From Class To Class
Irish UTM World
World Irish UTM
Irish Old UTM World
World Irish Old UTM
World World
Irish UTM Irish UTM
Irish Old UTM Irish Old UTM


The following differences exist between the two grid systems:

The Irish document uses some constants to keep values small for calculation using tables. This constants have not
been used. since they have no effect on machine calculation.

The UK document has equations IIIA and XIIA, extra correction terms.  They are not used in Irish Grid and are left out here.

The UK document cancels the minus signs in equations V and VI and  in the E equation, which the Irish Grid does not do. This has no effect, and is done according to the UK practice.

The Irish grid adds a last term in VI that is not present in the UK equations.  It is included here.

There is an erratum to the Irish document that gives the new  values of a and b. Old values are used in MiscIrelandOldUTMCoord objects,  new ones are used in IrelandUTMCoord objects.

For information on the underlying mathematics, refer to:

UK Ordinance Survey Information, "Transverse Mercator Projection, Constants, Formula and Methods",
March 1983

Ireland Ordinance Survey, "Tables for the Transverse Mercator Projection of Ireland", 1953, reprinted 1971

The equations are theoretically accurate to within 1 millimeter.  Extra decimal places are stored for the purpose of slowing error propagation that affects the numbers at the  millimeter scale, not because the extra digits are meaningful in and of  themselves.  If calculations require greater than 1mm accuracy, a different set of equations must be used:

Redfern, JCB, "Transverse Mercator Formulae", 1948, Empire Survey Review, 9(69) pg318-322



Method Types

Creating new instances + new
- finishUnarchiving



Class Methods

new
+ new

Create and initialize an instance of MiscIrelandCoordConverter if one does not already exist.  If one already exists, return it.  The new instance is registered so that it can also act as a subcontractor for other converters.




Instance Methods


finishUnarchiving
- finishUnarchiving

A finishUnarchiving message is sent after the MiscIrelandCoordConverter object has been read in from a stream. This method substitutes an existing one substituted for it.  self is freed and the existing  object is returned.