ABOUT NATMAP RASTER

COORDINATE SYSTEMS

Overview of coordinate systems

Coordinates are numbers which describe the position of a point. These numbers are always given relative to a system of lines or angles ("coordinate system"). There are two main types of coordinate systems used in mapping:

Geographic coordinate systems. Position is defined by angles measured from the centre of the earth and is usually measured in degrees. This is ideal for measuring position on a globe.

Latitude is the angle measured north or south from the Equator. East-west lines have the same latitude. Lines of latitude are always parallel.

Longitude is the angle measured east or west from an imaginary north-south line - often a line called the Greenwich meridian is used which passes through the Greenwich observatory east of London. Lines of longitude are called meridians. All meridians meet at the North and South Poles.

The closer you get to the Poles (i.e. the larger the latitude value), the distance between meridians gets closer and closer. This means that one degree of longitude actually represents a smaller distance the closer to the Poles you are. For example, at the latitude of Dawin (about 12 degrees S), one degree of longitude is about 106km, but at Hobart (about 43 degrees S), one degree of longitude is only 79km. In the diagram below, the red line at Darwin is longer than the blue line at Hobart, yet they both measure 10 degrees of longitude.

Note that one degree of latitude is basically the same no matter where you are on the Earth (depending upon the type of datum you are using as the basis for your coordinate system).

Cartesian or rectangular coordinate systems. Position is defined by a linear distance from an origin point. This is best for measuring position on a flat surface such as a piece of paper or a computer screen. In mapping, eastings are the X-coordinate (the vertical lines that divide the map from west to east), and get larger to the east; northings are the Y-coordinate (the horizontal lines that divide the map from north to south), and get larger to the north. Together, these are called grid coordinates. On 1:250,000 scale maps, these lines are 10,000 metres or 10km apart. You can accurately measure distances between points using a ruler on these maps.

Map projections

It is not easy to convert the round earth to a flat map. If you peeled an orange and tried to lay the skin out flat, the skin would tear or distort. The same thing happens when you try to convert latitude and longitude into rectangular coordinates using a mathematical formula called a projection.

Some particular properties of the globe, whether it is shape, area or distance, will be distorted. However, you can stop or minimise distortion for one of these properties depending upon which projection is used, and this will work well depending upon what the map will be used for. This is explained below.

What is a datum?

A datum is a mathematical figure that best fits the shape of the Earth. The simplest datum is a globe. All datums have a certain size, and a centre point or origin somewhere in space, as well as other properties not explained here.

Before GPS, NATMAPs used a datum called the Australian Geodetic Datum, or AGD. NATMAPs used a version of AGD called AGD66. This datum was used as it best represented the shape of the earth over Australia.

From the year 2000, all Australian mapping authorities are using a new datum called the Geocentric Datum of Australia 1994. This datum is based on a mathematical surface that best fits the shape of the Earth as a whole, with its origin at the Earth's centre of mass (hence the term geocentric).

The main reason for this change is the widespread use of satellite-based navigation systems such as the Global Positioning System (GPS), which is based on a geocentric datum known as the World Geocentric System 1984 (WGS84). For most practical purposes, WGS84 and GDA94 are the same.

A major implication of this change is that GDA coordinates (both latitude/longitude and easting/northing) differ from their AGD predecessors by approximately 200 metres in a north-easterly direction.

Datum used for 1:1,000,000 scale maps

Note that the lines of latitude and longitude shown on the 1:1,000,000 scale maps in the Mosaic product are shown relative to AGD66 - however, the map images have been projected to GDA94. Therefore, the lines of latitude and longitude shown on the 1:1,000,000 maps differ from their true position by around 200m (about 5") in a north-easterly direction.

For more information on coordinate systems and datums, please refer to the Geoscience Australia Geodesy & GPS pages.

Coordinate systems and map projections used in NATMAP Raster

All layers in NATMAP Raster use rectangular coordinates, and use the GDA94 datum. However, the main difference between the Mapsheets and the Mosaic products is the map projection used.

The maps in NATMAP Raster Mosaic uses what is known as an Equirectangular map projection. This assumes that one degree of latitude equals one degree of longitude. The maps use degrees as the unit of measurement. (Note: when a map is said to be "on GDA94", this means that it uses latitude and longitude as coordinates based on the GDA94 datum.)

It is simple to make maps using this projection, and is popular for using in conjunction with GPS or computer mapping applications. However, as described above one degree of latitude does not equal one degree of longitude in reality, and this means that shapes and areas of features become very distorted and most distances are incorrect. As you can see in the above diagram, the shape of Tasmania is severely distorted from its true shape using this map projection. A scale bar does not make sense as the units of measurement (degrees) are angles, not straight line units such as metres.

Therefore, maps in NATMAP Raster Mosaic should only be used in conjunction with GPS or computer mapping applications. They should never be used for measuring distances or areas. If you need map images for measuring, then you should use NATMAP Raster Mapsheets.

The maps in NATMAP Raster Mapsheets uses a projection known as Universal Transverse Mercator or UTM. In this projection, the world is divided onto 60 zones, and within each zone shapes of features are preserved and distances are accurate enough for topographic mapping. Mainland Australia and Tasmania fall into seven of these zones.

Within each zone, there is a rectangular coordinate system that uses Eastings and Northings in metres.

The main problem with UTM maps is that you can measure outside a zone boundary by about 80km without too much trouble, but beyond this, distortion becomes so bad that measurements will be significantly inaccurate. This also means you cannot join maps together that lie in different zones.

The name of the coordinate system used for these maps is the Map Grid of Australia, or MGA. This means that a UTM projection was used to convert GDA94 geographic coordinates to grid coordinates. Older NATMAPs use a coordinate system called the Australian Map Grid (AMG), which used a UTM projection to convert AGD66 geographic coordinates to grid coordinates.

Summary of coordinate systems used in NATMAP Raster

Product	Layer	Coordinate System | CD Layout	Default Units
Mapsheets	250K Maps	Map Grid of Australia (MGA94)	Easting & Northing (metres)
	5M Sheet	Geocentric Datum of Australia (GDA94)	latitude & longitude (degrees)
	20M
	Map Index
Mosaic	Satellite Mosaic	Geocentric Datum of Australia (GDA94)	latitude & longitude (degrees)
	250K Mosaic
	1M Mosaic
	5M Sheet
	20M
	Map Index

NATMAP Raster Viewer has the ability to convert between MGA and AMG grid coordinates, and GDA94 and AGD66 geographic coordinates, for display purposes within the Viewer only. All maps will keep the GDA94 datum (and MGA grid coordinates for maps in NATMAP Raster Mapsheets) when used outside the Viewer.