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Geospatial Information: Using Maps

Info on UWA's printed maps, online interactive maps, SLIP etc

Projection

 

 

The Australian Intergovernmental Committee on Surveying and Mapping (ICSM) has an excellent page on The Fundamentals of Mapping which includes information on map projection.

The United States Geological Survey (USGS) website also has an excellent page on Map Projections.

Another good source of information map projection is the Qgis website's page Coordinate Reference Systems.

Latitude - Longitude - Grid

 

 

Lines of latitude run east and west around the circumference of the earth. They are used to measure north-south positions. The equator is defined as 0 degrees, the North Pole is 90 degrees north, and the South Pole is 90 degrees south. Lines of latitude are all parallel to each other, thus they are often referred to as parallels. Each degree of latitude spans a north south distance of 111 kilometers or 60 nautical miles.

Lines of longitude run between the North and South Poles. They measure east-west position. The prime meridian is assigned the value of 0 degrees, and runs through Greenwich, England. Meridians to the west of the prime meridian are measured in degrees west and likewise those to the east of the prime meridian are measured to by their number of degrees east. The distance spanned by each degree varies between the poles and the equator.

To aid navigation many maps overlay a simple grid where the north-south and east-west lines meet at right angles. Grid lines may lay close to lines of latitude or longitude.

Definitions of latitude, longitude and grid lines can be found on the Australian Intergovernmental Committee on Surveying and Mapping (ICSM) site page and on The Fundamentals of Mapping - Marginalia Information (information about the map) under the heading Explaining Some Jargon.

Australian maps are included in the Australian Map Grid (AMG).  For more information about the AMG see the the Australian Intergovernmental Committee on Surveying and Mapping (ICSM) site page on The Fundamentals of Mapping - Some Commonly Used Map Projections under the heading UTM Map grid and the Australian Map Grid.

Scale

 

 

Scale is the relationship (or ratio) between distance on a map and the corresponding distance on the ground. For example, a scale of 1:100,000 on a map means, 1cm on the map equals 1km on the ground (0.01 meters * 100,000 = 1,000 meters = 1 kilometer). Scale can be confusing; the larger the reference number, the smaller the scale. A 1:100,000 scale map is said to be a larger scale than a 1:250,000 scale map.  A 1:100,000 scale map will cover a smaller area of the earth's surface than a 1:250,000 map of the same size.

Scale may be shown on a map as a representative fraction, 1:100,000 for example or by a scale bar such as the one illustrated above.

The Australian Intergovernmental Committee on Surveying and Mapping (ICSM) has an excellent page on The Fundamentals of Mapping which includes information on scale.

Magnetic Variation

 

 

Somewhat confusingly, maps can show three different versions of North.

  • True North
  • Magentic North
  • Grid North

True North is the direction to the North Pole and is determined by the Earth's axis of rotation

Magentic North is the direction to which a compass will point.  The North Magnetic Pole is close to but not collocated with the North Pole.

Grid North is the orientation of the map grid.  The grid will not always point to True North or Magnetic North as it is affected by the projection used to convert the curved surface of the Earth to the flat surface of the map.

For a more detailed explanation of these differences see the page, Magnetic North vs Geographic (True) North Pole on the GIS Geography website. 

Details of how Magnetic and True North varies in Australia and how they can be converted can be found on the page, Australian Geomagnetic Reference Field Values on the Geoscience Australia website.

Glossary of Map Terms

 

A useful list of words related to maps, cartography and geography - along with their definitions!

CONTENT LICENCE

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