Area Limts, Scale and Projection Menu

The purpose of this menu is to define the user data coordinate system, the region of interest and the scale at which this region is to be displayed. In the simplest from the user need only supply the left, right, top and bottom coordinates of the region and the map scale. However the user may also define a coordinate projection system to enable geographic information to be included in the display. This might be required for simply annotating the border with Latitude and Longitude values, or if some data is provided in a coordinate system which is different from that required for the output. Note that only one geographic projection system is available, so geographic data from multiple projections cannot be included in the display.

Area Limits Menu

When the Area Limits menu item is selected, the following menu form is displayed.

The Minimum and Maximum easting and northing coordinates define the left, right, top and bottom of the region of interest and the map scale defines how these coordinate are converted to paper coordinates. The Easting and Northing Range quantities define the extent of the region of interest in the east/west and north/south directions respectively. The range quantities are updated whenever the appropriate minimum and maximum values are changed. If the range value itself is modified then the maximum value is changed to give the area limits the specified range.

Also available here is the Frame type drop-down box for which the available options are

• Easting/Northing
• Longitude/Latitude

If Easting/Northing is selected the easting and northing values are used to define the region of interest and the border will therefore be plotted as a rectangular box. If Longitude/Latitude is selected then the longitude and latitude values are used to define the region of interest and the border will therefore be plotted with curved sides.

Note that the map is plotted using a Cartesian coordinate system with the origin in the lower left corner with the x axis increasing to the left and the y axis increasing from the bottom of the page to the top and distance units in cm. Similarly the easting and northing coordinate system is a Cartesian coordinate system with easting axis increasing to the left and the northing axis increasing from the bottom of the page and nominal distance units in meters. Any geographic data is converted using the currently defined projection into the equivalent easting and northing coordinates which are then converted using the current map scale into cm coordinates on the map.

Geographic Limits Menu

 If there is a currently defined projection, clicking on the Geographic Limits tab will change the menu to the equivalent geographic limits.

The minimum, maximum and range values behave in the same fashion as their counterparts on the Area Limits menu with the exception that the values need to be entered in degrees, minutes and decimal seconds format. Note that the minimum latitude and longitude values are computed using the minimum easting and northing values and the maximum latitude and longitude use the maximum easting and northing values. This ensures that the geographic and rectilinear limits remain unchanged when converted from one system to the other and then back again.

Geographic Limits Menu

When the Projection tab is clicked, the Projection Definition menu screen is presented to the user:

This menu allows the user to specify the projection to be used to convert between rectilinear and geographic coordinates. The Grid drop-down box is used to select the primary method to be used for defining the rectilinear coordinate system for the geographical region of interest. The available choices are:

• Australian Map Grid
• Universal Transverse Mercator
• Lambert Conformal
• Local Grid

The definition of the spheroid used to approximate the earth's surface is selected using the Spheroid drop-down box for which the choices are:

• Australian National Spheroid
• International Spheroid
• Clark 1886
• Clark 1880
• Bessel
• Airy
• WGS72
• GRS80

The Australian map grid (AMG) and Universal Transverse Mercator (UTM) choices are based on the same system of using 6 degree longitude bands to break the earth's surface into zones. In each zone the curved surface of the earth as approximated by the selected spheroid is mapped into a rectilinear coordinate system. The origin of the coordinate system is modified by adding a false easting and northing so that all coordinates are positive. These values are automatically included in the conversion code, but can be changed using the provided text boxes if required.

The Lambert Conformal choice is provided for situations where there is a large longitudinal extent for the region of interest. The Lambert Conformal Transform is based on mapping the curved surface of the sphere onto the surface of the conic which intersects the surface of the earth at the two specified standard parallels of latitude.

The Local Grid coordinate system has no direct relationship to geographical coordinate systems and so geographical options will be disabled if this system is selected.

Send mail to john.paine@onaustralia.com.au with questions or comments about this web site.
Last modified: April 10, 2012