There are many ways of measuring distances on a map: using dividers, a length of string, a ruler, a scale etc. Linear scale Measuring Distances on a Map The diagram below shows two examples of linear scale. This scale is drawn to assist in measuring distances and is worked out in kilometres and fractions of kilometres. So, for example, small scale maps (such as 1:250 000) are good for long distance vehicle navigation, while large scale maps (1:25 000) are ideal for travel on foot and RFS vehicles as these show the detail required by walkers and for fighting fires.ġ:100 000 Yass - Segment of a 1:100 000 scale map of Yassġ:250 000 Yass - Segment of a 1:250 000 scale map of Yass Linear scale The larger the scale of a map, the smaller the area that is covered and the more detailed the graphic representation of the ground. In this case, one centimetre on the map represents 50,000 centimetres or 500 metres on the ground. Figure 14 is a segment of a 1:50 000 scale map of Yass. The larger the denominator of the RF, the smaller the scale. 1 cm on the map represents 100,000 cm or 1 km on the ground.Ĭommon scales for Australian topographic maps are: If the scale is 1:100 000, every distance on the map is 1/100,000 th of the distance on the ground, e.g. This method expresses the distance on the map as a fraction of the corresponding distance on the ground. There are two main methods of expressing the scale of a map: Words This relationship is constant, in whatever direction the distances are measured on large scale maps such as the 1:25 000 series. The scale of a map can be expressed as a representative fraction or as linear scale. Remember your GPS may show locations as 200m different if the map is not based on the GDA. If it is not present, check the datum used. Look for the GDA logo on your topographic map. WGS84 is the most common default datum in GPS.Ī major implication of this change is that GDA coordinates (latitude and longitudes and eastings and northings) differ from their AGD predecessors by approximately 200 metres in a north-easterly direction. For the most practical navigation purposes WGS84 and GDA coordinates may be regarded as being the same. The primary reason for this change is the widespread use of satellite-based navigation systems such as the Global Positioning System (GPS), which is based on the geocentric datum known as the World Geocentric System 1984 (WGS84). This new datum was defined in 1994, and 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 AGD84 coordinates are based on the same datum as AGD66 and for map reading and navigation purposes can be regarded as the same.įrom the year 2000, all Australian mapping authorities are using a new datum, the Geocentric Datum of Australia (GDA). In 1984 some Australian States adopted an updated version of this datum, known as AGD84. This datum best fitted the shape of the Earth for the Australian mainland. In 1966 the Australian Geodetic Datum (AGD) was defined. Mapping and coordinate systems are based on a datum, which is a mathematical surface that best fits the shape of the Earth. Vertical distance between contour lines on that particular map Method of indicating the scale of the map Gives the title and number of adjoining maps Gives map production details including the reliability, grid(s) shown, datums adopted Gives a legend of the symbols used on the map to represent various features, together with their meanings Shows, for a given year, the relationship between true, magnetic and grid north and their variations from grid north over time Identifies the map edition and the map numbering system (map number)ĭescribes how to determine a six figure grid reference It may be the name of an important town or an area and indicates roughly the location of the mapĭescribes the type of map, e.g.
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