Map basics: a guide to the fundamentals of mapping
What is a map?
A map is a spatial representation of the Earth's surface showing how things are related to each other by distance, direction, shape and size.
A map has 5 key components:
- scale - the relationship between the real world and the map
- projection - a mathematical formula that helps project the curved earth surface onto the flat paper surface
- reference system - a coordinate system to help maintain spatial accuracy
- datum - a base from which coordinates and heights can be measured
- presentation - cartographic practices that make the map easy to read.
A scale is the relationship between the distance in the real world and the distance on the map. Scale can be shown as:
- ratio (e.g. 1:1,000) - one unit on a map represents 1,000 units in real world
- words - 1cm equals 1km
- graphic - a line marked with the distance of the real world indicated on the line.
Small scale maps (e.g. 1:1,000,000) show less detail and cover larger areas such as a whole country.
Large scale maps (e.g. 1:1,000) show more detail and cover smaller areas such as a suburb.
A cartographer's (or mapmaker's) fundamental issue when making a map is how to represent the curved surface of the Earth on to a flat surface. Have you ever tried to wrap a soccer ball without creasing the paper?
A cartographer uses mathematical formulas to reduce the distortion on a map. These formulas are called projections. Cartographers apply different projections to suit the purpose of the map.
Different projections can be applied to ensure 1 or more of the following remain accurate while potentially compromising those remaining:
- angles of direction.
The Robinson projection of the Earth. Image © www.icsm.gov.au/mapping/map_projections.html
For example, in the Mercator projection, direction remains constant and the shape of small objects is preserved. However, distortions appear as you move towards the poles as landmasses (shapes/areas) like Greenland, appear larger than they are.
Reference System Geographic coordinates (latitude and longitude)
Latitude are imaginary lines running around the Earth parallel to the equator. The lines of latitude are measured in degrees (º), minutes (') and seconds ("), or decimal degrees, north or south of the equator.
Convention says the equator is zero degrees (0º). The lines of latitude are called parallels with the extents being the North Pole at 90 degrees north (90ºN) and South Pole at 90 degrees south (90ºS).
Globe of the Earth showing the 4 significant lines of latitude. Image © www.icsm.gov.au/mapping/coordinates.html
Globe of the Earth showing lines of latitude and longitude. Image © www.icsm.gov.au/mapping/coordinates.html
Longitude or meridians are a series of imaginary lines running north–south around the Earth, connecting the North and South Poles. By convention, the prime meridian runs through Greenwich in England and is at 0º longitude.
Meridians, also measured in degrees, minutes and seconds, or decimal degrees, are either east or west of this prime meridian up to 180º (approximately the International Date Line that runs through the Pacific Ocean).
Combining the latitude and longitude gives us a reference for any point on the Earth's surface. For example, the Brisbane GPO is about 27º28'03"south and 153º01'40"east.
Grid coordinates (eastings and northings)
The other common coordinate system used is Universal Transverse Mercator (UTM) or grid system. Based on a mercator projection (common for world maps), the UTM system divides the surface of the Earth up into a grid.
To reduce distortion, it uses a series of 60 zones in the east–west direction (6º wide).
UTM or grid coordinates are expressed in metres measured from a false origin. Drawn down the middle of each zone is a line known as the central meridian.
In each zone the central meridian is assigned a false easting of 500,000 metres. An easting value greater than 500,000 indicates it is to the east of the central meridian for that zone.
Northings are measured in metres from the South Pole in the southern hemisphere and from the equator in the northern hemisphere. About 10,000,000 metres of northing are in each hemisphere.
Map of Tamborine showing grid coordinates. © Department of Natural Resources and Mines
The coordinates are represented with the zone number first followed by the easting and northing in metres. There are no negative numbers in eastings and northings.
In Australia, the local version of the UTM projection system is the Map Grid of Australia (MGA94). For example, Brisbane GPO is Zone 56 Easting 502744 Northing 6961782.
There are different types of north.
- True north is based on the Earth's axis or the direction of a line of longitude which converges on the North Pole.
- Grid north is based on the grid coordinate system and varies only slightly from true north.
- Magnetic north is determined by the Earth's magnetic field and is the direction your compass points.
The difference between true north and magnetic north is called magnetic variation. Maps are generally based on the true or grid north because they do not change over time like magnetic north.
Datums are the base reference for coordinate systems. The vertical datum is a base height from which all heights are measured. The horizontal datum is the base used to fix a position.
In Australia, the vertical datum is the Australian Height Datum (AHD), which is the mean sea level. Generally, all heights are measured from AHD. The horizontal datum in Australia is the Geocentric Datum of Australia (GDA94). This datum is internationally compatible and is suitable for GPS coordinates. Coordinates from previous datums (AGD66 and AGD84) can be converted to the new GDA94. All maps now produced in Australia are based on GDA94, however users should check the map to confirm what datum is being used.
Earth's true shape. Image © www.icsm.gov.au/mapping/datums1.html
A map tells a story symbolically.
Features (e.g. towns, roads and rivers) need to be shown and labelled to effectively display spatial information. Cartographers use symbols to assist with the representation of the features. These are explained via a legend. Placement of the symbols and labels rely on human input and experience and can become complicated.
Over the centuries cartographers have developed this art so that points of interest can be clearly identified even on the busiest of maps.
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