Map projections in the Antarctic and their use
Maps are flat, but the surfaces they represent are curved. Transforming three dimensional space onto a two dimensional map is called 'projection'.
Projection formulas are mathematical expressions which convert data from a geographical location (latitude and longitude) on a sphere or spheroid to a representative location on a flat surface. This process distorts at least one of these cartographic properties: shape, area, distance, direction and often more.
It is important that anyone who uses a Geographical Informational System (GIS) to map or analysis their data, has an understanding of which projections distort which properties, and to what extent. Likewise an understanding of map coordinate systems and datums is important.
The Australian Antarctic Data Centre holds master topographical and thematic GIS datasets for the Australian Antarctic Territory and the subantarctic Territory of Heard and McDonald Islands. The Data Centre also holds datasets for Macquarie Island. All of these datasets are held in geographicals, i.e. latitude and longitude. This allows users to project data on the fly to a suitable projection. It also allows for the easy plotting in a GIS of geographical data, in particular, data from a Global Positioning System (GPS).
The most commonly used projections in the Australian Antarctic Program are: Polar Stereographic; Universal Transverse Mercator; and Lambert Conformal Conic.
These projections are conformal, that is, they preserve shape and angles over small areas. When working with precise referencing and directional relationships, it is necessary to develop a plane coordinate system, preferably one that is conformal.
The Polar Stereographic is a planar perspective projection where the south pole is viewed from the north pole. True scale is represented along the standard parallel. For example, 71°S was chosen for the 1:20 million scale map of Antarctica. This projection is good for mapping a hemisphere, continent and medium scale, (for example, mapping the whole of Antarctica,) as it preserves its shape. The true scale equals 0 if the plane is tangential to the Earth's surface at latitude 90°.
Universal Transverse Mercator
The Universal Transverse Mercator is a cylindrical projection made up of sixty zones which cover the globe. Each zone is six degrees and has its own central meridian. It has a limit of 80°S.
Lambert Conformal Conic
The Lambert Conformal Conic is a conic projection usually based on two standard parallels. It can represent the pole as a single point. This projection is good for mapping a region, continent and at medium or large scale.
Projections and Satellite Imagery
The most common question asked about a satellite image is probably "Where am I on this image", followed by "How accurate is the location". The answer will vary depending on the details known about the satellite, the instrument and control points on ground. Even then, the image could be distorted by topography, cloud or snow drift and time of day. It will also depend on the location of the control points. For example, control points on an AVHRR image may be a coastal feature like a cape, as opposed to a rock outcrop on a MSS image. The certainty of the location of control points can be difficult. Snow drift can result in a rock outcrop looking different from one image to another.
AVHRR imagery does not have a specific projection. A polar stereographic projection is applied using coastal features as control. This projection allows all the images along the coastline to be joined into a mosaic.
Landsat TM or MSS imagery has its own projection, the Spatial Oblique Mercator. This projection is nearly conformal and has little scale distortion within the sensing range. This projection incorporates the Earth's rotation with respect to the orbiting satellite. Scale is true along the ground track. This projection was designed to minimise distortion of the imagery as it orbits the rotating Earth. It is good for continuous mapping using Landsat imagery.
Properties of the Spatial Oblique Mercator:
Shape - Shape is correct within a few parts per million for the sensing range of the satellite
Area - Varies by less than 0.02 percent for the sensing range of the satellite
Direction - Minimal distortion within the sensing range
Distance - Scale is true along the ground track, and varies approximately 0.01 percent within the sensing range.
From Map Projections, a manual published by ESRI, page A-108
Also see Reference systems of maps and geographic information systems of Antarctica by Jörn Sievers and Heinz Bennat published in Antarctic Science 1 (4):351-362 (1989)