Where Am I?

On this Oblate Spheroid called Earth

GRID REFERENCES / COORDINATES

To accurately describe a location on Earth a north/south-east/west grid is overlaid the area from which a Grid Reference is then calculated.

The most common grid referencing systems are:

Degrees Minutes Seconds (DMS)
Universal Transverse Mercator (UTM)
Military Grid Reference System (MGRS)

There are two variations of DMS:
Decimal Degrees
Degrees, Decimal Minutes

The term ‘coordinates’ is often used interchangeably with ‘grid reference’ and, practicably, there is no difference.

PROJECTIONS – Globe to Plane

Because the Earth is a globe (oblate spheroid) and a map grid is a flat plane the means of depicting the information on the surface of the globe to the flat grid is achieved by a Projection.

The three common types of projections:

Cylindrical

The most common projection seen is the Mercator. This is the projection used by Google Earth and Google Maps. It is useful for straight line navigation over distances that do not span the globe.  It represents lines of constant true direction.

The Gall-Peters or just Peters Projection has extreme distortion in the polar regions, as any cylindrical projection must, however, it is a more accurate representation of surface area (though not shape) of the globe.

Conic

The distortion in a conic map makes it inappropriate for use as a visual of the entire Earth. Conic projections are usually used for regional ⁄ national maps of mid-latitude areas – such as Australia and the United States of America. The Lambert is a common Conic Projection.

Geoscience Australia used a Conic Projection to determine the Centre of Western Australia.

Azimuthal

Generally used for maps depicting the North or South Poles and are often known as polar projections. A compass direction is only correct from the centre point to another feature – not between other features.

Best Projection

A newer map, called the AuthaGraph, may be the most accurate map projection created to date.

The only ‘projection’ that has all features with no distortion is a globe.

TECTONIC PLATES

The Earth has seven major tectonic plates (African, Antarctic, Eurasian, Indo-Australian, North American, Pacific and South American).

These plates lie on top of a partially molten layer of rock and move relative to each other at different rates, from 10-160mm  per year.

The continent of Australia is on the Earth’s fastest moving tectonic plate and is drifting north-east about 70mm per year.

Tectonic plate movement or ‘continental drift’ has no effect on the actual Centre of Western Australia although, because it moves in relation to GPS satellites used for geo-location, it certainly affects the way the Centre is determined and needs to accounted for.

In respect of the Tropic of Capricorn in Australia (an arbitrary line of latitude) tectonic plate movement is having a major effect.

DATUMS

In simple terms, a datum is a reference point, a base line. A geodetic datum is used to precisely represent the position of locations on Earth.

Because Earth is an imperfect ellipsoid, local datums give a more accurate representation of an area of coverage than a world datum (WGS 84).

Australia is using a datum known as GDA2020.

GPS

GPS is a system of 30+ navigation satellites circling Earth that constantly transmit signals at varying frequencies. A GPS receiver uses these transmissions to calculate its distance from four or more GPS satellites, thus determining its location.

A more detailed explanation of the operation of GPS is here.

 

© Kim Epton 2024
598 words.

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